Designing Model Aircraft

Designing

MODEL liRe

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Designing

MO EL I C

Designing

© Peter Miller 1995

All rights reserved . All trad emarks and registe red nam es ack nowledged . No part of this book may be co pie d , reproduce d or transmitted in an y form w ithout the written co nse nt of the Publi shers. The information in this book is true to the best of our knowled ge at the time of co mpilation. Recommendation s are mad e witho ut any guarantee , implied or otherwise, on the part of the author or pu blish e r, who also disclaim an) liability incurr ed in co nnec tion w ith the use of data o r specific informa tion co ntaine d w ithin this publication . I , Peter Miller , hereb y asse rt and give no tice of my right unde r section 77 of the Copyright, Designs and

Patents Act 1988 to be ide ntified as the author of this book .

First published by Trap let Publi cation s Limited in 1995 Rep rinte d 2002 Trap let Hou se , Severn Drive, Upto n-upon-Severn , Worceste rsh ire . \'(1R8 O]L United Kingdo m.

ISBN 0 9510589 6 7

Front Cover Qua rk is a C02 p owered modelfor tbe CETO su b miniature radio, total, ready to fly weigbt is 2 ou nces . Inset is Pee Tee Spo rtsterfor .40 to .45 fo ur stroke engines. Ba ck Cover The author witb bts u np ublished "Belgarion 'tfo r Sa ito 90 twin . Tech n ical Drat oings by Lee Wiseda le Cm 100ns by Simo n Bates

TRAPLET

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Printed and bo und by Step he ns & George Limited , Merthyr Industrial Estate, Dowla is, Merthyr Tyd fil, Mid Glamo rgan CF48 3TD

About the Author ete r Miller started modellin g in 1952 w ith rubber pow ered kits before moving o n to pow er mod els in 1954. After several attempts to learn to fly co ntrol line mod e ls with out ac hieving o ne co mplete lap he design ed his own train er w ith whi ch he finally taught himself to fly. Being tot a lly obsessed with aircraft, Pet er spent 12 years as as airfra me fitter in the R.A.F. whi ch removed some of h is illusions a bo u t a ircra ft. As Pe ter puts it, "Most aircraft are de sign ed by sad ists and yo u need to be a cross be tween an oc to pus and a co ntor tio nist to work on them ." Dur ing this time he co ntin ue d to build models from kits, plan s and his own design s. In 1974 Peters first plan was published , a co ntrol line aeroba tic bipl an e . Over the next 8 yea rs ano the r 9 co ntrol line plan s found their way into British and Amer ican magazines togeth er w ith several articles. 1982 saw th e pu b lica tion of the a utho r's first R/ C design wh ich has been fo llowe d by a w ho le host of va rie d de s igns p ublis hed in most o f th e magazines in Britain and the USA. Peter is not only known as a designer, he wrote the "Engine Bay" co lumn in RC Mode l Wo rld fo r 7:;' years a n d cu rre n tly has tw o co lum ns runnin g w ith o ther magazines. As well as th e co lum ns, Pe ter has writte n articles o n a w ide ran ge of mod ellin g su bjects a nd produ ced nume rou s kit and produ ct rev iews , as we ll as writing "Fo ur Stroke Model Engines", the first book o n the subject of the modern four stro ke eng ines wh ich wa s publi shed by Trapl et Publication s.

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Contents Page

Chapter 1 Chapter 2 Chapter 3 Chapter 4 Chapter 5 Chapter 6 Chapter 7 Chapter 8 Chapter 9 Chapt er 10

Introduction Why and What With? Essential Aerodynamics The First Design Layout Lightness Equals Strength Fuselages Wings and Tails Undercarriages and Other Bits Scaling Up Drawings Flight Testing Selling Your Plan Usefu l Addresses Notes

9 11 15 22 31 39 46 .54 64 69 73 78 79

Introduction ne of the first questions any specta tor asks a modeller is 'Did you make it yourself?' and there ca n be few modellers who do no t fee l a touch of pride when they say 'Yes, I did.' Imagine then the pr ide you would fee l it yo u could say, 'It is my own design.' A response which brings a look of respect to the spectators eye . Q uite apart from the ego factor , designing your own mo dels adds a treme ndo us amount of interest to the hobby and, no t on ly can it save you money it can actually be profi tab le. Th ere are other books o n designing mod els but this book brings the subject down to the simples t possible ter ms and methods. No maths tha t cannot be ha nd led on a basic calcu lator, the minim u m of th e o ry w hic h is reduced to the barest facts in understandable terms. Even if you do no t want to design models, knowing a little about design will he lp you to improve on kits, spot poten tial weak spots, mod ify o ther designs without m ining them and it w ill he lp yo u to understand why things are done in a particular way. I have not tried to simplify the su bject for the book , what I have done is lay o ut the methods tha t I use , no more and no less, to pro duce my own designs. Judge for yourself how successfu l my methods are, 48 published plans in 10 magazines in three countries over 20 yea rs at the time of wri ting with another 8 awaiting publication, and I don't know how many designs tha t have never been sent in to magazines.

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Designing Model Aircraft

9

Chapter 1

Why and What With? hy bother to design yo ur own mod els? After all, look at all the kits arou nd. Isn't it e no ugh that yo ur fellow mod ellers wi ll a d mire yo ur ar ti sti c pl a cin g o f th e s tickers o n th e seve n tee n th "Maniac" in the pits. Wh at is wrong w ith pa ying £5 0 for £1 2 wo rth o f materi als in a fan cy box, after all, it is all cut o ut for yo u and yo u o nly have to ge t the parts to fit. Of co urse , if yo u ar e fussy yo u might have to repl ace some of th e pa rts like the one ult ra so ft fuselage side (or the rock hard o ne if yo u prefer). Want so me thing a little mo re adve nturo us? O ne of the plans publish ed in the magazines, fine , lots of them . I lik e peop le who build from pl an s , it me an s that th e magazines will bu y my design s. Actu ally I wonde r why I am wr iting this. I'm cutting my ow n throat doin g this at th e pri ce . Ju st ca ll me "Cut Me Ow n Throat " Mille r. (With apologies to Terry Prat chet t.) Of co urse you are limited to building wha t o the r people like and ther e is a good choice but do n't you eve r want to do so me thing o rigina l? Don 't yo u have an urge

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to create a trul y beauti ful model. Isn 't there a sca le subject that yo u lon g to build but there are no plan s for it? J ust once wouldn't yo u like to have a mod e l that no one else has? If the answer to ALL th ese qu estion s is "No" all I ca n say is, "\V'atch ou t, th e tran spl ant surgeo ns are looking fo r spa res ."

Perhaps you think it is hard Th e reaso n that peo ple do n't design their ow n mode ls is that they believe that it is difficu lt. Bull . .. Er . . . Cod swallop. I know 16 ye ar old youths who have designed excellent proje cts fo r the ir G.C.S.E., I kn ow of so me that have th en gone o n to se ll the plan s to magazines. I imagine t hat most modell ers ha ve as mu ch int elligen ce as a b right 16 ye ar o ld . All yo u need to kn ow to design a mod el are a few simple facts. If yo u have built a few kits or the odd plan yo u alrea dy kn ow most of wha t you need , you just don 't realise it.

Tbe a u th o r 's firs t RIC design was tbis Luton Minor.

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Designing Model Aircra ft

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Tim Rose designed 'R b u ba rb ' as a scbool project. Plans tben published ill RCMlV. Do yo u un de rstand the co-efficie nt o f lift o r the co efficie nt of dr ag? Do you understand neutral points and polar mom ent s of inert ia? Do yo u stud y the gra phs su pplied with the co-ord inates of wing sectio ns? You d o? Well , yo u d on 't need to read this bec au se I don't and nor will the readers of this bo ok . You w ill learn all that you ne ed from this book, tru e rule of thumb designing and th e first rule of thumb is, do n 't hit it w ith a ha mme r be cau se you can 't hol d a pen cil afterwa rds. O ut of the goodness of my heart and be ca us e I am ge tting paid for it, I am goi ng to tell yo u how to design mod els the easy way witho ut a ny of the mys te ry or formulae.

What you need

16y ear old Gm). Gooderbam designed tbis model for bis GCSE project, bis first d estgn. ftet o ve l)' welL

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You can desig n a mod el wi th a pe ncil, a ru ler, a tab le and a sheet of brown paper an d an erase r (do n't forget the eraser). It isn't ve ry ea sy but it ca n be done . At the o the r e nd of the sca le yo u ca n ha ve an AO drawi ng board wit h parallel moti on , a full se t of drawing instruments, French cu rves, ass o rted templates , se ve ral pencils and pen s and Mylar film at £.50 a ro ll. All ve ry nice but it won't mak e yo ur design s a ny bett er, easier perhap s but not bette r. Most people will sta rt o ut w ith a bit mor e than the most ba sic and slowly build up as they progress. Le t's look at a simple and chea p kit of eq uipme nt. You need a drawing board , it mu st be big e no ug h to draw a co mplete fusela ge o f the s ize yo u pr efer and a t least o ne w ing panel. You d on 't have to do eve rything on o ne sheet so it does n't need to be that big. Let 's say ab o ut 48" by 36". You might eve n be a ble to use the ba ck of yo ur building board . You can buy a nice piece

Desig llillg Model Aircraft

o f y," plywood or bla ckboard for abo ut £.4. Ma ke sure that the edges are sq ua re , sa nd pa pe r it smooth and yo u have an excellent board , I used o ne like that for man y, man y yea rs. You need a Tee Squ are , a chea p o ne w ill be fine , if yo u ca n ge t o ne as lon g as the board , good , but a sho rter o ne is OK. You need a couple of rulers, a Metre rule which mu st be straight. You ca n find the m in Wo olwo rths or DIY stores at very se nsible pri ces (under a fiver) bu t check for ,stra ig htness or yo u w ill ha ve funn y sha pe d w ings and strange incid en ce ang les . Yo u also nee d a 12" or 24" plastic ruler, bot h is eve n better. A good large se t sq uare is vital, ge t a 12" o ne as this w ill enable yo u to extend lines from a s ide view right up to a pla n view as we sha ll see in a fut ure chapter. I also use a sq uare 5" prot ractor , th is is a wonde rful too l and can be o btaine d from pilot 's su ppliers as it is used for navigatio n. It will cost about £.4 , it isn 't essential but it is ex treme ly use ful and you do need a pro tractor . Pe ncils a re as ess en tia l as a mod ell ing kn ife is to building. Use a 2H as th is will give a dar k eno ugh line but will ho ld its po int , buy a good sharpe ne r to go wi th it. Yo u can buy Pentel prop elling pe ncils wit h different size leads, 0.50mm is ab out right , they are not expen sive. You w ill nee d a good pen cil e raser, ge t a nice big soft o ne , a quality on e , the nasty little thi ngs so ld for sc hool use will soon ge t dirty and leave di rty smea rs o n you r draw ing, an untidy d rawi ng is no t as nice to work o n and ca n lead to mistakes. A Frenc h curv e is usefu l, you can buy se ts but I find that I o nly use o ne (see fig 1.1), this is q uite expens ive but chea pe r o nes whic h are sup posedly the sa me sha pe do no t have quite the same flow from curve to curve. You w ill need this fo r airfoil sections . A pair of co mpasses is nee ded for drawing whe els

First model design by retired full s ize designer , ouer strong but not too beauy, needed more pourer but was 11ice flier. an d the front of sp inne r rings. O ne that ca n be co nve rted to di viders is useful. Ge t a reason able pair, not o ne of those things that you stick a co mp lete pen cil in. You now have a se t of eq uipme nt that wi ll p rod uce almost any design tha t yo u want. There are one or two o the r item s but they w ill be men tio ned late r and e ithe r do n't cos t mu ch or alterna tive meth od s ca n be used. You sho uld be ab le to get all the above eq uipme nt for far less than a cheap kit an d you will save that with yo ur first design. It is wort h goi ng to car boot sa les and auc tions w he re yo u ca n so me times find items . My AO d raw ing board co mple te w ith parallel mot ion and stand ca me from a co lleague via a ca r boot sa le , it cost me 3 ou nces of tob acco .

YOIl,. ruler must be straight

Design ing Model Ai rcraft

13

Paper Fig 1.1 -Tbe most useful French curve

Drawing paper and tracin g paper ca n be ex pe nsive whe n bou ght o n a roll o r in shee ts, but yo u are go ing to be draw ing, co rrec ting and altering, a nd qu ite often yo u will te ar a fusela ge side vie w off th e board and sta rt aga in, then , wh en yo u have d on e the dr awing yo u are goi ng to be building on yo ur plan so yo u wa nt so me th ing chea p and tou gh . On e so urce of pap e r is the ba ck of oth er plan s, the o nes th at yo u bought be cau se you th ou ght th at you might build them onl y to find that they were design ed by a sado -masochist wh o would not use o ne piece of wood whe n he co uld use ten . For a reall y good so urce of dr awing paper go to a DIY sto re and find their lin ing pap er , tou gh off white pap er and the pri ce of a roll will be che ap . .. I p ick min e up fro m a local sho p at about 40p a roll. Another typ e of paper which is harder to find is white she lf lining paper, this is thinner and smo o ther, I like it but ca n rarel y find it these days. On ce yo u a re in to d e s ignin g yo u c a n g o on to "Detail" pap er , this is a thi n wh ite p ap er wh ich will also ac t like an o paq ue tracin g pap er, it does cos t nearly £20 a roll though . In futur e yo u may also wa nt tra cing paper so that yo u can have prints run out from your plan , it also allo ws yo u to keep a co py whil e yo u build on the sc ru ffy original , thi s is useful it yo u w ant to se ll th e plan to a maga zine after the test flight s.

Tbe basic needs ill tbe way of dratoing instruments,

My favourite sbape of French curve. Figure is to illustrate uibat you call do tuitb French curves . . . maybe 1 sbould re-phrase tbat!

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Tbe writer's bargain drauiiug board, sbeer IIlXIIIJ' after )'ears toitb pieces ofplywood.

D esigning M odel A ircraf ]

Chapter 2

Essential Aerodynamics e rod ynamics is a vas t and co mp licate d subjec t but the bas ic theory of flight is much simp le r, . and what yo u need to kn ow to desi gn a mo de l is eve n simpler than that. I kn ew a University grad ua te who design ed a ser ies of bea utiful co ntrol line stunt models, all the form ulae , the w orks and they flew ve ry we ll ind e ed . O ne day he needed a stunte r in a hurry and design ed a simple bo x and plank design for quick buildi ng ... it flew far better than the fancy o nes. Do n't think that all the co mputers and wind tunnels and expertise of the full size wo rld a re infallible , they ge t it ho rribly wrong as well at times. I am goi ng to tell yo u the minimum that yo u need to kn ow to design a model , if yo u wa nt to understand ae rod ynam ics be tter I ca n recomme nd "The Mechanics of Flight" or "Flight with out Formulae" by Kermode , o ld bo o ks bu t st ill tw o of the easiest to unde rstand . Th ey sho uld be ava ilab le from your lib rary or aviation book sho ps.

A

All you uiant to hnoui about flight An ae ro p la ne fl ie s b e ca u s e th e w ing is mo ve d through the air and ge nera tes lift, th e lift is ge ne rated from o ne o r bot h of two action s. Reason o ne is that be cau se the s ha pe o f the cross section (called the airfo il from now o n) mak es the air travelling ove r the to p sur face move faster than the air mov ing under the lower surface, this redu ces the pr essure of the ai r ab ove the wing. The second reaso n is that the wing me e ts the air at a n a ngle fo rcin g a ir down an d th e refo re th e w ing is pu shed up. Thi s an gle is called the ang le of attack. I cOllkk~_ i nto de ta ils as to w hy th is is a nd ex plain it a ll but yo u do n 't need to kn ow that and it w ill tak e up va luable space. The wi ng needs to be stab ilised in flight and for this we use a tailplane , we co uld use a forep lane and buil d a ca nard or even a flying wing but that wou ld be running be fore we ca n wa lk .

Tips)' N ip per is bigbly a erobatic, note umtsual toing sect io n and lar g e in cidence angle compared uiit b tailplane and engine tbrustline.

Designing Model A ircraft

15

\Ve a lso ne ed a p ow e r source to pro pel th e w ing through the a ir. On a glider this is simply the weight of the aircraft, it is literally sliding downhill all the time. Yes I k now gliders ca n gain height , look at it this way , if yo u are in a lift and launch a paper glider which tak es five seconds to reach the floor fro m six fee t and the lift is goi ng up at ten feet pe r seco nd the glide r will have go ne up 44 feet in the five seconds it took to reach the floor. Th ere are four forces ac ting o n an aircraft; weight , lift, dr ag arid th rust. Wh en the aircraft is in stable flight , Le. travelli ng at a co ns tant speed in a co ns tant di re ction , these for ces will ba lance out. Tru st me , they do and you don 't really need to worry abo ut tha t at the mom ent and I w ill ex p lain as and when yo u do ne ed to kn o w .

The Airfoil Section I have said tha t th e shape of the airfo il produces lift and it does but the re a re hund reds of airfoil sections , wh ich do we use? For o ur ve ry simple minds w e ca n divid e them into four types. Flat bottomed , se mi-symme trica l (o r Bi-convex as th e purists ca ll th e m), symmet rical and unde r ca mbe red . Flat plate can be considered sym me trica l an d curve d plate is under ca mbered . These are rath e r ine fficie nt sectio ns for wings and will onl y be mention ed in passing, althoug h flat pl ate section s are very co mmon fo r tailplanes and the flat plat e is a lso used for very si m p le model s w ith a solid s hee t wing.

Fig 2.1 -Airfoil Types

c Flat bottomed

Bi-convex

E Symmetrical

'-

-

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Under cambered

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Desig n ing Model Ai rcraft

Flat bottom ed sect ions are used fo r so me trainers and are q uite commo n o n ce rtain sca le typ es, Clark Y is the grea t classic fiat bott omed section . They are easy to build becau se they ca n be placed flat o n the building boa rd. Semi-sym metrical sec tions a re very co mmo n, they are used o n many full size d a nd mod el aircra ft and are good for the faster tra ine rs and sports aeroba tic models. Full sym me trical sec tions are for true aerobatic models as they w ill fly in exactly the sa me way u pside down as the right way up if they are se t at the correct a ngle. Under cambered sec tions are used o n vintage mod els and so me floate r type glide rs, they. are for slow flying mod els o nly as they do produ ce a lo t of dr ag bu t they also provide a lot of lift. For o ur purposes we w ill o nly use the first thr e e types o f sec tion.

Angles ofD ... Different Sorts In describing how a wing produ ces lift (and inciden tally, g iving all aero dyna mics expe rts ap op lexy ) I mention ed 'Angle o f Attack ', forget abou t it. Th e a ng le of attack is co nstantly variable and the o nly time you need to worry abo ut it is wh en you exceed the angle when the w ing stalls and eve n that varies with spee d, but we are not talking about /lying so , as I sa id, forge t it. The angle that we are co nce rned with is the ang le of incide nce. Th is is the angle of the wing to a purely arbi trary line called the datum. The datum is normally taken to be a ce ntre line d rawn through the fuselage to which all other ang les are related . You cou ld have a datum line at 45° to the fuse lage but it wo uld make drawing difficult.

Higb iotng cabin models ca ll be slee k and beautiful, tbe author's Lady Boioyn d esign tohicb is ill RClIflV plans '·allge.

Fig 2.2 - Terms Explaiued

Cbord lille

\

Ang le of attack

Direction of atrfloto (variable) A ng les

C!.Jord Une

17Jis lin e parallel to

Desig n ing Model A ircra ft

d~tum

/

Angle of in cidence

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O the r ite ms which are se t at so me angle in relations h ip to th e d a tum lin e a re th e th ru st lin e a n d th e tailplane chord line. The o nly othe r angle th at we are co ncerned wi th is the d ihedra l ang le w hich affe cts stab ility a nd a ilero n resp on se, bu t this w ill be d iscu ssed w hen we get down to putt ing pen cil to pap er.

WillgAreas

This model is based loosely 011 control line stuuter layout,jlew well. Four strokes are best in ligbt models. T he an gle of the w ing is mea sur ed from the chor d line and the cho rd line is a line drawn from the centre of curv ature of the leadin g edge to the ce ntre of the trailin g e d ge . So me times th e cho rd lin e is ta ken as runn ing alo ng the bott om of a flat bottom e d wing.

The w ing area o f mo de ls va ries depe nding o n the type o f model. Powered g lide rs wil l have la rge wi ng areas for ve ry small e ng ines. My Falco n design has 620 sq . in . and is mild ly ae roba tic o n a .12, w h ile Te q u ila Su nr ise h as o n ly got around 230 sq . in. for similar po we r and is fully ae ro ba tic but naturally do es not have th e same low speed perfo rma nce o r glide . T he mod el with th e sma lle r wing w ill have bett er pe netr ation in a wind becau se the dr ag is less in pr oportion to th e powe r. Fit a .40 to a Falc on and it would handle eve n stronge r winds, but it would then have to be struc tura lly stro nger to tak e the power a nd sudd e nly it is not a powe re d glide r any mo re . I h ave g ive n a ta bl e o f suggeste d w ing a reas for most sizes of e ng ine . Thi s ta ble is fo r average sports

Full size Skybolt and model Vollesplane obey tbe same rules forfligbt.

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Desigllillg Model A ircraft

aeroba tics models w hich is th e most popular typ e of model and is o nly intende d as a guide. Bip lanes w ill have more area, so me ligh t types o f sca le mod els will 'have mu ch more area and pow ered g lide rs eve n more . Goi ng be low the sma ller areas would not be a good idea for any typ e of mod el as it wo uld be very difficult to keep the wing load ing within the bo unds of reason. .09 .20 .29 .40 .45 -

.15 .25 .40 .45 .60

200 350 450 550 650

to to to to to

350 sq . in. 450 550 650 800

Areas and Loadings

Small, ultra simple design call be built quickly and still provides fun and satisfaction:

O ne of the mos t importan t factors in any design is the wing area and the win g loadi ng. Too high a wing load are designing a mod el for a .40 engine, a re alistic weight ing and you will have a mo del that is a pig to fly, if it for this wo uld be up to 6 lbs maximum . A se ns ible wing w ill even get of the gro und . load ing wo uld be up to 24 o unces per square foot. This is whe re we have to do a little math s, no thing 6 Ibs = 96 o u nces . 96 di vided by 24 e q ua ls th e co mplicated, you don 't re ally need a calculator unl ess desired nu mber of sq uare feet = 4. Multiply by 144 = 576 you are a pro duct of the mod ern ed uca tion syste m. sq uare inches. That is the size of the wing bu t we ca n Wing area is quite simply the chord or w idt h of the brin g it up to 600 for safety. wing multiplied by the span o r len gth of the wing. Egad! I had planned to include a tab le of suggested w ing How basic can I ge t! Wing lo ad ing is the Competition FilII Fly models take low aspect ratio and low icing loading to extremes for incredible performance. figure tha t yo u ge t w he n yo u d iv ide th e es timated we ight of the mod el in ounces by the w ing area in sq u are feet. Let 's ta ke a n exa mpl e . Th e wingsp an is 50", the cho rd is 9" so : 50 x 9 = 450 s q ua re in che s . Divide by 144 wh ich is the nu mb er of s q ua re in ch e s in a sq ua re foo t and that is 3.125. Ass ume th at we a re design ing a mod el fo r sp o rts ae robatics with a wing o f th e size g iven abo ve , if we build fairly light it sho uld come out at ab out 4 Ibs in weight. 4 Ibs is 16 ounces x 4 = 64 ounces. If we di vide 64 o unces by 3.125 square fee t we ge t 20 o u nces p er sq uare foo t wing load ing. After yo u have built th e mo d el a n d found out wh at it does weigh yo u ca n see how close you go t. The sa me system will allow yo u to decide on h o w bi g a wing yo u need . Assum e that yo u

Desig ni ng Model Aircraft

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loadings for diffe re nt sizes of model , but when I began to lay it out I reali sed just what a wid e range there are, eve n for one size of eng ine. For exa mple, a Competition Fun Fly mod el might be pow ered with a .25 to .40 and have 700 sq uare inc hes of w ing ar ea , a wi ng loading of ar ound 10 o u nces per sq ua re foo t, but a s ports model mi ght go up to 25 o unces pe r sq uare foot which w ould defin itely be the top limit for the .25 size of eng ine . Lets lo ok at so me sca le models. The Sig Pip er Cub is 72" span and ca n be built to we igh aro und 4lbs , it flies like a dream o n a 20 four stro ke . Th e Grea t Planes Pip er Cub is the same size, bu t th ey claim that the ir kit will weigh 61, to 71, lbs and need s a .40 to .60 tw o stroke . Th e o nly co mme nt that I will mak e is th at th e real Cub is light and flies very slow ly. Runn ing th ro ug h so me o f th e figures fo r my own de sig ns, I suggest that , for norm al spo rts aerobatic types yo u do not exceed 20 ou nces per sq uare fo ot for models up to .15 eng ines and 22 ou nces per squa re foot fo r .25 size mo de ls, up to .40 yo u can go up to 26 o unces and for .60 you might go as high as 30 o unces per sq uare foo t. Goi ng a little over these figures will not be disastrous, but keepi ng below them will help improve performance a nd han dlin g of the model.

Sailplanes use bigb aspect ratios, tbis Condor MS bas all A.R. of about 20. Powered gliders, lightwe ight sca le models like Piper Cubs ete. sho uld have much low er w ing loaclings.

Fig 2.3 -Aspect Ratio

AIR 20:1

AIR 6:1

AIR 5:1

AIR 4:1

20

Desigu inp. Mod el A ircraft

Full size Kitfo... is m icrolig b t, velJ! like a model ill construction.

Wing Volume Loading

Aspect Ratio

Giant sca le mod e ls us e mu ch higher w ing loadings an d if you start working out the loadings for full size aircraft they becom e astron om ical in co mpa rison with their mod el counter parts. This is becau se of the claim ed extra efficiency with large r sizes . Many yea rs ago Dave Platt did so me compariso ns and found that if o ne took the vo lume of a wing as a unit of loading instead of the area and co mpared that at differen t sca les of a particular mod el of a subject the figures very close ly matched . Dave 's figures sho we d th at increasing the size of a sc a le w ing from 500 s q u a re in ch e s to 700 s q . in . increa sed the area by 40%, but increased volume by 98%. 'Dave went on to show that a loading of 1 ounce per 5 cubic inches was about right for a hea vy sca le mod el and that this co incide d nicely with the heavier load ings possible with larger mod els. It would seem likely that if this wa s tak en to it's logical co nclus ion, the vo lume loading on a model would be very close to that of the full size prototyp e . I don't suggest you try work ing the vo lume out, but it does ex plain wh y larger mod els can use high er loadings. Presumably the computer "Whizz Kids " could write a pro gramm e that would calculate the desired figures but for o ur simple purposes it would be ver y co mplica ted bearin g in mind that eac h aerofoil sec tion wo uld give a different volume .

Yet ano the r of the secret term s of the designer. All it me an s is the sh ap e of the wing, for a p arall e l cho rd wing it is simply the wing span divided by the cho rd . Example , from the ab ove , 50" spa n d ivid e d by 9" cho rd = 5.55 , a nic e figur e . Nor mal ran ge of asp e ct ratio s for paralle l cho rd win gs is abo ut 4.5 to 6. Glid ers use A.R.s as high as 20:1. For a tap er ed wi ng we add the tip chord and root cho rd , d ivid e by 2 whi ch gives the aver age cho rd and then di vid e that into the spa n .

D esign in g M odel Aircraf t

Balance Point or Centre of Gravity O ne of the mo st vital thin gs abo ut a model is whe re it balances. This balance po int or ce ntre of gravity (CG) will affec t the stability to suc h an ex tent that if it is just a little to o far ba ck it w ill be impossibl e to fly the mod el. Th e CG has to be in front of the po int through which the lift ac ts. Now wor king o ut th is latter point is not o nly co mplica ted but rath er hit o r miss as it dep ends on so man y factors so we ado pt a rule of thumb fo r the time be ing. On co nve ntiona l mod els the CG sho uld be be tween Y, and Y, of th e cho rd back from the leading e dge of the w ing , 25% to 30% if yo u prefer and yes, I kno w that Y,rd is more than 30%. Th e nearer the forw ard limit the safe r the mod el .

21

Chapter 3

The First Design Layout n Cha pter two we looked at the basic requirem ents o f an aircraft , we talked ab out win gs and ang les and ba lan ce points and so on , the aerodynamic side of th ings. Th e typ e of mode l that we want will decide the aerody namic layo ut a nd the ae rod ynamic layo ut will have so me co ns ide ra ble effect on the sha pe o f the model, th us all basic trainers are high wing and all pattern ships are mid o r low wing. Now yo u kn ow why eve ry airliner loo ks the sa me as all the res t and why mod ern owne rs of full size sa ilp lanes ca n't tell one from the o ther. T he rea son tha t military ai rcraft all look different is b e cause th e y are d rawn to specifica tio ns de cid ed by co mmittees who don't kn ow wh at they want anyway. Wh at sha ll we design ? Well , I imagi ne that yo u are p ast th e trainer stage a nd th e mo st p opul a r typ e of mod el is the sports aerobatic type so that is the way we w ill go . It can be for .25 size eng ines or .40s, I w ill give figures for both but I suggest the sma ller size as it is cheaper to build .

I

Ultr a boxy camera plane is tbe easiest type of model to d es ign. Tbis model w o u ld u s e flat bottomed w ing o r semi-sym metric a l s ection.

Fig 3.1 - Tb e Th ree Most Usef u l Sectio ns.

~KY

22

Desig ni ng Mode! Aircraft

Specification Before w e ca n s ta rt drawing w e n e ed to d e cid e o n th e s ize o f th e m od el, th e wi ng section that w e want to use and the sha pe and po sition of the wing . For a .25 size model a w ing o f a b o u t 4 50 s q .in . is a b o u t righ t. No w w e p lay a bo u t with s pa n and c ho rd until we ge t a n as pect ra tio that is sensibl e , I took 50" s pa n and 9" chord fo r an as pect ratio of 5 .5 but yo u c o u ld vary this from a span of 45" a n d c h o rd of 10" (AIR 4.5) to a span of to 52" and 8.6" chord (AI R 6) . The s h o rte r win g span will roll faster. Experimental design by Geo rge Strobm does not baue a single curve ill it, evelJ I You ca n round fig uiing s ection is a d ia mond. Flew a t b lg b speed but was d iabolical a t loto speed. ures up and down , a little more area or less is In this cha pter yo u will find three airfoil sec tio ns , yo u nothing to worry about. For a .40 s ize model the area can have these enl arged o n a photocopier to th e size wou ld be 550 or more sq . in. th a t yo u need , take th e measuremen t of yo ur w in g Th e wing can be located in the sho ulde r, mid or low cho rd and d ivide that by the measurem ent of th e sec tio n positio ns. The mid-wing pr es ents structural problem s printed here , that is the enlarge me nt factor to set on the and takes up valu ab le space , the sho ulde r win g is nice cop ie r whi ch mu st hav e a full zoo m ca pab ility. e .g. Your but can make for extra work later, it is ide al wh en the mod e l chord 9.5", in th e bo ok , 5.375, yo u ne ed to model has to be hand launched and also mak es wo rking on the model eas y as it will sit o n its unde rcarriage . e nlarge by 1.76. You could des ign yo u own sec tio n, I have shown th e Th e low wing model is mo re commo n and has man y ma in points that you need to watch . If yo u go this route ad vant ages but may present problems when it co mes to you will probabl y not no tice the differen ce . You could locating the undercarria ge in the ide al position. We will eve n use th e MILLER T.L.A.R. (tha t looks about right) cover both shoulder wing and low wing in the exe rcise , after all, this is yo ur first desi gn and you are go ing to sectio n . build it. We also ne ed to decide on the w ing sectio n . He re the choice is bewildering. You co uld use a semiP aper and P e ncil symm etric al wing or fully symmetrical, yo u could eve n Tak e yo ur piece o f pap er a nd attach it to your drawuse Clad, Y se t at true 0 incidence as this works like a ing board , I sug gest masking tap e at the co rn ers but thin semi-symme trical wing inve rted but it is a bit thin . drawing pins or even proper drawing board clip s can be I favour a th ick symme trica l sec tio n with a blunt lead ing ed ge, this will slow the model slightly a nd giv es ver y used . Now draw a line about 10" inches from the bottom of gentle stall characteristics. Fig 3.2 - Ouin Desig n Section Max thic knes s at 3 0% cb ord

Miller T.L.A.R.

st« lit L.E.


Fla t fo r easy build tng

23

th e paper right along . Th is is your d atum lin e and th e w h ole m od el will be drawn ro und thi s. Th e datum lin e is arbitra ry , it ca n be o n th e thrust line or it ca n be the to p of th e fuselage side, it co uld eve n b e o utside th e model but thi s is not w ise . \'\fe will make it the top o f the fusela ge side althoug h yo u w on 't know it at th is stage. Abou t 12" in fro m th e left side o f th e p aper drop a line down from th e da tu m , thi s will be the L.E. of the w ing. Now we draw in th e chord lin e . If thi s w as a trai ne r w e w o uld p ut it w ell above th e datum line , fo r a shoulder w ing mod el we ca n leave it on th e d atum and o r a lo w w in g m od el it mu st , n aturall y, b e belo w . Put it ab o u t 3)1," be low an d paralle l w ith th e d atum. Now, o n th e d atum, measu re back fro m th e L.E. th e dis ta nce th at yo u h ave de cid ed o n fo r th e chord and d rop ano ther line , th is is th e T.E. No te that yo u always wo rk o n the d atum and drop or raise lines . \'\f e n ow h av e two m o re d e ci si ons to make , th e length o f th e nose and the tail mom ent arm or th e dis tanc e from th e T .E . o f th e wi ng to th e L.E. o f th e tailpl an e . Th e length of th e nose de p en ds o n th e weigh t of th e eng ine, the w eight of th e rear end and th e le ngth of the tail moment and yo u can't w ork it o ut so , for most typ es of m odel , we ca n w ork to a distan ce from the p ro p drive r to th e L.E. o f about o ne chord . Becau se four stro kes are he avier you co u ld re duce it a bit if yo u are go ing to use o ne. Th at chor d dimen sion is a ve ry handy o ne fo r giving p roportion s for any size o f mod el. Fig 33 - Basic Layouts

Th e tail mom e nt ar m and th e size of the tai lp lane are lin ked w ith sta b ili ty . A long m oment arm an d sm all tailplan e give the sa me stab ility as a sho rt ar m and b ig ta ilplane, howeve r, contro l is smoother with the lon ger arm. A good co mpromise moment ar m is about one and a h alf chords w ith a tailpl an e abo ut 18% to 20% the area of th e wing . Patt ern sh ips have longer arms , so me of th e w ilder fu n fly mod els use less . Havin g decid ed on th ese d imensions we d ro p a lin e d own 9" in fro nt o f th e L.E., th is is th e front o f th e mode l an d we d rop one down about 13" behind th e T.E. thi s is th e L.E. of th e tailplan e . Th e tailpl an e s iz es h a v e to b e ca lc u la te d no w . Per centage of th e wi ng area is easy , divide th e w ing area by 100 and mu ltip ly by what ever p e rcent age yo u wa nt . E.g., w e have a w ing area of 450 sq .in . so 450 d ivid ed by 100 eq ua ls 4.5. If we go for 18% we multiply by 18 an d ge t a tailplan e are a of 81 an d so o n , most calculators h ave a pe rcentage b u tton which mak e s life even easier. I pr efer a little mor e area so we w ill try 20% and ge t 90 sq.i n . \\Te now want to know th e cho rd of th e ta ilp lane , not quite as simp le , w e may have a p arallel chord w ing but the tailplan e is much n icer if it is taper ed. Play w ith yo ur ca lcu lator aga in . Hit 90 and div ide by 5, th at gives us a sp an of 18" and a n ave rage cho rd of 5". Do odle arou nd o n so me scra p pa per an d see what lo oks nic e , a taper o f two inc hes looks good , right, root chord of 6" and a tip chord o f 4". Yo u ca n vary th is an d yo u ca n put more taper o n the L.E. o r o n th e T.E., just ke ep th e area about th e sa me.

--

-------Higb wing

' " Datum line

p =1 SbOO'de

I

1.,r--.....

__

I

----

Low wing

24

Design ing Model A ircraf t

Fig 3.4

-s

Aerodynamic Layout

Prop driver

I.E.

T.E.

1

I

-------l...I+------cHORD (C)-----.'
TIP I.E,

1.5 - - - ----.

Datum

Put the tailp lan e's 6" root chord o n yo ur datum line . You can decid e wh ere the tailplane will sit now, I like putting it o n top of th e fusela ge s ide .

Variations

011

the Theme

\X/hat yo u have d rawn ab o ve is the ae rod yna mic layo ut for a sp ort s ac ro batic mod el. With a mod e l of this typ e the w ings, tailpl an e and e ngine w ill norm ally be se t at 0° to the datum, this wi ll give neutral stability or, in othe r word s, the model w ill stay point ing in any dir ection that you have placed it in. Some oth er typ es need to be different. Tra iners need som e positive inciden ce o n the wing to mak e them pull out of a dive if the learner releases the s tic k. Th ere ar e so ca lle d train ers that ha ve neutral

stab ility and so me that a re ac robatic . . . the y a re not basic train ers whatever the box top says . We talk ed bri efl y a bo ut th e balance o f for ces a nd dra g in Part 11, now we need to look a little furth er. Ta ke a high win g mod el, the w ing produ ces lift but also d rag and it is high . The e ng ine is low er d own and produces thru st, as you ca n see from the d rawin g this te nd s to pull the nose up so we tilt the engine d own and this co unterac ts the imbal an ce of forces. Likewi se , it w e mount an engine o n a pylon , o n a se aplane o r a powered glider for example , the thru st is well above the dra g and so th e model will tend to nose div e and in this case we would use up thrust. Qu ite hon estly, it is imp ossible for the average modeller to calculate the exact angles need ed and so provision mu st be mad e for adju stment during the flight test period .

Higb 1II00m ted pusber engine on tbe Polisb bomebuilt needs some 'up' thrust but, because it is pushing tbe engine is pointed downwards s ligbtty.


25

Fig 3.5 - Thrust Line Adjustment Nose up fo rce counter urith d oum thrust

Drag

D r ag

Nose doum force

Fitting the Gear It may so und strange to talk of fitting radi os and tanks when you don't have more than a few lines o n the pap er but now is the time to plan it, before yo u are restricted by sha pes. Start off by placing your win g sectio n o n the plan , if yo u have co pied a section from the cha pter you ca n stick it d own o n the draw ing , just ex tend the chord line and the line d ropped for the L.E. and use these to line the section up with yo ur draw ing. Now yo u ne ed to d raw in th e a mo u n t that yo ur aileron servo w ill stick up above the w ing. I find that cutting ou t card sha pes the sa me size as my servos, receiver an d batt ery pack is useful, yo u ca n jugg le them aro und and be sure that all the co ntro l ru ns will clea r each other. Don't forge t that the ailero n servo will be beh ind the main sp ar w h ich is normally about 30% of the chord back from the L.E. You also have to locate the engine at th is point. Is it goi ng to be up right or side mounted ? You ca n also position the fue l tank, this sho uld have its ce ntre level with o r just be low the ce ntre line of the carboYou mu st allow ro om for the eng ine mount and a forme r between the tank and the eng ine . You d o not have to draw these part s in detail, just an ou tline is fine.

26

O nce yo u have worked out where all the internals will go you ca n, at last, dr aw the side view of your model. Here you ca n let your imag ination have free rein , racer , jet style, wh atever yo u like . I would suggest that you keep it simple at this stage, you still have to draw all the o the r parts and then build it.

Sta rt with a Bo x 90% of all model a ircraft are based o n a pl ain box fuselage . Some , like the Stik famil y don 't go any furth er, othe rs have a simple turtle deck and o the rs have the box hidden unde r stringers and fairings . Your box will look so me thing like the drawing , th e slo pe up to the ta il may vary but at thi s stage in the learning curve little e lse will. What you do w ith it is up to you , a simp le turtle deck will be easy to design and ca n be varied in he ight , position of co ckp it and so on . Draw it in lightl y, th is is wh en the era ser co mes into its own as yo u cha nge things until yo u get a shape that you like. Of co urs e you should have had a fair idea of w ha t you w anted from th ose ske tches on the back of e nve lo pes, but ge tting it to look right at full size can tak e a little time . A very simp le jet style mod el is sho wn in Fig 7 this uses th e bo x and a few pieces o f shee t to crea te an attractive looking mod el.


Fig 3.6

-s

Bquipmeut Layout and Outline

~fE -=tfR8f~+---

--~

You can draw in the fin, this sho uld have about half the area of the tailplane, more if you have a very bulky front end of the fuse lage . Fin shape is not critical, you can have any sha pe you like, just keep the areas correct and rem ember that part of it has to be hinged fo r a rudder. The rudder area is no t critica l, highl y aeroba tic models will benefit from large rud de rs to kic k them over in good stall turn s and produce crisp flick man oeu vres an d spins , spo rts mod el do not need as much . A good comprom ise is about Y3rd the tot al area of the fin and rudder. The rud der is norm ally ex tende d down to the bo ttom of the fuse lage .

With th e si de vie w dr awn we ca n d raw th e p la n view. Draw a line ab ove or below yo ur side view , th is line mu st be pa ra lle l w ith th e da tu m lin e . Now d rop lines from the various critical location s like the front former , the L.E. and T.E. of the w ing a nd the tailpl an e L.E. and the end of the sides . You now ha ve to de cid e on the width of the fuselage , this will dep end o n the size of the se rvos and tank. Always leave so me roo m to play with and d o n't forge t to allow for th e th ick ness of the sides and any doublers. The fuselage ca n be pa ra lle l fro m th e nose to th e wing T.E. and tap e re d from the re to the e nd . You ca n

Planning installation during designing will save a lot of'problems later.

Design ing Model Aircraft

27

Fig 3.7 -Styles

011

same Basic Box

o

o

o

o use a s moot h , n atu ral curve o r a tighte r c urve a nd straig ht tape r. To d raw a smooth sweeping curve yo u need a spline, this is not ex pens ive be cau se it is just a str ip of Y.." sq uare balsa or spru ce . Using mask ing tape hold o ne e nd d o wn to the stra ig ht po rtion of the fusel age and then pull the free e nd to line up w ith the ce ntre line at the rear of the fuselage. A light pen cil line alon g each s ide of the spline and

28

yo u have the plan view of a curve d fuselage side d rawn . The alterna tive is to use the Frenc h curve to d raw a ge ntle be nd behind the w ing a nd then add the stra ight tape r back to the en d . The o nly o the r task at th is stage is to measure down from the thru st line a distance , that will a llow plenty of clearance for the intended prop eller and draw a line paralle l with the datum , thi s is th e ground line a nd will define ho w lon g the und ercarriage is.


That takes ca re of the fuselage outline .

Make s u re tbe ra dio toill fit

The Wing T h e wi ng c a n b e draw n as a re c tang le b e c a u s e we a re n o t goi ng to g o in for tap ere d w in gs at th is stage . Straight w ings are eas ier be cau s e a ll the wing ribs a re the sa me with jus t m ino r va riation s. At th is stage we ca n design the sha pe of the t ip s a n d we c a n al s o p lo t the ailerons . Th e tips ca n be an y shape that yo u want but yo u really wa nt to keep them simple and light at thi s s tage . T he s hap e show n o n the drawing is as good as a ny fo r a first mode l. For ease of co ns truc tio n strip a ilerons a re ha rd to beat. I have foun d that 14% of the wi ng a rea is a pre tty good rule of thumb, so take yo ur wing area , le t's say 450 sq. in. and punch that into the ca lculator and we ge t 63 sq .in. , if we have a spa n of 50" divide that into 63 and

we ge t 1.26" so a I X" wide ailero n is what we ge t. Draw a line I X" forwards of the T.E., that will be o ur aileron hinge line . Th at is the sha pe of the wing, everything e lse o n the dr awin g w ill be struc ture which we sha ll co me to in due co urse .

T1J' different outlines, something sligbtly unusual call produce an eye catching model.

Desig n ing Model Ai rc raft

29

Midwest K its use a basic box and uitng for a ,.a llg e of "SOI·t of' s ca le models.

Tailplane

area , so it is a matter of jugglin g the tap er into a sha pe that looks nice . Slig htly more swee p o n th e L.E. lo o ks b e st a nd gives a se nsi ble sort o f s ha p e to th e e leva to rs. Don 't w orry too mu ch abo ut keeping to exact areas and pe rcen tages , m o d el s wi ll fl y wit h wid e vari ations . You sho uld now have a dr awing that lo o ks like Fig 3 althou gh the wings a nd tail may be on separate p ieces of paper.

What have we got?

We wo rked o ut the size of the tailp lane and its shape when we were drawing the fuselage side in Part Ill , so now it is a simple ma tter to lay it o ut. Draw a line whic h w ill be the hin ge line and drop a verti cal throu gh it which will be the ce ntre line. The eleva tors sho uld be between 20% and 25% of the tail pla ne area, I pr efer th e larger figure be cau se it is m ore e fficie nt to mo ve a la rge r ar e a o ver a sma lle r ang le. We kn ow the roo t chord and the tip chord and the

Th e result of o ur work so far sho uld be a drawi ng of the shapes of all major co mpo ne nts of the model and we know the relatio nsh ip between the m. We know that the radi o and tank w ill fit in the mod e l and we know wh ere the eng ine w ill fit. Th e next stage is to de sign and draw in the structure but before we d o th a t yo u will need to th ink a bo ut weight and struc tures.

Fig 3.8 Aero dyna m ics and Sh ape Outline -s

I

I

I I

\I------_-+---_----jl/ I

i

Straight lilies tangent to cu r ue

RIC omittedfor clarity

30

Desig ning Model Aircraft

Chapter 4

Lightness Equals Strength OU have all seen those models that are built like mod el, ther e is les s ine rtia and and so the struc ture ca n a bri ck built sh . . . chi ck en coo p . You hav e co pe with the load s. Another effec t of inert ia is to mak e a model more d ifseen them cras h and dem olish themselv es and ficult to fly accurately. Tak e a w ing and hold it in the perh ap s you ha ve wonde red wh y suc h a stro ng model middle and roc k it as though it wa s being put into a broke in a relatively minor cras h. bank. Now add some lumps of modelling clay to each I will tell yo u wh y, becau se th e idea that large and he a vy lumps o f w o od ar e s tro nge r is wro n g When one bit stops, tbe rest try to ca,.,y on a n d in fac t the y c o ntribute to the damage .

Y

Inertia Sir Isaac Newton sa id th at a body had a tend en c y to rem a in in a c o ns ta n t s ta te unl es s acted o n by a n o utside forc e , Le . th e apple kept falling until his he ad got in the way and sto ppe d it o r, a m o re m od ern e xa m p le , s it ting in a cha ir until the wife tells yo u to go and mow th e lawn . .. this is inertia . In th e sa me w a y a mod el ha s ine rtia a n d eac h pa rt of that mod el ha s iner tia and wh en it is m o vin g non e of it w ant s to s to p . Wh en one part sto ps th e rest wa n ts to ca rry o n an d the heavier eac h part is, the more iner tia it has. No w ima gin e yo u r ni c e s tro n g , he a vy mod el. Th e s p inne r sto ps be cause it meet s th e g ro u nd but everything else want s to ca rry o n until it too , m e ets the ground and so yo ur he avy wingtips hit th e ground and the wing is in two (o r a lot more) bits and the tail tries to ge t up front with th e e ngine and your solid 'lo o king mod el is a he ap of splinters. Now tak e a light e r


-

t,.~

----

ddIl'\t'

ti ff • .

\~ .

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~h ;\1 \

... . ~""o.l ....

~ \\ I

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.

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. r:

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~ "~t· ( ,.. .. ....,Hft. . .

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.

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~ -a

31

Plying a uintage model is tbe best illastration of bow nice ligbt models are.

Front view of a Piper Cub fin, about -%" thick, bracing wires are II0t mucb thinner. Note also waJI fabric is used to fair fin to fuselage, something tbat is often missed Oil scale models. tip a nd re peat th e ac tio n. Did yo u not ice how much harder it is to sto p the rota tion o nce it is moving? We ll th at happen s in flight and the sa me th ing happen s in the p itch mode w he n the eleva to rs are used . I o nce ha d a ve ry spectac ular dem on strat ion of ine rtia . I was flying a .60 power ed , se mi-sca le Fairy Firefly co ntro l line stun t mod el. This aircraft had w ing mounted und e rcarr iages with big, heavy wheels. I was flying inverted a nd ca me down a bit too low an d mad e a hard , heavy but flat inve rted land ing in the grass. T he model sto p ped in a co u p le of fee t wi th o n ly mi no r dam age , but both unde rca rriage legs depart ed from the wings and e nded up se ver al yards fur the r o n, co mplete with sections of ribs and bits of spa r and L.E. sheeting .

Weight ill Flight We have seen that , fo r level flight, lift has to eq ua l weig ht. If a mod el is heavier it has to fly faster or at a high e r an gle of attack. Increasing power is not a good o p tio n becau se you nee d four times the power to do ubl e the speed an d it w ill not help the lan din g speed . In cr e asin g th e a ng le o f att a ck in cr ea ses d rag and slows the mod el until yo u reach a stage w he re the w ing is flyin g just be low the stalling po int. From th is it can be seen that the mor e weigh t th e lo wer the ac tua l flying speed. At the sa me time the stalling speed w ill rise w ith the increase in weight. It is q uite possible to ge t a sta lling

32

spee d hi gh er th an th e ma ximu m flyi ng s pee d of a mod el , th is is a safe mod el becau se it ca n 't fly but that is not the ob ject of the exercise. Conve rse ly, of co urse , a light model w ill fly fas ter and slower an d this w ide speed e nvelo pe is mu ch sa fer . A ligh t model w ill accele ra te and decele rate faster, th is mean s q uic ke r take- o ffs beca use th e mod el will reach flying speed in less time . It a lso me an s grea ter safe ty in landi ngs. Conside r a heavy mod el co ming in to land in a 15 mph wind, the model has a stalling speed of 30 mph a nd is a pproaching the landing at 35 rnph. Now the wind is not constant, a no minal 15 mp h w ind is dropping to 5 to 7 mph and rising to 20 mph . If the w ind d ro ps to 5 mph the mod els airs peed is sudde n ly re d uced to 25 mph , 5 mph be low sta lling speed , the heavy mod el ca nno t accele rate fast eno ug h to mak e this speed up . . . instant sc rap balsa . A light e r model w ith a stalling speed of 20 mph ca n make its approac h at 30 mph o r less and w ill accelerate q uickly enoug h to prevent a disastrou s stall. Light e r mod els are safer and fly bette r.

Structures Full Size Classification \V'e can le arn a lo t abo ut mod el struc tures fro m the full size. Most mod els, kits or plans , are poorly design ed from the struc tura l po int of view. This co uld be because it is easier to man ufacture the kit that wa y o r sim p ly du e to ign o ran ce. Fu ll s ize aircra ft struc tu res are d ivided into three classes. Primary, secondary and tertiary. Primary structure takes all the main load s, the flying load s, the landing load s , everything . It is very stro ng . Spars, lon ge ron s, e ng ine mou nt ings and u nderca rriage mountings wo uld all be p rimary struc ture. In so me cases the ski ns of the aircraft may be p rima ry struc ture. Any dam age to the primary struct ure w ill gro und tha t aircra ft. Seco ndary struc ture ca rries some load s an d may have some of the loads on prim ary struc ture tran sferr ed to it. Giving exa mples of secondary structure is mu ch hard er , o n a mod el we migh t co ns ider lea d ing e dge sheeti ng as second ary but as in a ll cases w he re so me thi ng lie s bet ween two ex tremes it is harder to de fine. Te rtiary structure is no n load bearing, wing tip s and fairings are o bvious exa mp les , very often w ing ribs are tertiar y str ucture o n fu ll size aircraft, lead ing edges a nd trailing edges are also terti ary. Damage to te rtiary structur e wo uld no t preven t an ai rcraft from flying apa rt fro m


aerody na mic co nsideration s and the possibility th at th e da mage might ex te n d to seco n d ary structures . Ke ep th e a bove in mind when d e s igning your mode l. Afte r a ll, wh y mak e yo ur w in g tips o ut of roc k hard X" s hee t w he n so me so ft ~" with a little X/ th ca n d o th e s a me job . Yo u wi ll s a ve w e ight a n d make your mod el much n ice r to fly w ith lig ht tips. Even ligh te r wi ng tips ca n b e made b y laminat ing strips o f X,," sheet, these are re mar kably strong but still very lig h t. No , t h ey won 't tak e a full power win g tip lan d ing b ut if the y did the wing spar might Uniden tified eleva to r banging 011 a w a ll, 1I0te bow ligbt all tb e m embers are and go instead , ask yo urse lf Iigbtening boles ill ribs. w hich is easier to repair. Tai ls ca n b e mad e petition fun fly models have tails built up from X" x Mr," mu ch light er than man y wou ld believe . The Piper Triand X" x ~" strip, ve ry ligh t an d tlex ible, they survive Pace r fin look s so substantial wh en it is covered but it co ns ists of a stro ng tub e w hich acts as hin ge post a nd so me very violent aeroba tics but they are pron e to nutte r the rest is ~" tube and cha nne l sec tion , it loo ks positively badly. flimsy. On o ur simp le q uick bu ilt model we may prefe r soli d To take a mod el exa mp le, the w ild and woolly co mshee t for ease of co nstructio n but we ca n use soft shee t

Fig 4.1 - Tapering

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Balsa trebler

Desig n ing Model Aircraft

Ply doubier

Balsa side

33

T he p a rt s rh at . do n e e d to b e s tro ng are the e ng ine m o unting , th e u n d e rcar ria g e mo un t, the wi ng attac hm e n t area sa n d th espa~

but e ve n th es e ca n be mad e a bit lighter w ith so me thou gh t.

The Rotten Carrot Syndrome T h ere a re ve ry few m o d el s t hat are des ign ed to spread th e load s prope rly. We see d oubl ers th a t e nd in a s tra ig ht lin e just a t th e rear of the w ing , ce ntre sec tio n s h eeti ng th at jo ins the L.E. shee t and T.E. at a right ang le an d This 15 powered aerobatic model used tbe tapered spar toitb webs mid velJI so o n. Th is sudden cha nge ligbtureigbt structures, note centre section sheet blended to spars and tips. Flew of sectio n alwa ys results like a dream. in a high stress area an d tha t is w here the mod el w ill brea k in a cra sh. The co rand pu nch holes in the surfa ces. A strip of harde r wood fo r the hinges w ill also act as a spa r to stiffe n th ings up rect way to design any stru cture is to tap er the change of a little. sectio n and to taper the streng th to suit the load s.

This Tiger Motb kit fuselage could be improved at tbe [unction bettoeen sbeet and built up section

34

Desig n ing Model Ai rcraft

Ultra light structure Oil lVaco PG -2jilselage showing curved ends to doubler,

.049 powered aerobatic model, velJ' light,flew well, guess where the toing broke ill the end.

Look a t a main spar. The no rma l model u ses Yl" sq ua re top an d bottom which may be we bb ed and the n is sheered over the L.E. Th e stre ng th is the same from roo t to tip but the load is highest at the roo t an d low est at the tip. The ideal spar would be laminated from Xl" x Vi", three lamination s over th e first th ird , tw o over th e second third and one from there to the tip and the cha nge of lamin ation sho uld be tapered . I have on ly found a spar like tha t in one kit. Now th is will no t save weigh t but it puts the streng th an d weight near the roo t and keeps the tips ligh t. Fig 4.1 shows this along with ot he r ideas for ta pering strength. Th e we bs may be X." near the roo t and }:"nd over the next th ird w ith no webs at the tip. Th e L.E. shee ting can be om itte d an d a sma ll su b spar used to help pr eselve the section o r the sheeting can be very so ft X,," or even red uced to X,nd sheet. Loo king at the fuse lage, any doublers mu st e nd in a long curve or tap er and if there are cut-o uts in the fuselage suc h as a cockp it ope ning and wing o penings the d o u bl e r mu st ex te nd beyond th em . Doublers ca n be th inn er than yo u may imagine , X,nd ply bonde d to Xl" ba lsa is qui te strong enough for a ny .40 size d e ngine an d if trebl ers are used right at th e fro nt it w ould be ade qua te for .60s. If yo u ins ist on Vii," double rs yo u ca n cut hol es in th e m because th e ce ntre of a ny sheet co mpo nent is und e r far less load tha n the edges . One of my favo uri te fuse lage si des consis ts of Vi,," b alsa , YJ," pl y doubl ers and Yl" x Xi' longero n s an d uprights a t fo rm er loc ati ons . I have flo wn .46 fou r strokes in a fuse lage w ith this type of construc tio n, Le. my Turbulen t in RCMW plan s se rvice amo ng o thers . Believe it or no t a slig htly flexible structure will wit hstand crashes better than a roc k so lid one. Even mod ern full size jets have flexible wings , just loo k ou t the window and watch them go ing up and down in fligh t and twist ing wh en reverse thru st is applied . I have a came ra plane design, the front of the fuselage is a very so lid box to pro tec t the camera and rad io, the rea r is bu ilt up from strip . In crashes it was found ' that th e rigid box wou ld bre ak but the re a r p ortion wo uld be undamaged . We don 't wa nt a floppy struc ture but so me "give " is qui te acce p ta b le. Now re ad thi s aga in a nd d ig e st because now we are goi ng to draw the co ns truc tion on our plan .

Materials

Design ing Model A ircraft

To build struc tures yo u ne ed ma teria ls. Yes , I know, pretty o bvious but the choice of materials is important. You co uld double the weight of a mod el qu ite eas ily w ith the wrong choices. Th e re are th ree main types of material that we use , balsa wood , pl ywood and hardwoo d a nd, by careful choices, we ca n bu ild the lightes t and stro ngest poss ib le struc ture. Th is is w he re th e mod el built from a pla n scores over th e one bu ilt fro m a kit , the q ua lity contro l is in yo ur hands and th e designer w ho understands his mat erials w ill do the best job of all. We w ill loo k a t each type of wood in turn and see how th ey can be used to best a dva ntage.

Balsa O f a ll woods , bal sa is th e mo st varia ble , th is is because it grows ve ry fast and so a very dr y or part icu larly wet season has a mu ch mor e dram at ic effec t o n the w oo d . You can be prett y ce rtai n th at any plank of oak w ill be mu ch like any o ther, but a shee t of balsa ca n be rock hard o n o ne side and, in the sp ace of two inches, go to very soft. More often yo u will find less var iation than that in any give n sheet but any stack of shee ts in th e mo de l sho p w ill s how th at de gree of va ria tio n be tw een th e hard est and softes t sheet, and fo r ha rd and so ft you ca n also re ad heavy and light. Thi s varia tio n in hard ness is ve ry usefu l, we ca n use hard shee t fo r he avily load ed areas, medium fo r less stresses p arts and so ft fo r th ose th at do not nee d th e streng th. . No t onl y do sheets vary in hardness, they also vary in stiffness , this is co ntrolled in part by the hard ness but also by the way it has been cut. Quarter g rain sheet is rel uc tant to bend across it's w idt h an d so is used fo r th ings like fuselage sides and w ing ribs . The softe r, easi ly curled sheet is best for L.E. sheeting an d turt le de cks where it w ill curve wit hout spli tting. Str ip wood shou ld a lways be h ard , it is used for spa rs, lon ge rons a nd stringers , the first two have to ta ke the main loads of the mod el w hile stringers take ha nd lingIoads and soft stringe rs w ill break very eas ily whe n you p ick the mod el up . Solid Tail surfaces sho uld always be mad e o ut of so ft

35

shee t, yo u ca n't affor d the weight of anything heavie r at the tail e nd . If it is very so ft yo u ca n glue hard strips round th e e dge or a hard spa r to tak e the hin ges and it ca n pay to let in a stiffener to prevent the sur face warping . As yo u design yo ur mod el yo u sho uld loo k at each part a nd de cid e th en what gra de of wood yo u w ill use for each part , co ns ide r if it will do th e job in the light est way. Think abo ut the tailplan e , this is a large co mpo ne nt fa r ba ck fro m th e C.G . No w ass u m ing th at yo u are goi ng to mak e it o ut of sheet, whi ch is lig hter, a %." sheet with hard er edges and stiffene r o r a pie ce of Yo"

shee t, o r wo uld it be better to use ha rd strip wood ? It w ill depend o n va rio us factors suc h as sha pe and size e ve n the mom ent arm . Neve r forget , a surp lus o unc e at the ta il needs up to four o unces in the nose to co mpe nsate . Ano ther factor that ne ed s to be take n into account is the g rain, we a ll know that wood splits a lo ng the g rain and ye t I have seen part s in kits cut-o ut w ith the gra in running across the nar row dim en sio n. It may be th e o nly wa y to get th e part in a nd so saves the ma nufacture r so me mon e y but it w ill cos t yo u th e mon e y to re pla ce . At le ast w ith yo ur o w n des ign yo u ca n ge t it right.

Inside vie w of Taylor Monoplane fus elage sbototng W' sq wood and ply co ve ri ng: Gtrssets are formed by solid blo cks g lued illto co rners ofjoints.

Unidentified bomebuilt fus elage under co nstructio n. Square cut-out is for toing, 1I0te large gussets spreading load ill bigl» stress area. Tbe fram e will be covered tuttb ply late,'.

36

Plywood Plyw o o d is mu ch stronger tha n ba lsa but it is a lso mu ch he avier a n d s o mu st b e u s ed with car e . T he re a re m a ny ty p e s o f p lywoo d a nd se ve ra l usefu l thi ckn esses , most have thei r uses o n o u r mo de ls. Th e most co mmo nly u se d s izes a re Yt. ,", y.,", X,,", X" a nd Y.". Th e th inner s izes are ofte n used fo r d oubl ers as th e y ten d to be rather fle xibl e a nd b onding th em to b a ls a mak e s a very stron g, stiff laminate . Doubl ers a re o fte n mad e o ut o f X,." ply but th is is thicke r th an yo u rea lly nee d . y'," p ly la minated to ha rd W' ba lsa is q ui te stro ng e no ug h to ho ld a powerfu l .40 e n g ine , w hi le 10 ." p ly a nd .y,," b a ls a is a deq ua te fo r the nose struct u re o f m od el s wi t h e n gin e s u p to . 15 , incre ase the balsa to X" for e ng ines up to .20. I o nce sa w a mode l whi ch u s ed o b ec h i ve nee r ins tea d of pl y fo r th e d oubl ers . Th e gra in was runn in g in t h e sa me direct ion as th e b a ls a g ra in a n d o bec h i is n ot mu ch stro nger than balsa anyway so I form ed a ra t he r lo w o p in io n o f the design. Th e thick e r s ize s o f pl y a re used for forme rs . T h e form er o n w hi c h t h e e ng ine is mou nted must be birch

Designi ng Model Aircraft

pl y and , if 1'." pl y is u s ed for th e lar g er e ngines , thi s sho uld be multi pl y with a t lea st five layers. Th ere are s e ve ral typ es of plywood , so me o f this is wh at used to b e ca lle d "Do u b le Thi ckn e s s Ce n tre Laminati on ", it is ofte n mad e from mah o gan y, it is easy to re co gn ise b e ca u s e th e ce n tre is thi ck with th e outsi de layers being ver y thin . I find that it cu ts badl y , tends to se parate and is n ot reall y stron g en ough . Lit epl y is a mu ch bett er material and ca n be used for so me forme rs . Man y kit s u s e lite ply for mu ch of th e co ns truc tio n , incl udi ng a ll th e fu s ela ge p arts . Thi s is hea vy and harder to cut-o ut and so we would onl y use it for a few parts. Formers ca n be cut a way lea v ing ju st a n o u tli ne a n d th e a reas wh ere o the r p art s are a ttac he d, s uc h as th e plates fo r the nuts th at hold the win gs o n . l1H" ply is a ve ry useful mate rial , it fe el s so w eak and floppy but in th e righ t place it ca n be bett er than balsa. My Velie Mon ocoupe has a fu sel age b uilt of Wt sq ua re s pruce w ith an inn er skin of )(,," ply in th e ce ntre area, thi s w as pow er ed by a Cox Qu e en Be e .07 4 a n d w as full y aero ba tic . In th e fin al cras h whi ch demolished the top p ortion of th e ca bi n, th e wing , buried the eng ine and snappe d the cranksha ft, the ba sic fusela ge was not bro ke n. l-f, ." pl y can a ls o b e Metal bomebuitt, 1I0te oarying size of' tubes, biggest is 0111)' W '. Turtle deck is us ed for turtl e de cks as fibreglass, rest is fabric cooering. it will roll so easily a nd Hardwoods I have used it to shee t ce ntre sectio ns o n w ings. I a lso Th ere are two main hardw ood s so ld for mod el co nkn o w of so me mode ls wher e th is ply is use to shee t th e struc tio n . Bee ch for e ngi ne bea rer s and undercarri age L.E. of th e win g. mo untings , a nd sp ruce w h ich is use d for s pa rs a nd Use d carefully, y", " pl y ca n ad d va luab le s tre ng t h with very little we ight pen alty. stringe rs.


37

Take th e ave rage all w ood home-built. Her e we ha v e a n a irc raft w h ic h is built of a frame wo rk of ro" squa re and covered with ~" ply. T he re a re o ne o r tw o th at use eve n thinn er plywood. Lo ok a t th e s ize o f the tu be on a metal airframe w ith fabric covering, 'X" tube is la rg est a nd most are eve n thine ngi ne n e r. Even mounts use q u ite sma ll d iame ter tub ing carefully arranged in trian gles. Wh en yo u lo o k a t co ntrol surfaces you can see th at th e y a re built up fr om a fe w thin pi e ce s o f wood a n d a One offour bolts bolding doum a Cessna fln; pound coin gives scale. Note rivets spa r. holding bracket to fill. Tbis is primal)' structure. Where parts are bolte d tog e th er th e b olts Few design s use hard wood bear ers th ese days, the a re oft en s ma lle r th an o ne mi gh t ex pect. To run nylon e ngine mount is so mu ch easier to use . through so me exa mp les . Th e Taylor Monoplan e tailThe ha rdwood bearer had o ne big ad vantage , it tied pane is held on by four W' bolts th rough a co u ple of th e front of the mod el together as it ran through at least sma ll met al brack et s. Th e fin o f a Cess na 152 is held two forme rs. Th e disad vantages were th at th ey mad e down with four X" bolts th rou gh four small brack ets. positioning a large fuel tan k difficult and they we re ve ry Pe rh aps a n aero batic aircra ft w ill have mor e s ubdifficult to re place' if the y go t brok en in a cras h. stantial bolts. No. The To p win g of th e Steen Skybo lt, a ve ry ae robatic ma chi ne , is held o n by two );,." bolts but Both these disadvantages are overco me if the bearers of co urse th e flying wi res tak e so me of th e loads. are glue d to the fuselage sides and the eng ine is mounted o n a Paxolin plate , wh ich will snap in a cras h proStru t b raced wings o n a ircra ft lik e Cess n as a n d tectin g the bearers and the e ngine . Being sprea d apa rt, Aust ers are held to th e fu sel age by two sma ll bolts fittin g a tank between the bea rers is easy. th rou gh brack et s, th e stru ts tak e a lot of th e lift but not Spruce is often used for spars and stringers and can all. b e used fo r lon ge rons. Th e we ig h t p enalty is low becau se it is used to repl ace hard balsa and o ne can use The Application of Models a sma ller sec tion . Spruce is idea l for sca le mod els whe re the framewo rk Mod el s d o s u ffer stresses th at full s ize d a irc raft is visible and the members wo uld be over sca le if ba lsa co u ld not s urvive. We pull "G "s th at wo u ld h ave a was used , this applies es pecially to stringe rs. pil ot sq uas he d fla t on th e cockp it floor a nd of co urse Referring again to my Velie Monocoupe , the rear porman y, if not mo st , of o ur landings sho u ld be calle d tion of the fusela ge was mad e from W' sq uare spruce co ntro lled cras hes, but we ca n still mak e ou r models a lon ge ron s with ~" sq uare ba lsa cross members, the cras h bit lighter. did not damage that part of the mod el either. Even allowing for th ese high e r load s we co u ld use sma ller s izes of mat eri als that w e normally do . An incredi bly stro ng fuselage can be built with spr uce if yo u g usse t ea ch joint wi th y,;," ply . I ac tually built a My Monocoupe 110 Specia l has the w ings held o n fuselage fram e with hard 1(6" balsa and gusse ted eac h w ith four 4mm nylon b olt s, I co uld have used 8 BA stee l and still have been co mp lete ly safe. I had a .40 joint. I decid ed to scrap the fuselage and I used it like a club again st a met al cha ir and the bench , holding it by powe red co ntro l line Pitts specia l o nce, a nd th at used the tail end an d smashing it down . I had to hit it aga ins t two 6 BA bo lts to hold th e top wi ng on . Tak e th e Tay lor Monopl an e , a fa vourit e s u b jec t the ben ch an d the cha ir as hard as I co uld several times becau se I spe nt a lot of time doin g the sca le d raw ings of befor e it even started to break up . the full size aircra ft. A Y. sca le ve rsion wo uld only be 63" spa n a nd th e fuselage fram e co uld be %, sq ua re hard Strength and Lightness in Full Size balsa an d W,," ply or X6" hard balsa skin for a ve lY stro ng scale struc ture, suc h a model wo uld be quit e happy o n Aircraft .40. By th inn ing abo ut weight a nd stre ng th, usin g mateI have menti on ed using full size class ificatio n of strucrials to th e best adva ntage it is possible to buil d a light tu re but let 's look at so me ex amp les of full size conmod el th at will withst and a ppa lling p u nishme nt and struc tio n. You w ill be surp rised as jus t how lightl y built o nly need min o r repairs. most aircraft are .

38


Chapter .S

Fuselages an y people draw the ir plan s with single lines to represe nt pa rts, fo r exa mple just the front of the spa r and the rear of the T.E. Th ey also use single lines for the for mers and fusela ge sides. This is bad policy in my view for two rea son s. The most immediate reason is that you have to draw all the parts using previously d raw n item s. You need the fuselage side and plan view s to draw the forme rs. It is all to o easy to ge t mixed up wh en taking these mea surements if you only have o ne line . Imagine that you are o nly using a single line for eac h former. The fron t form er would have to be represen ted by the front face becau se that is where the e ngine sits, but the next one back wo uld have to be sho w n by the rear face which is wh ere the w ing L.E. locates and the former at the T.E. would aga in have to be show n by th e fron t face . . . See ho w easy it would be to ge t confused ?

M

Tbe majority of models u se tbe basic box fuselag e .. . this is Miss 5 7/"0111 tbe R CMlV Pia ns s eruice . . .

Fig 5.1 -Collstrllctioll Layout optiollal sub spar

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Design ing Model A ircraf t

39

... some just leave it like that . . . Blue Movie 2 from tbe RCMlV Plans Seruice, tbere is not mucb else you call do witb Correx . .. The second rea son is that yo u just might want to offer yo ur pl an to a maga zine and the y will ne ed more th an a few thin lines. A typ ical pl an is show n in the drawing Fig 5.

Fuselage In th is p art I s ha ll tell yo u how to lay out yo ur drawing but I won 't cover everyth ing in gr eat detail be cau se a nyo ne designing a model sho uld ha ve built o ne o r tw o kits and so will have a little ex pe rie nce . For exa mple, yo u sho uld n't ne ed me to tell yo u that th e e ng ine mount ing form er mu st be pl yw o od a nd mu st be firml y attac he d to th e side s. De sign er s use

. . . some dress it up II bit . . . Triuial Pursuit sbotos that angular turtle deck call look smart . . .

40

many standard techniqu es so , if so me thing is not cove red in detail , use a method that yo u have se en e lse wh ere . Th e sid e view will sho w a ny d oubler s and th e forme rs. Ind icate the dou bler , not forgetting to make it end in a cur ved line aw ay fro m any o the r changes in section (Yo u did read th e last chapte r, didn 't yo u). Form ers come at logical points, the e ngine mounting bul kh ead , at the L.E. and T.E. of the wing and at the L.E. of the tail plan e . You w ill need at lea st o ne between the wing and the tail and the turtle deck may need a co uple of ex tra formers. Drop lines to the plan view and dr aw the doubler and formers in to this. If you are fe elin g lazy yo u ca n indi cate the th ickness of the sides and dou ble rs in the plan view at the former locat ions o nly. Now to plot the formers. Draw a lon g datum line and dr op ve rticals from th is, also ex tend the ve rtica ls upwards . It is at this p o int that sq ua re protra ct o r co mes into its ow n. Now, usin g a pai r of divid ers tak e the distan ce from th e d atum on th e s ide v ie w to th e b ottom a t th e e ng ine fo rme r and tran sfer thi s to th e first former, re peat with the top d imen sion a nd then using the plan view tak e the width fro m the ce ntre line to th e side and tran sfer this to each side of the former drawing. Us ing yo ur sq ua re protractor pl ot o ut th e sq ua re sha pe of th e ba si c forme r. Th e top p ortion will be plott ed later. Rep eat fo r all the o the r formers. The re a re var iou s styles o f turtl e de ck , yo u ca n le a ve it flat (Ug h l), yo u ca n mak e it ro u nded or a sharp ang led deck . Th e sha rp a ng led top is easier to plot as yo u can use fixed points o n each former. Th e ro unded turtl e


d e ck lo o ks b e st a n d Fig 5.2 - Former P lotting ca n b e pl otted u sin g th e h e ight a n d width a n d drawin g it with co m passes . A p icture be ing wo rt h a th ou sa n d w ord s yo u w il l find two d raw in g s I 1Vl s howi ng th e m eth od ---+= elsewhere in this cha pter. Fig 2. The me th od s ho w n for pl ottin g round to pped fo rmer s works we ll , but so me times I yo u may want a diffe re nt sha pe to the turtl e deck and yo u will find that interm edi ate form e rs ca n no t be p lotted easily. Th e simpl e wa y is to lea ve them oversi ze and s a n d down I with th e aid o f a I straig ht edge until they I blend in . Curve d turtle deck s be w ill n e ed to plank ed a nd thi s method of co nstruc tion WI allows yo u to use fo rlIT m e rs th at vary qu it e d ra maticall y. Th is ca n ma ke for very attractive s ha pes w hic h a re not lIT o ften seen these days. When drawin g forme rs o ut don 't for get to allow for the th ickn ess o f th e s ides a nd top , and rem ember that the double rs do not ex tend to the tail, all these are good reason s for drawing the model in full. Oth e r it ems t hat h av e to b e added to th e fu s e lage d rawing w ould b e win g bolt pl at e s , a ny rei n fo rcement for att ach ing th e undercarriage , perhaps a sheet inse rt for gluing do wn the tailpl an e and a ply mount for the tail wheel and sk id . Draw all th ese p a rts in on bo th drawings. Very often yo u will spo t possibl e pro blem s w hile doing the d rawin g w hich is a lot bett e r tha n finding th em w he n co ns truc tion is well under way. For o ur in itial d esign we ca n use a very sim p le unde rcarriage , the be nt dural U/C bo lted to the bott om of the fuse lage so incorporate a ply plate jus t in front of the win g an d use trian gul ar stoc k to reinfor ce the join ts b et w e en th e pl at e a n d th e s ides a n d a lso between the e ng ine mounting form er and side s. This wh ole area is primary struc ture as defined in Cha pter 4.

~I

I

~

-

!L

t r1----

Design ing Model Airc raft

~il'

HT2

,,

,,.

-

11 11 rI

,

HS2

JL i-__tJ2 _ 11

l1i72

11 11

Advanced Fuselages The design of yo ur average sports mod el is prett y basic and there a re limits to w hat o ne ca n do with a plank w ing and modifi ed box fuse lage with dural U/C. Add round fuse lages and tape red wi ngs to the pe rmut a tions, th ro w in a variety of under carri age types a nd yo u can pro duce some rea lly differe n t look ing mod els. You may fee l that suc h co mplications are not worth the effo rt a nd for sports mod els this could be true , but w he n it co mes to sca le mod els so me of th e nicest subjects are mo re co mplex. The aerody na mics rem ain th e sa me , the probl em s co me in design ing the structure so that it is still light and relatively easy to build.

41

Rounded Fuselages Fig 5.3

Rounded so«

If yo u study 3 views and photos of full size aircraft you will see that a ll fuselages are variations o n a few methods. Rounded box Th e slab sid ed fuselage has been dealt with but a few , like the Piper Cub hav e a stringer or two down the side wh ich just gives a little more sh ap e. More rotund fabr ic covered fuselages will be the simStringers Sbeeted ple bo x with sub form ers and stringers. For this typ e of fusela ge it is well worth building the box up from strip _0 , like a vintage mod el, as the stringers will add stre ngth and yet the whole thin g will be no heavier than a plain '- .... box. Fig 3. The next typ e is the metal cove red fusela ge and here careful an alysis ca n pay off. l et's take two examples , the B-17 Flying Fortress and the Ryan STA or PT se ries. Th e first C/ l 3- 17 that I built was plunked . a very lon g and ted iou s task but then I found better drawings and stud ied mor e ph oto s and I realised that the fuselag e is a cylinde r and a cone, the o nly double curvature wa s at the no se . You ca n do the whole thing w ith rolled Strip frame sheet. This is wh at I did with my seco nd 3-1 7 and the no se sheeting was slit and curve d and pulled in just like short planks. l ook at the Ryan STA and you will see that it is the sa me , straight lines whi ch are very easy with balsa. It is possible to get so me d ouble curvature into balsa sheet. The method is quite simple . You need a fram e work of form ers with substantial stringe rs about 3" apa rt. Tak e soft balsa sheet and so ak it in water for a whil e, if you add a little ammonia the soa king time is reduced to about 15 minut es. Now take the sheet and ca refully bind it to the frame work with bandage and lea ve it to dr y. Tw o o ppos ite pan els ca n be d on e at a tim e . On ce the panels ha ve dried the y can be trimmed to fit and glued into pla ce . \X'h en building a fuselage like the B-17 it would be a waste of weight to have mu ch internal struc ture , as the skin and Y; sq. lon gerons would be qu ite stro ng eno ug h, but some sort of jig is n e ed ed to bu ild th e Internal view of rear fuselage of all Auster undergoing restoration. Tbln outline fuselage. ply formers support stringers, nearestformer is coaming tobicb takes cabin The simple answer is gtaxing: Mai"framework is steel tube. to bu y a pie ce of wood about X" ( I umm) by 3" or 4" a nd c u t s lo ts to tak e the form ers whi ch c a n b e lin ed up and s p o t g lu e d in pla c e . O nce most of the fus ela ge is built it ca n be crac ke d aw ay from th e jig. Fig 4. Thi s typ e o f jig ca n be used on oth er mode ls, th e Hirth Acrostar has a fuselage that go es from slab side d to round o ver a ve ry s h o rt di s tan ce and the o nly wa y to build it is with the jig describ ed . On e o f th e mo st beautiful se ries o f a ircraft were the Lockheecl Veg a, Sirius , Altai r, e tc. -i

~

---

I

42


Th is range o f a irc ra ft us e d. o ne mon o coqu e pl y ". fu s e lage s he ll form ed in a co nc re te mou ld a nd a s ta nda rd wi ng ;w ith slight variations o n dihedra l. Fro m th ese co mp on ents Lo ckh e e d b u ilt th e p a ra s o l w in g Air Exp re ss , the s h o u lder w ing .Ve g a , th e lo w wi ng Altai r, Sirius and O rio n , so me w ith a nd so me without retracting undercarriages. These aircra ft w e re v ery fa mo u s , Wi nni e Mae for exa mple hel d the reco rd for a ro und the wo rld flight. I h a ve o fte n w ondered w h y s o me kit ma nufactu rer has n ot prod u ced a series of m od e ls of t hese a irc ra ft. Even th ou gh I d isli ke fibregl ass a n d . . . toitb stringers Oil tbe s ides of tbe box . . . like Tequila Sunrise . . . fo am I ca n see t h e adv a ntages he re . O ne Meamobile, Back at the Drauring ma in GRP moulding and a few small o nes , o ne basic foam wing and a varie ty of sheet tail surfaces a nd he Board wo uld have five diffe rent kits. The average sports mod el has a hard life (and a sho rt From our point of v iew the Lockh eed se ries wo uld ne ed to have the fuselages p lanked , whic h is the rea o ne so metimes) , and so we ma y not want to go to suc h so n that I have never built a mod el of o ne of these airle ngths as describ ed above but we can use the techniqu es. craft. . . . tobile others bide tbe fa ct co mp lete ly . . . Pee Tee Sportster is built as sb o um on figure 53

Desig ning Model Aircraf t

43

This Auster has velJ' box)' fuselage but three stringers add a little shape. . . . but a feto do not use tbe box at all: B-17 built as show" in figure 5.4 .. .

44

. .. a feto use a box at tbe front . . . like tbe Hirtb Acrostar.


You ca ll make some really attractive s h apes

The slab side d sports mod el can be tran sform ed by adding stringe rs dow n eac h side, instant ro und fuselage . A nice dou ble curvature turtle deck ca n be planked (NO! The man who said ho llow block, it's heavy an d ve ry exp en sive).

Combinations of all the methods can be used to produ ce so me really bea utiful fuse lage shapes . The ph otos and drawings will give you so me ideas. Just rem ember the rules, kee p it light, no sudd en change of sec tion, spread any load and tert iary structure does not carry any loads.

Fig 5.4 - Simple Fuselag e fig

Side view ofjig

B-1 7 Jig, only two Iongerons sboum fotn s kill

Oil

longerons

I Typical former

~

I : I I

J ig sboum dotted '-_!

Desig ni ng Model Ai rcraft

45

Chapter 6 '

Wings and Tails Wings T he w ings a re cons ide ra b ly easie r to d raw o u t, th e cross-sec tion is alrea dy deci de d by the airfoil section so it is just a matter of decid ing o n the size and positio n of s pars etc. O n a mo de l the norm al location for the main spar is a t the th ickest point in the sectio n , aro und Y.rd back fro m the L.E. O n la rge r w ings yo u may wa nt to put ano ther spar at Y.rds chord . We have talked abo ut tap erin g the spars and whil e this is the ide al I predi ct that few peopl e will both er , pr eferrin g the standard type of spar, Le. X" sq uare top a nd bottom possibly with we bs. Leading edge sheet does add strengt h and provides a smooth surface over the most important section of the wing but it is not essential on sma ller and light er mod els suc h as powered glide rs. Nose ribs between the full ribs or sma ller sub spa rs ca n give equa lly good result s and are cheaper and lighter. For the mo re highl y load ed and stresse d mod els the '0' box is much stro nger and w ill pre ve nt wa rps an d

The simplest uiing of all, parallel chord, 110 ailerons and 110 sheet, suitable for velJI small light models. This is American Dragon forB diesels but has been scaled up to 60" span for .40follr stroke.

Fig 6.1 - lVillg Sketch Page

POOR Splits here Common with or rottbout sheet

c. ::

Good with or tottbout sheet

Front view

c =======

Best for sbeeted L.E.s

c:

Light models, small aerobatic, gliders

46

Design ing Mode l Aircraf t

distortion of th e wing . Be warned th at it is a lmos t imposs ib le to rem ove a built in warp from this type of wing. Rib spa cin g will depend on th e typ e of mod e l, th e size a n d loads. Closer ribs at the ce n t re e sp e c ia lly if a win g mounted und ercarriage is used and wid er spacing out beyond the ce ntre sec tion shee ting. All s hee ting s hou ld foll ow th e rul es about tap er ing off the strength, large radii at all corners o r ce ntre sectio n sheet tap ered into the L.E. and T.E . over two rib bays will prev ent that sudde n Ellip tical tips O il p a r allel cbord look nice, inset ailerons are velJ' effective Oil change o f se cti on th at tbis Mo nocoupe 110. snaps so easily. Trailing ed ges can be made with a strip of W' sq . and tap er the bra ces o r the win g will snap wh ere they end. two pieces of XI," shee t or of mor e substa ntial strip withStrip a ilerons are normally so lid, quit e often ready out the sheet. The T.E. does hav e to take the loads of sha ped T.E. stoc k can be used but do not compromise the aileron hinges and so must be strong enoug h for the the airfoil section by using the wrong size of wood , better by t~I1' to sh ap e yo ur ow n stoc k from thicker wood . task. Leading edges do not need to be very strong . O ne Tips must be kept light , rem ember inertia ? A co uple very commo n type of L.E. is Y, sq se t o n one co rne r, this o f sugg es ted meth od s are sho wn in the drawings. is not good becau se a slight tap will split the ribs. A better L.E. is shee t glue d flat to the front of the ribs. Dihedral My favourite L.E. is a variation of the abo ve whe n used with L.E. sheeting. The L.E. is ~" sheet, it ca n be Dih edral is use d to p ro vide lat eral sta b ility , o n a quit e soft, this is glued to the ribs and sa nde d to blend mod e l with no aileron s it is esse ntial but not on o thers. A mod el w ith no aile ron s ne eds abo ut 5° o r 6° o n in pe rfec tly. Th e sheet is glue d on and trimm ed back eac h panel, this will give reason able rudder control, less flush with the front of the L.E. and then a caps trip of ~" d ihedra l will reduce the efficiency of the rudder. sheet is glu ed o n the front a nd sanded to the co rrec t sectio n . Thi s sys te m is very stron g and ve ry Fig 6.2 - lVillg Shapes ea s y to mak e and alwa ys looks ve ry neat. Standard tapered tuing J oinin g tw o w ings ca n be done in various w a ys . Som e Ame rica n kits ha ve us ed g lass --tape and resin just like joi ning foam wings , it work s if yo u don 't mind the mess and e ffo rt of ge tting the joint smo ot h. Small mod els can just hav e th e w ings butt join ed and rely o n good Spar glue joint s and the cov" e ring materi al, but thi s is o nly for mod els with engines up to .09. Th e normal method is to use d ihedral braces o n th e s pars a nd L.E. and T .E . Th is s yst em has work ed for as long 12° Washout as model s hav e b e en built , just rem ember to

l

-

)


11:===4

47

Tapered Wings O ne of th e easiest mod el struc tures to design and bu ild is the pa rallel chord wing with strip ailero ns, however, the tapered Wing is a bit more co mp lex be ca use eve ry rib is d ifferent and the d read ed tip stall is more likely to occ ur. Of co urse if you really wa nt to make life hard you can go for a fully elliptical wing and if you are a ce rtifiab le sado- masochist yo u co uld design a wing like Carl Go ldberg's famo us (I'd say infamous) Valkyrie , whic h has each rib buil t up from st rip o n that vas t elliptica l wing. My o nly co mment is, if yo u are read y for that , w hat the heck are yo u do ing wasting yo ur time reading this?

Tapered all sheet co ue red uiing does not need spars 0" smaller sizes, tbts is Sbrike Commander: Even more dihedral is needed for a low wi ng model wi tho ut ailero ns , u p to 10° which looks pret ty ridiculous w hich is w hy there are so few low wing mo de ls witho ut ailero ns . With a ilerons di he dra l is not essentia l but a slig ht amo unt w ill make the mod el slightly more stable and so easier to fly. As the dih e d ral is increased the ailero ns become less effective , for no rmal use about 1° or 2° is about right. A low wi ng mod el with no di hedral w ill fly very nicely and need not be very unstable, altho ug h it ca nno t be left to itse lf for ve ry lon g, however this type of mod el is normally used for aerobatics and so this is no t a problem Wings w ith no dihe dra l do look as thou gh they are d roo ping and so it may be desirable to use a tou ch of d ihed ral for cos metic reasons. On a tapered wing it is e nough to make the to p of the w ing flat with all the dihed ral o n the bo ttom of the wing .

The Straight Tapered Wing The only ex tra work in designing a straigh t tapered w ing is that all the ribs are d ifferen t. The spars can remai n in the sa me relative location o n the rib and this will mean that the L.E. sheeting w ill be tap ered wh ich , as we have seen, is a good th ing. The actual tap er ca n be on the L.E., the T.E. or both, it is much a matter of taste but it is a good idea to ke ep the mai n spar straight at least o n yo ur ea rly design s as this makes joining the spar easier and also provides a da tum to work to . Ta pered wings are mor e prone to the d re aded tip stall and the grea ter the tap er ratio the worse this ca n ge t, so for your spo rts mo de l it would pay not to go too w ild, a tip of Y. the roo t chord loo ks right an d is not excessive . Tapered w ings will ben efit fro m wa sh out, a slig ht twis t so that the tip is at a lower ang le of incide nce tha n the roo t, one o r two degrees is fine. A use ful tip, X,," at abo ut 3W' eq ua ls 1 degree , so if we take a roo t chord of 10" and a tip cho rd of 7", an d we wa nt two degrees of was hou t we divide 7 by 3.5 = 2 so

Homebuilt biplane top using structure, note ve,,' ligbt construction of ribs and beauier compression member betuieen spars. Ribs are tertiary structure.

48


Laminated strips make strong, Iigbt tips, 1I0te sbeet L.E. blended ill.

co m p ute r. Th ere a re peopl e who can do the job for yo u at rea sonabl e cost. Th e next method is to use a full zoom ph ot ocopi er , just tak e yo u r o rigina l drawing a nd e nlarge or reduce it b y th e co rre ct factor for each rib , thi s method w ill not allow yo u to change sectio ns a long th e w ing . It will also cost lOp per rib a nd yo u will hav e to add the spa rs etc afterw a rds. The a bove method ca n be used to good advantage if yo u ca n ge t hold of Martyn Pressn ell 's bo ok , Ae rofoil s Fo r Aeromodellers w hich wa s publish ed by Pitm an . Thi s book has large nu mb ers o f aerofoils drawn in different s izes a n d s o m e are al s o drawn w ith th e )'I,, " s ki n removed . Most are intended for gliders a nd F/F power but th ere ar e several u seful sectio ns fo r R/ C p owe r. Th ese do reduce th e num ber o f pho tocopies th at yo u will have to ha ve done . Thi s boo k is o ut o f p rint but yo u ma y be a ble 'to find co p ies a ro und . Anothe r so ur ce of tapered ribs is a range o f sheets ava ilab le from the Aviation Booksh op . Th ese are th e o ld Ae romodell er s hee ts and cover quite a wi de ra nge of sectio ns in useful sizes for the sma ller model but ag ain , th ese are mo re suited to glide rs and vintage type mod els a ltho ug h NACA 0018 is a useful sectio n for aerobatic models. One method of drawing ribs that I hav e read about in a 1939 b ook was to tak e a ply outline of a rib a nd a long p iece of w ood , you fit a torch bulb at the top a nd

Taylor Monoplane aileron is top hinged and bas massive differential.

Sbeet of tapered uiing ribs from tbe Auiation Bookshop, one 0.1a series uiblcb are velJ' useful to tbe designer. o ne degree w o u ld eq ua l Yx a nd two wou ld be Y; so the T .E. would be rai sed by ~ " a t th e tip to give yo u 2 degrees wash out. Th ese are 'Rules of Thumb' a nd ca n b e b roken, for exa mp le, the Great Plan es Patr iot kit design has a root cho rd of 15" and a tip of 7" and the whole w ing is swe pt, it only uses Y, a de gre e of was ho ut an d , so long as the CG is in the specified rang e , tip stalling is not a p ro blem .

Ribs for Tapered Wings Obviou sly every rib is different on a tapered wi ng a nd so each rib ha s to b e drawn separ at el y a n d b e ex actly the right size and sha pe so that the w ing tap e r is perfect. Som e w ings on sc ale models have th e ad de d co mp lex ity that the sec tio n cha ng es from root to tip . Th ere a re a number o f m ethods fo r producing a d rawing of a se t o r win g ribs, for the most part th ey all have d isadva ntages , they may be ex pe ns ive , time co nsuming o r limited by the ran ge of sectio ns avail abl e . Th e qui ckest and best w ay is to us e a co mp uter prog ramme, this will draw th e ribs with all th e spa rs a nd she e t, it will ch an ge the sectio n fro m root to tip a nd it is . unlimited in range because yo u ca n feed in the coord inates o f an y sectio n . It will cost yo u quite a lot o f money for the programme a nd even more if you have to buy a co mputer , but it is worth it if yo u already ha ve a

D esign ing M odel A i rc ra f t

Inset ailerons are more complicated to build but are essential on scale models. Bxtra gusseting at corners would be all improvement.

49

Fig 6.3 - Inset Ailerons

~;/\\\ ,/

:-\

'.

Spar may co ver aileron only or

I

. If\ \

i ,.\ '.

,'

~ Without

Ply bellcranh mount

Full span

~ gussets

H inge

Pusbrod

slide the outline up and down until you get the correct size of shadow and then draw rou nd it. I haven 't tried it but it migh t work. There are ways of draw ing the req uired ribs, o ne is to plot the o rdina tes for each rib and then draw it o ut . .. This way lies insan ity . . . then the re is ano the r meth od w here by yo u d raw th e tip rib and root rib a nd then about seven thousand lines (we ll, an awful lot anyway)

Aileron connection 011 all Isaacs Fury set-up for lots of differential; just like a servo arm:

Top hinged

and co nnect the m up an d draw each rib o ut. It does work, it takes a very lo ng time an d you wo uld be better off buying a co mp uter or having a foam wing cut.

Elliptical Wings Elliptica l w ings ca n be treated in the sa me way as tapered wings bu t the shee ting is much harder because it is dou ble curve d . A straight wing with elliptical tips is one so lution an d the re are Spitfire models wit h a straight tapered wing wit h ellip tica l tips a nd curved strip ailerons, they are quit e co nvincing in the air. . Nice, light elliptical tips can be made by laminating strips of wood over a pattern, this is ideal for sca le and se mi sca le mod els. See the picture of my Monocoupe wing. Elliptical wings do tend to increase the likelihood of

Tiger Moth aileron pusbrod sbotoing connection for differential aileron ... just like a seruo disc.

50

Designing Mode l A ircraft

tip stalling and so washout is essential, eve n the full size Spitfire had washout.

Ailerons The strip a ileron is the mos t co mmo n typ e o n models because it is ver y easy to mak e , ex treme ly simp le to

Fig 6.4 - Linkages for Differen tial A ile rons

connec t to the servo and ge ne rally make s life easy for the designer. It also makes life easy for the manufacturer of foam w ing s but it is no t the mo st efficie nt typ e of aileron. Inset ailero ns are working out at the tip wh ere they can apply most leve rage , the portion close to the fuselage is doin g very little work.

No te: Top d ra uiing ill each pair is s ta nd a rd, lower dratoing s h ows differen tial

Strip Aile rons

Seruo disc

N ote: all high uiing models connection is movedto X

x

A

B

Ins et A ile rons

Bellcrank 90°

Bellcrank 45° 1

j/~I~+7c::============================~JU 1 1 1 1 1_1""""'-----.11 1 1.....------...1


51

they do add some stress areas in the w ing, however, th e y are essential for sca le mod els. I kno w that there is a so called scale Piper Cub kit that uses s trip ailero ns but you are not going to sink as low as that whe n you design a scale model are you? Th e main things to rem ember whe n designing inset ailero ns is that it s ho u ld b e 1 \1," to 2 times the width of a strip a ile ro n a nd th at yo u s ho u ld use g usse ts to . .:. s p re a d th e lo ad s and ~ .:, ... rem o ve the stress are as ,.',' ~ . . . in the corners. Fig 6.3 sho ws a typiTail toith laminated outline is vel)' strong and light, tbis one is Triuial Pursuit ca l in set a ile ro n . Th e forLBcc engines, spars could be added forlargermodels. aileron bellcrank can be re versed a nd th e horn Another snag with strip ailerons is that they can twist, can be fitted o n top of the aileron if desired . Note that the this will reduce the angle out at the tip where you need be llcranks poi nt in o ppos ite direction s in eac h wing so th e deflection most. You ca n use mu ch harde r wood that o ne is pulling as the other pushes. which will resist this tw isting but that is needless we ight It is desi rabl e but not essential to have diffe rential just where we do n't want it. aileron movement , this means that the up goi ng aileron One way to overcome this is to co nnect the controls to sho uld move up farthe r than the down going one . This is the ce ntre of the aileron by means of a system of bellbeca use the down go ing ailero n crea tes more lift, more cra nks and pu shrod s or, alternatively by using a servo o ut lift eq uals more drag, more drag tries to pull the aircraft in each wing. This allows the aileron to be used as flap s towards the down aileron which is the exac t opposite of via a mixer, probably the only aerodyna mically valid reawhat we wa nt. son for using strip ailerons. By mak in g th e up go ing a ilero n m o ve fu rth e r Inset ailerons are much more complicated to make and we incr ease th e d rag so th at it is equa l to or mo re tha n th e do wn aileron. Tail assembly uslng built up and core methods. Core systems is vel)' goodfor Th e Ta yl or mon oscale control surfaces. plan e hom ebuilt , specifies aileron travel as "250 up and almos t no down movem ent. " Thi s differ ential ha s another adv ant age . The down go ing ail eron in effect increases the angle of attack of the wing in that area , at slow speeds this can initiate a tip stall o n th a t side, reducing the throw w ith differential helps to delay the tip stall. Fig 6.4 sho ws how to set up the aileron bell c ra nk for d ifferenti al , Chart Micro Maid make a n adjusta ble bellcrank which is useful for this. \

Tail Surface For in itia l d e si gn exe rcises the shee t tail is the best ')Tay to go but yo u can build them up

52


with so mething like X" x )1," outlines and ~" x X" strip ribs. Used medium shee t for the solid tails and hard stock for built up tailplanes. Of course the draw ing for solid shee t tails is do ne as soo n as the outlines have been drawn and I hop e that I don 't have to describe how to draw the bu ilt up system. It is rare for a model to finish up nose heavy and so anything that can be don e to keep the tail light will he lp, don 't forget that an ounce of sur plus weig ht at the back end will need three to four ounces up the front, this is useless weigh t which we need like a hole in the fuel tank . Tail surfaces can be built up , there are three co mmon meth od s. Ordinary straight strips of wood with ribs as sho wn in drawing 6.4 of this chapter. More curvaceo us shapes can use the same method but the outline can be don e with lamination s as described for wing tips. Another meth od which is very goo d and popul ar is the core meth od , a thin shee t core is cut to the shape of the tail and then ribs and spa rs etc are glued on each side , wh en covere d it looks just like a fabric cove red tail but is ve ry s tro ng and al so light. Co mb in ing thi s Fig 6,5 - Tailplanes method with th e lami nat ed o utline mak es a wonderfully strong light structure. Weight can be saved over a so lid tail b y build in g th e struc ture from strip and coveri ng it with Yl,nd balsa, this makes what is, in effect, a b o x struc tu re with webs which is incredibl y stiff for its weight, but you must use soft wood throughout. One method that was very popul ar some years ago was to build an outline with diagonal bracing (\'(farren Girde r) and then put a spar top and bottom a nd fill in th e ribs with small triangular s ha pe d pi e ces . Thi s meth od was used on the aerobatic mod els of the d a y and is light and strong. The tail need s to be stiff. enoug h to preve n t flu tt e r. Th e top Co m pe titio n Fu n Fly model s use inc re di b ly light tail sur faces w ith very large co ntro l surfaces bu t these mod els are not design ed to be flow n fast a n d m o s t wa rn th e builde r th at flying straight and level at full throttle w ill cause the mod el to flutter and self destruct. I have see n it happen seve ral times.


Auster tail assembly. Note bow fabric on fin is used to make a fatring to fuselage, typical of most fabric covered aircraft.

Ltgbttoetgbt, could be covered tuitb 'l$z sheet

Sheet core

c --W®F

Vintage aerobatic e.g. Astro Hog

53

Chapter 7

Undercarriages and Other Bits T

he main parts of the model are the fusela ge and the wing but the design of the undercarriage and its mounting is of major importance, this area is prim ary struc ture. The cow li ng is al so important b e cau s e a b adl y desi gn ed co w ling ca n ca use th e engine to o ve rhe at w hich will dama ge the engi ne and may cause a crash.

Undercarriage Th e undercarriage is o nly used for a sho rt time at take-off o r landing, however , it is a vital piece of eq uipment. It sto ps the propeller bein g worn down and the belly of yo ur model bein g scra pe d alo ng the ground so it mu st work properly and not co llapse. Simple dural

54

To work we ll the wheels must be in the proper place or tak e-offs will be diff icult o r downright impossibl e. The undercarriage mu st also be of the right tJpe. Tryin g to use a tricycle unde rcarriage on a nything othe r than sho rt grass or tarm ac is a dead loss, the lon g grass will hold the no se down and even fold the nose leg back e no ugh to allow the prop to ge t into the grass, slowing the e ngine . If the main wheels are too far back not eve n the mo st powe rful elevators will get the nose up . . . No take-off. The ma in wheels on a trike sho uld be just behind the CG, so close that if the mod el is rocked ba ck ont o its tail it will stay there . In this wa y the e levator will be able to lift the no se and allow the mod el to take-off. The nose leg takes an awful pounding so it mu st be

vie can be held 0" with elastic bands 01' •••


Fig 7.1 -VIC location

\ \ \ \

\

CG behind wheels uib en model is tipped back

Tricycle VIC

"Ta il dragger" VIC J

IN

Effect of'foruiard VIC uiben model stoing CR

=

Centre of drag of wheels

IN

=

Inertia acting through CG

Note: Larger moment of leverage 011 foruiard VIC increasing suiing


55

... bolted 011 toitb

11)'1011

bolts. ( Numberplate bolts from Halfords are good).

strong e no ug h and it must be mo unted firmly, no t on ly tha t but the form er must be so lidly attached to the fuselage. The re is a ce rtain kit around whi ch has a trike U/C , it also has a tank hatch o n top of the nose . I have seen severa l o f these kits a nd in quite ge ntle landings th e leve rage of the nose leg has pulled the front former out, becau se the forme r was o nly held at the sides and not across the to p . The owner of o ne of th ese mo dels drill ed two W' ho les in the top of the front forme r and 1'-2 and e poxied hardwood dowels between the form e rs to try and tie the front former in place. It worked. Tail d ragger U/Cs need to be position ed with care . Th e idea l locatio n is just under the L.E. of the w ing . Slightly furth er forward is better o n grass but do not be tempted to put it too far forwa rd for th a t is the Extra cosmetic struts dress wa y to gro und loops. M o u n tin g th e U /C ca n be o n the fuselage o r the w ing. Fo r sports mod els / prefer fuselage mo u nt ing be ca use it is ligh te r, it ca n be mo unt ed o n the ply form er o r bo lted to a sma ll p late in t h e bo tto m of th e fuse lage . Th e w ing mou nt ed U /C n e ed s h a rd w o o d bloc ks and ply doublers o n t he rib s w hic h a re he avier but it can loo k be tter a nd d o es all o w for torsi on s pri ng ing . O n low Wi ng trikes it is the o nly o ption in most cases. T he u nd erc arria g e tak es trem endo us loads, just try tw isting a Dura l o r 10 SWG u n d erc ar-

56

riage leg ba ck into sha pe and see the effo rt that has to be used . A lot of the landin g loads have been absor be d by that tw isting but much of the load has been transmitted into th e fuse lage o r wi ng . T hat load has to be spread. Look at the d rawing of the w ing mounted un dercarriage, note how the doublers on the ribs spread the load an d tran smit it to the main spars. Note that the torsion springing will absor b so me of the loa ds but not all. O n th e fuselage mounting not e th at th e mo unt is braced with triangu lar stock to spread the load in the joint and that the stresses are then transmitted to the ply doublers. O ne of the best metho ds of mo unting a dural V/C o n a tra iner is to attach it with e lastic bands, if the landing is too hard the U/C will flick back wards and the mod el will se ttle o n its belly with minim um da mage .

up a normal single leg wire Ur'C:

D esig n i ng M odel A ircraf t

Fig 7.2 - Undercarriage Sketch Page Mount for dural

Wire

u/c

Triangle stock brace . .. iiJl>.,rIZ--~...- - - - -

'A Pl)' 'A6 P l)'

Typical Li/C mount ill using Liteply rib or':-7"--J~~~~ ply doublers both sides of rib

/~7J.,----,,~ ,, ~~ ,~~"'--

Ply doublers

Sheet omlttedfor clarity

Cowlings Cowlings may no t be so importa nt from the struc tura l po int of view as they ca n be classed as terti ary struc ture, but they ca n have a very se rious effec t on the engi ne coo ling if not design ed properly and it is velY frustra ting if you canno t ge t in eas ily on the field to sor t out so me minor pro blem. Cowling the eng ine neatly is ofte n a proble m, man y mo d el s jus t u s e an exposed engi ne w it h ju st th e crankcase cove red by a cowl that is p art of the fuselage.

This engine is normally cotoled, note bow baffles take air from top of cylinders dounuoards, forcing it through tbe fills.

Designing Mo del Aircraf t

This system has man y adva ntages as almos t every vital req uirem en t is covere d. The vital req uirements are, good cooling, good access to the engine and controls and sec urity of attachment. Good cooling needs a flow of a ir d irected ove r the cylinder fins and good outlet for the hot air, this sho uld be bigger than the inlet. Many people think that a nice big radia l cowl is perfect, but in fact the vas t intake and limited outlet makes for poor cooling. The answer is to fit a plate with dummy engi ne in the cow l and this forces air round the cylinder. The plate has no se rio us effec t o n p e rfo rm an ce as p ro ved by my Monocoupe 110, this mod el was 44" span with a cowl that hid the 0.5. 15 FP engine, over 4" in diameter. One might ex pec t the plate blanking the cowl off to act as an airbrake, not a bit of it, that mod el was very, very fast in fligh t just like the full size . Contro l line team racers and speed mod els are operating at the very limits of eng ine perform an ce , good cooling is essential and yet, if you look at the cowls o n these mod els they have tiny air inlets. Of cou rse they are flying very fast so there is a lot of ram air bu t the main reason is that that air is forced throu gh the fins and right round the cylinder which provides the most efficient cooli ng. Eng ines in large cowlings would ben efit from so me ba ffles to g uide th e air flow ro und th e cy linder a nd throu gh the fins . Full size eng ines use these ba ffles, the ba ffling o n a tw in row rad ial like the Bristo l He rcules is so mething to be ho ld.

57

How radial cowls are made.

Tbe inside of a well designed cowl, note shape of duct round cylinder and crankcase cooling scoop ill otber applecbeek. Note also doubled tuire for VIe, just about tbe best uiire VIC going. Tbls is Moolldallce,..

O ne ar e a th at most p eopl e forget about is th e crankcase of an eng ine, this need s co o ling just as much as the cylinde r, it may not sho w up until you ge t a really hot day. Two exa mp les : My biplane design , "Yu ppy Love" had a PA\XI 1.49 d iesel , this engi ne overhe ated badly eve n thou gh the cylinde r was out in the o pe n, I fitted a piece of tube to ac t as a n airscoo p to dire ct ai r over th e crankcase, end of probl em . A friend has an anc ient Junior 60 powered by an old O .S. 30 , it has b e en flyin g re gul arl y fo r 14 yea rs . Sudde nly it started pla ying up . The day was roas ting hot , the e ng ine was well ca rbo ne d up o n the o utside but it

Fig 7.3 - Couiling Sketcb Page

Cooling air floios through gap

Sheet baffle dummy cylinders call be aUacbed

Ideal cotolfor enclosed cylinder F,.ollt

-----

Fit small scoop

P_'_l),_-----,~----___;r ~ Balsa

. :

7

Two laminattons

of~z

balsa

Crankcase needs cooling alrfloto

58

Designing Model A ircraf t

Tbis uncotoled VlVe"gi"e ill a turbulent needs baffles to ensure tbat rear cylinder is properly cooled. had been flown in this state and these cond itio ns before. What was different ? My friend had fitted a spinne r which covered the slot betw een the crankshaft and the co w l, cutting off the flow of air to the crankcase. Instant ove rheating. The reason is that the incoming charge is p ie-heated in the crankcase, this reduces e fficiency but also se nds hot mixture into the cylinder, that mixture is part of the

coo ling o f th e e ngi ne, th e e ng ine ge ts hott er , the un co ol ed cra nkcase ge ts eve n hotter a nd soon th e viciou s circle build s up to the point wh ere the eng ine will not run . O ne of the hard est thin gs to design is a cowl that can be rem oved easily a nd yet will not co me loose in flight. Hardw ood blocks and se lf tapping screw s ca n be one so lution whil e cycle spo kes ca n also be used to good ad vant age. O ne thin g to rem ember , cowls are go ing to hit the gro und first and the y will break regardless and they are tertiar y struc ture a nyway a nd so d o not need to be trem endou sly strong . Having sa id that , the radi al co wl shown in the drawing is remarkabl y tou gh , I had o ne on my first Teq uila Sunrise design , wh en that mod el cras he d it smas he d the carb back whi ch broke the crankcase and bent the carb spigot. The cowl wa s und ama ged . Cow lings ca n also be moulded in plastic in the sa me way as cano p ies are .

Canopies and Cockpits Your spo rts model will look a lot better w ith a smart coc kpit, this ca n be an o pe n coc kpit or a bubble canopy or so me othe r sor t of e nclos ure. This is a styling matter and will probabl y hav e littl e e ffec t o n th e fli ght performance .

Higb drag but good cooling 0" America" Dragon.

D esigning M odel Aircraft

59

I di d sa y 'Pro ba b ly' but m y T ur b u le n t d e s ign ( RClVIW pl ans ) has a big w inds creen , if this go t kn ocked off o r b adl y b roken I w ould have to wind in masses of trim o n the e lev ato r c le v is to mak e th e mod el fly properly and th en take it o u t aga in when the w in dsc ree n wa s replaced. In th e c ase o f a m ould ed ca no py th e imp ort ant thing is to design it to suit w ha t is a va ila b le , as th ere is nothing more frustrating than tryin g to find o ne that fits wh at you have drawn , in dee d yo u mi ght eve n h a ve to mould yo ur own. Tbe simplest form of couii firmly attached, good access and good cooling. Tbis line is from Gerard Peeney "Callisto" design. Th e u se o f vac uu m form in g pl astic a llo w s you to de sign an y sha pe of ca no py, it also allow s you to where that will do the formin g for yo u but this is not as produ ce involv ed and tight fitting cowlings , spa ts and difficult as you might imagine. fairings . The plu g or patt ern can be mad e out of hard balsa if it is only to be used fo r o ne or two parts , ha rder mater ial Most peopl e avo id this tech niqu e because of the need to make pa tte rns . The o ther probl em is findi ng somewould be in orde r for production runs .

Monocoupe bad vel)' small outlet area so front of cotoling is btanked off, model was still vel)' fast

011

all

O.S. FP.15.

60


The finish is imp orta n t , es peci a lly fo r cano p ies as these need to. be crystal clear. Most norm al finishe s react to he at and so th e w ood grain sl{ows .thro ugh o n the ve ry first d raw , this d o e s..n ot mat ter on a thick cowl but a cano py w ill be ruined . I .bave found th at M.D. 'Products Filler will give a ~ perfect finish and the heat does no t raise the grain of the wood , . resu lt, e xce lle nt finis h for at least two formings and the n another application of th e filler will rest or e the fin ish . Th e pattern must be slightly smaller than the finished part as it is inside after the formi ng process. To ge t it out there must be A similar arrangement uiitb no undercuts bu t rathe r outlet bere. a s lig h t taper to ai d remova l of the pattern. Radial cowlings can be rem oved by d rilling a hole in the ce ntre and app lying so me air pressure to this whi ch will blow the pattern out of the cowl. There are home mad e vacuum formers which use the suction from a vacuum clea ner , q uite fran kly these are not much use for a large, thick cowl or cano py; (A) the suction is not strong e no ugh and (B) it is d ifficult to ge t the wh ole of a large shee t of plastic up to the right tem perature. So wh ere do we go from here?

Toumend ring

011

Triuial Pursuit, No problem tottb

Back to sc hool. Most seconda ry schools or upp e r schools and eve n so me middl e schools will have a very good vacuum forming machin e in their Craft Design and Techn ology Depa rtme nt. This w ill be capa ble of forming thick plastic about 10" x 18" wh ich is big eno ug h fo r mos t norm al mod el uses. You might wo nde r how to ge t the use of o ne of these machin es. O f co urse if yo u have ch ild ren there is no probl em at all, if yo u don 't, we ll, there are ways .

Tequila Sunrise cowl s u rviv ed toben all eng ine was turitten off inslde it.

Design ing Model Aircraf t

61

Isaacs FlIIJ' Lycoming engine bas baffles like tbe VlVellgille.

Tiger Motb air inlet is not ve/J' large. Internal bafflillg directs air tbrougb fills and bettoeen cylinders.

All COT departments are sho rt of mon ey, the y are alwa ys grateful for materials which may be scrap or offcuts, timbe r, metal, plastics, goo d card boa rd, electro nic co mpo ne nts, anything to help the pupil s make their projects. If you can supply anything in those lines you will be welcome. Even the offer of goo d old fashion ed mon ey.

To approac h the head of the dep artm ent you sho uld tel ephon e just after th e lunch o r final bell has go ne whe n he is still there but has got rid of the pests, I mean pupil s. You will NOT be popular if you ph on e during classes, the school recepti onist will tell you the bes t time to ph on e.

Tiger Motb air outlets are large, and there is one heated and expanded uolume of air.

62

011

eacb side and another underneath. To allow for tbe

D esign ing Model A ircraf t

Open cockpits ca ll look really smart 011 tbe right model. You may have to wa it for your moul ding to be done, the de partment may not have a full time tech nician w ho is the person w ho will be lum be red wi th the job. Be p atient, re me mber they are do ing yo u a favour and teac hers are very busy people. I know, I use d to be a school tech nician . You may also nee d to sup ply yo ur own ma terials altho ug h sc hools norma lly stock polystyrene in l mm and 2mm thickn esses. They rare ly stock .clear plastic for mo ulding .

Tbe Auster's Gips)' engine is hung 011 tbese bearers. I baue seen beauier bearers 011 models!

Desig n ing Model A ircraf t

Failing the school, you co uld try the Yellow Pages, bu t th ese co mpa nies ma y no t have a sma ll e nough machine and may not want to do a sma ll one -off job . Some of the mo del manu factur ers such as Chart may be w illing to help out. All th is may seem a lot of hassle but a well moulded cowl or canopy can make a mod el stand out from all the others.

Canopies baue to be moulded uibicb call present problems.

63

Chapter 8

Scaling Up Drawings T

he re are tw o reasons for sca ling up d rawings. O ne is to enlarge 3 view d rawin gs to a suitable size and th en design th e struc ture fo r a flying model, the second reason which is not rea lly classed as designing is to enlarge a plan to suit a different size of engi ne , this is mo st co mmo n w ith vintage models and it will be discussed here becau se it is not a straightforward 'Blo w it up two times and let 's build it' process.

Scaling Up for Larger Engines Yo u may co me across a plan that yo u just have to build , fo r exa mp le a full size plan in a magazine that is design ed for sma ll e ngines and micro RIC , an d wh ich yo u would like to buil d for your .40 e ngine . Ge nerally speaking this is a pretty simple job wi th mod ern ph otocopi ers because the enlargeme nt factor is onl y go ing to be abo ut x2 at the most. Many of my own sma ll mod els have been enlarge d by modellers in this wa y and all of them ha ve been highl y delight ed . O ne modeller eve n took my Trivial Pursu it up to 10 foot sp an

and built it in ca rdboard, you can no w buy a kit of the development of this model . . . I can 't co mme nt on the kit becau se they never had the co urtesy to let me have o ne . Most full zoom ph otocopi ers will enlarge over a wide range and definitely up to x2 . All that yo u need to d o is decide o n the en large me nt fac tor. Now, assumi ng tha t you wa nt to go twice size you just cut your plan up into AS size p ieces and ge t it taken up to A3, Bingo , you are there. Most pages are A4 and if yo u take A4 up to A3 that will be 1.41 times, w hich is pretty close to I ll, times enlargeme nt. Anoth er way to look at it is this, decide o n the spa n desired fo r the larger mod el, let's say 55". Now look at the ac tua l spa n of the original, let's say 33". Divide 55 by 33 = 1.66 times. A good pho toco pier will co pe with that. Now it is not just a matter of sticking the bits of the plan togeth er and build ing it becau se your material sizes are go ing to be ridiculous , take a %," sq . spa r enlarge d twice, it will be %" sq w he n a X" sq . spa r is the correct size .

Trioial Pursuit bas bee" scaled up for .25 sized engines and eve" enlarged to 10ft tuingspau and kilted toith modified construction and sligbtly altered tail surfaces.

64

Design ing Model Aircraf t

American Dragon is a 30" span for .Bcc engines but bas been scaled up to 60" spa" andfitted tuitb aileron Modellel' was delighted toitb performance and handling.

New Zealand modellers scaled Tequila Sunrise up.40 and .60 engines, .40 version was modeller's first low wing plane.

D esigning M odel Aircraft

65

Th e best way to deal with spar sizes is to co nside r the cross-sec tion, a X,," sq. spar is 3 x 3 = 9 sq X,,", a X" sq. spa r is 4 x 4 = 16 sq X,," and a X" sq spar is 6 x 6 = 36 sq. X"ths, four times the cross-section of the orig ina l spar but the X" sq. spar is just und er twice the cross -sec tiona l area . Sheet sizes are the sa me, X,," sheet o n the L.E. can remai n the sa me size , the sides sho uld be increased to the size nor mally used for that size of mod el, so X." sheet sides would go up to l{;" but y.!" side wou ld still only go up to l{;th. Th e resu lt of all this is a mod el that sho uld we igh abo ut two to thr e e times the o rig ina l weight for fo ur times the win g area, but if you double all material sizes it will weigh 8 times the o rigin al's we ight for o nly four times the win g area. Which will fly bette r? Which w ill be stronger? Go back and rea d cha pter 4 again. As a pra ctical example, I took my O-Fo urn ier which is a 56" span powered glide r with a Cox Texaco .049 and with three channe l micro R/C and which we ighed about 25 o un ces , I scaled it up o n a photocopi er 1.41 times, I red uce d the result ing 79" span to 73 to make the bes t use o f sta nd a rd wood si zes a nd fin is hed up with a

mod el that weigh ed 52 ounces, tw ice the we ight, twice the area. I fitted a Supe r Custom 12 for power and four cha nnel radio using sma ll but not micro se rvos .

Enlarging 3- Vie w Drawings The ave rage 3 view drawi ng is e ither ~!n d or X"th , the re are o the r sca les and the same pr inci p les apply. Most R/C sca le mo dels are from ~t h sca le up ward s, this means that to e nlarge a 3 view to mo de l size we are look ing at factors of 7 times upward s. Th er e are co mpa n ies who ca n do e nlargements to these fac tors but let me tell you, it is a ve ry expensive process. You co uld do multiple ph otocopi es but aga in, it wo uld be ex pe nsive a nd all photocopie rs distort so at the e nd yo u wo uld have so me rath er non scale shape s. You wo uld also have so me ver y thick lines. Be lieve it o r not, the most accurate wa y is to scale the draw ing up by hand . You can enlarge the 3 view slightly so that you have an exact en largem e nt facto r because it is eas ier to e nlarge 5 times th an it is to e nlarge 5.76 times.

Fig 8.1 -Tbe Effect ofDoubling Matel'ial Sizes

-U6 Sq.

-'h Sq. A ll dimensions doubled Cross -s ection area = 4 times Volum e = 8 times

O-Folll'llier pow ered glider bas been enla rged 1.41 times untb twice tbe toing area aud exactly t wice tbe uieig b t.

66

'A Sq. Lengtb double Cross-section x 1.3 Cross-s ection area = 1. 77 times volu m e = 3.55 times

Proportional dividers are expensioe but s ave hours of tedious work, tbls pair is 3 0 y ears old.

Desig ni ng Model A ircraft

Fig 8.2

-i

Scattng up a 3 View

~Ve,.tiCal

datum

I Horixoutal datum Grid metbod

Diuider 0,. multiplication method

a

+-- - - _.

You decide o n the spa n of it by the spa n of the drawing good. Divide the mod el spa n by the spa n of the 3 view and factor for the 3 view .

the finished model, divide which gives you 5.76. No by 5 and divide the result that gives the enlargeme nt

Tools for the Task Enlarging with a ruler and calculator is not o n, there is an easier way. You use divide rs. Set your d ividers on on e dim ensio n, say the spinn er ring to L.E. on the 3 view . Now using your dividers, step that measur em ent off on the datum line of yo ur drawing. Now tak e the me asurem ent from the da tum to the cho rd line at the L.E. and step that off o n your drawing. All main points are p lotted in th is wa y and the n connected by lines until you have co mpleted the outline of all parts of the aircraft. I ca n hear screa ms of, "It will take a yea r to do that." No it doesn 't, you will be surprised how much yo u can ge t don e in a co uple of hours o nce yo u ge t the hang of it. You can save a lot of time and increase accuracy if you bu y a pair of Prop ort ion al Dividers, these have a sliding joint and a scale and ca n be se t to any e nlarge ment factor from 1 to 10. They are very , very ex pensive, u p to £30 last time I loo ked, bu t they are a tool that saves hours of work and will last for yea rs, mine are still goi ng strong after some 30 yea rs. You can buy chea pe r on es mad e from tw o pieces of wire . . . you ge t exactly what yo u pay fo r in terms of qu ality, accurac y and ease of use .

D esig ning M odel A i rc ra f t

Enlarging We have talked abo ut the basic meth od of enlarging with d ividers and this is pre tty standa rd for the co mplete mod el but there are a few places whe re differe nt techniques are need ed . Plotting the shape of a elliptical w ing tip or fin and rudd er ca n be done by p lottin g seve ral points and joi ning them up with a Fren ch Curve, but you ca n also draw grids over the sha pe and a s imilar grid o n yo ur o wn pla n a nd use thi s to plo t the sha pe.. Man y Ame rica n scale drawings actua lly had these grids printed on the plan, Nye and \V'ylam draw ings for exa mple. Possibly tbe ultimate design ch a lle ng e, drato YOIl,. 01011 3 uieurs from full size plans and pbotos and tben d esign tbe model Tbls is tbe author's Mong Sport control line model.

67

done this for a co uple of mo dels yo u will know w hy I bu ild flat sided models. You can have some of the smaller parts en larged on a photocop ier but do remember th at the lines w ill be ve ry th ick and the re may be d isto rtion. O ne metho d whic h I have not tried is to take a 35mm slide o f the 3 view and p roject it o nto a sheet o f paper. You w ill need a superb camera to ge t the qu ality and yo u will still have very th ick lines . I would suggest that you choose a n ice simple subject with lots of straight lines for your first atte mpt at sca ling up a 3 view draw ing , there w ill still be p lenty to do b u t the shape w ill appear q u ite q u ick ly w h ich is rewarding and ther e is less chance of errors and even these w ill be seen mo re ea sily.

Taylor MOlloplalle, tbe autbor prepared scale 3 uieuis from full siz e construction dratoings and then designed model Witb s ca le homebullts you can follow tbe original construction uiblcb is often just like a model Mos t p lans provide fuselage cross-sections, in fact it isn't mu ch use if these are no t shown. Unfortu nately these sections are rarely jus t where we want them and even worse there are not enough of them , so we have to plot th e intermediate sections, you are goi ng to have fun now . The m etho d I use works well. Plot and d raw the largest cross-section and then the sma llest o n top of it. You w ill have four common lines connecting these se ctions , th e da tum lines. Using a p ro tractor pl ot more lin es fro m the centre outwards. Now d ivid e the lines into the required nu mber of sections between the two exis ting ones and co nnect th ese p o int s an d you w ill have the in termed iat e sectio ns w hich will be yo ur fo rm ers . Cut th em a little oversize just to be o n the safe side. When yo u have

Scale Construction Th ere is no di ffer e nce between a sca le model and a s ports m odel as fa r as constru c tio n goes , th e sca le mo del may have a more complex shape which w ill ca ll for some of th e fancier tec hniques but it is still jus t a model aircraft. The average sca le aircraft has a large r fuse lage in relat io n to the wi ng area th an th e equiva lent size sports mod el , this ca n add weight w hic h is bad news . There is a nat ural te n d ency to wa nt to add d et ail to a sca le model , th is will also add weight an d is even mor e ba d news . Fina lly, the co lour sc heme on a sca le model may tempt you to do a fancy pa int sche me an d thi s w ill add ye t mor e weight which is even worse news. Bear all this in mind and mak e a co nscious effort to save weight and you might keep th e m o del somewhe re w ith in reasonabl e limits but it is diffic ul t. Console yo urse lf wi th the thou ght th at if th er e w e re no weight lim its at all for competitio n sca le mod els, the build ers wou ld still b e moani ng th at th ey co u ldn 't keep wi thin the limit.

Fig 8.3 - Plotting Formers from Two Knoum Sections

1

200 Equal diuisions

1 1

~I

~----'''"--t.I4--=---...I.''--='-------'' '1

F6

F7

F8

F6 Plot points F6 & F9 k'IOWIl F7 & F8 plotted

68

011

radii and connect uittb French curue

Designing Mode l Aircraft

Chapter 9

Flight Testing O U stand in th e workshop a d miring yo u r first ever own design model , you picture it takin g off and flying ro und, you imag ine th e aerobatics and then the approach and greased o n landing. This prob ably takes pla ce at about ten o'clock on a Saturday night and you are go ing to fly her in the morning . You are? Well you have quite a bit of work to do befor e that.

Y

Flight Test Stage One

Th e first th ing to check o n is the CG , don 't tak e "about right " as go od enough, mak e sure that it is where you wanted it and preferably a bit furth er forward . With a small tailplane the CG wants to be well forward , I favour starting with a CG at 25% cho rd for suc h models and go ing back as far as 30% for ae roba tic models with large tailplanes, any further back is askin g for trouble on a first flight. Chec k that the incidences are as you design ed them you sho uld have chec ked this d uring build ing but check again. I always remember th e o ld RAF safet y poster whi ch said "Don't Assum e , CHECK". You will have set up the control th rows , chec k them again and check for co rrec t sense . I like to se t full rates at a little more than I think the model needs and low rates at a little less, I take-off on low rates. If you have ex po ne ntial rates these are even better. Che ck the under carri age for cor rec t track ing and if you have no sewheel stee ring, check that, I know that it is OK, I had a model and I knew that it wa s OK, fortunatel y I checked becau se the steerin g was reversed on the nos e w hee l. Now cha rge the radi o and hope for decent weath er.

On e is always a little nervous before a first flight and this re ally is a first flight. You have a co mpletely new mod el, not onl y has it never flown but no on e has ever flown this particular design. The worst thing you ca n do is to worry ab out thin gs . Rem ember , it is only a mod el and if it is a co nve ntiona l sha pe and it follow s the rules set out in this se ries and has been built according to plan there is no reason why it will not fly. Having sa id that you mu st g ive th e model ev ery chance beca use it ca n be badly ou t of trim , th e CG cou ld be a fraction too far back and the engine might decide to play up at the wron g mom ent so be careful. Don't attempt to fly if the wind is too strong for your likin g, I hav e a w ind meter and I w ill not test fly in w inds of ov er 12 mph even if the model is a p roven design , it is better to wait for two o r three week s than write a new model off. Do a range check o n the field and get someone to double check that the co ntrols are working in the correct dire ction. Run the engine up and hold the no se up , be very, very sure that the engine is go ing to keep going and that it will respond to throttl e properly. Som e less ex perience d mod ellers get another pilot to fly their mod els for the first time. That is up to you but be sure that the pilot that you choose is ex pe rienced . Expe rience is not just a matter of being a good pilot, for test flying it also me an s bei ng able to recogni se a problem instantl y and take the correc t action, for example, if the model goes into a spin and will not recover do es your choice of pilo t kno w wh at to try in an attempt to save the model?

DOli Kicbot, lA scale Polish bomebuilt cruises past, note IIp trim needed for slow flight. A lovely model to fly.

Taylor MOlloplalle needed a dab of'full doum for spill recouery. It takes expensively earned experience to knoto what to do at fiftyfeet.

Final Checkout


69

both . If the ailero ns are to o se nsi tive yo u ca n land a n d a d jus t th e throw s or carry on tes tin g , se nsi tive ail e ron s are not as da ngerous as e leva to rs wh ich c a n ca use s ta ll s, s pi ns o r flicks. If th e c o nt ro ls are sluggish yo u ca n go to high rates and see how th e model p e rforms in that state . Contro l sensitivity is a matter of perso na l tast e . I lik e ve ry powerful ailerons wh ich o t he r cl u b m embe rs ha ve troubl e co pi n g with . On ce yo u are happy that the mod el is in trim Fantasy (lievel' sent for publication] flew like a dream. First model that I ever and flies reasona bly you flew inuerted at 2foot altitude several times, s ho u ld gain some The First Flight he ight , yes, more height , and throttle back , slowly feed in up e levator and watch for any tenden cy to d rop a It is often a good ide a to fly ano the r mod el first, just w ing. Feel out the low speed handling be cause any to ge t re laxe d and feel the co ndi tions, yo u will have vicious habit s will catch yo u out o n lan din g if you are bro ug ht a second model just in case anyw ay. not awa re of them. It is a good ide a to ask the other modelle rs for a slot While you still have plenty of height (reg ain it if neeto yo urse lf for the first flight or at least ask them to fly essa ry) , and are flying slow ly with lots of u p elevator , hit well clea r of the take-off area. If som eon e has a video full rudder, the mod el sho uld drop into a spin, allow it ca me ra as k the m to film the flight for you , this ca n be to turn three times and then centralise the co ntro ls, the useful later. mod el sho uld stop sp inning , if it do es no t, give a dab of Now , go throu gh all your fue lling and start up procefull down elevator w hich should stop the spin, opposite dures calmly and carefully, have the nose of the model rudde r may also b e ne ed ed. Yo u ca n also open th e raised to che ck aga in for correct running, throttl e back and have the mod el placed on the strip . Check that the throttle , if none of these stop the sp in you are in deep co ntrols move . trouble, but I have yet to find a model that did not sto p spinning with down elevator . \\then yo u are satis fied that all is well, look aro un d, o pe n the throttle and take-off. Don 't try and haul the On ce the sp in has stoppe d be ca reful an d pull out as ge ntly as the rem aining height w ill allow becau se if the mod el off the ground too so on, hold it down and ge t as model was relu ctant to recover from the spi n there is a mu ch speed as possible before easi ng in up eleva tor. Be gent le, at this stage you don 't kno w how the mod el will risk that it could go back into the spin as you pull out. react. Climb to a good he ight mak ing any tur ns slow ly. Th e cures for a model that do es not want to sto p You will alrea dy have so me ide a as to how se ns itive spinning are, CG forw ard , add washo ut o r, to be mor e the mod el is to the co ntrols, if it is ve ry se nsitive to eledrastic, a bigger fin an d rudd er. vato r it would pay to throttl e back and land and eithe r If yo ur nerv es will stan d it, yo u ca n now try loops move the CG fo rward or reduce the eleva tor throw o r and rolls and then landings.

George Strobm's e....- perimental model uras fine at bigl» speed but deadly as SOOIl as it was throttled back,'

70

First Tequila Sunrise flew well but bad poor spill recovelJ' due to POOl' cboice of totng section, sbape and uiing loading.


The Test Programme The elatio n of having made the first fligh t w ith your firs t d e s ign wil l h a ve yo u ' wa lking aro und about six inches a b o ve t h e ro u nd a nd you will wa nt to fly the mod el agai n an d again . The first thing to do is exa mine the model , jus t as yo u wo uld a ny new mode l b ut wit h even mo re care as yo u w ill wa nt to be certain th a t th e s tructure th at yo u design e d is u p to th e stresses an d strai ns imposed on it. In subse que nt flight s yo u ca n fl y th rou g h ae roba tics bu t w hile you are en joy ing yo urse lf yo u sho uld no te if th e mo de l rolls with eq ua l s pee d in bo th d ire ctions , if loops and bun ts a re equa l, ho w mu ch down trim is need ed in inverted fligh t. Now you ca n eva luate your mod el , Is it as ae robatic as yo u h a d hop ed , is it un stable , do a ll th e contro ls ..,h a ve ro ug h ly t he sa me powe r? In s ho r t, a re yo u h ap p y wit h the model, Ge t other p eopl e to fly it and see wha t they say. You may no t ag ree wi th them but yo u are getting feed back which ca n be help ful for your n ext design . You may wa nt to try so me modifications , bigger co ntro l surfaces or perhap s cutting them down , maybe you will try fitting a bigger eng ine . T he most likely thing that you will do is just canyon flying the mo de l witho ut any . major changes bu t with the knowledge of things th at yo u wi ll change on yo ur next d es ign , becau se the best way to le a rn to d es ign is b y practical experience . T he mo re th at you fly your new design the


AIlalyse what toent wrollg

....oJlI,.. . \LI, ·

...•u. . ... ..sIf.,liJI• .• .

. P' -

....... ........

..

~

~

.. .

"tK)to . ...... ·

Second uersion Of Tequila Sunrise was excellent and has been publisbed here and ill the USA and enlarged with success ill New Zealand.

71

One scale lip tb a t didn't work, Aztec Two -step scaled up to 45" span for two . 12s, came out grossly overuieigbt at 600111lCeS, tip stalled and fliclsed as SOO1l as it was clear of tbe ground.

more yo u will learn abo ut it from the aerodyna mic and stru ctural as pects. On e word of warning, never get complace nt. No matter how man y mod els you design ea ch mod el is differe nt and you can still have failure s. \':Ihen yo u do have a failure, anal yse what went wron g becau se you can learn a great deal from your failures. Failures can be divided into three main gro ups, aerodynami c, struc tura l or pilot e rror, we will loo k at each of these in turn .

Pilot Error Thi s is th e most co nte ntio us g ro up because if the model is mar ginal the pilot will usuall y blam e the mod el and d raw ing th e line between a situatio n that a pilot could have save d and one he co uld not ca n ca use bad feelin g between peop le . This is the reas on tha t I always test fly my own mod els and hat e being aske d to test fly othe r people 's mod els. It mu st be said that a we ll designed mod e l, pro perly se t-u p and flown by an ex pe rienced pil ot sho uld be sa fe, so o ne of the oth er failure groups will almost ce rtainl y be a contributing factor in any failure .

Aerodynamic Failure Most e rro rs in aerod ynam ic design w ill s how up immed iat el y after take- off. Ask yo urs el f exac tly w ha t

72

yo ur model did . Did it stall, did it flick roll or did it just refus e to lift off? Stalling or flick rolling on tak e-off ind ica tes a CG that is too far ba ck o r that th e model had not rea ch ed flyin g sp eed before be ing hau led off th e ground and this , in turn ca n be caused by too high a w ing load ing . Aerod yn ami c failure ca n b e ca us e d b y w a r ps, incorre ct an gles , wrong CG, excessive wing loading, po or cho ice of wing sectio n or comb ina tio ns of these . If this boo k ha s b e en fo llowed a n d th e m od e l ch ecke d ca re fu lly th en a ny e rro rs s h o u ld b e of a minor nature a nd failures sho uld not be a catastro phe .

Structural Failure Th ese ca n happen and w he n they happen at height th ere is nothing that yo u can do ex ce pt stan d ba ck a nd enj oy the cras h . Struc tur al failures are rar el y caused by major design faults , it is normall y th e sma lle r things lik e a wing atta ch me nt that has not be en rei nfo rced sufficie ntly o r th e use of inc o rrec t grades of wo od . Failures after a heavy landing o r minor crash will often show up wea k po ints in the co ns truc tio n , ver y often ca used by that sudde n cha nge in sectio n . Th e impo rta nt thing is to learn from th ese things and go ba ck to the dra wing board a nd d esign a new mod el o r modify the last on e .


Chapter 10

Selling Your Plan T

here are three monthly R/ C magazines and ab out five bi-monthly magazines in Britain which publish at least one plan in eac h issue, most of them will publi sh two plans. This adds up to a lot of plans every year. Som e of the magazines will want o nly du cted fan or electric powered mod els, or perhaps glide rs but they still want plans. Where do they ge t the se plans? From mod ellers like you, people who have design ed a mod el and se nt it in. Do they bu y every plan su bmitte d? No, but if yo u se nd the right design , prop erly prepared and pre sent ed you ha ve a ver y go od cha nce that yo ur pl an will be publi shed.

.W hat Sort of Design just abo ut an y typ e, but it sho uld have so mething to

attract the editor and potential build ers. It ca n look beautiful, it can be ultra che ap to build or velY qui ck to build. Scale mod els are always popular bu t do call for ca reful se lec tio n of a subject , p ro to types that hav e be en don e to death are less likely to be accepted but equally the very obscure typ e for whi ch there is no docum ent ation would ne ed so me th ing s pecia l. Hom e builts are always popular. The one important factor is that it must fly well , if the perform an ce of your mod el is poor o r it has very nasty hab its , don 't se nd it in becau se it will do nothing for yo ur reputation and co uld affect the sale of futur e plan s. My first Teq uila Sunrise was pretty good but it would flick into a spin at the slightest provocation and it need-

Blue Movie 2 (RCMlV Plans Seruice], is a best seller because it is velJI cbeap and quick to build and almost indestructible.

Designing Model A ircraf t

ed a lot of height to ge t it o ut, pret ty soon it came to grief be cause of that. I design ed a new model whi ch was fine and has been publi shed here and in the USA. Some time s a design may have a characteristic that is dan ge rou s but un avoidabl e . My Waco PG-2 (RCMW Plan ) pow ered glide r has two e ng ines and no throttles, if o ne e ng ine cuts you have to be very careful not to let it lose speed. In this case it is acceptable becau se it is a co mmo n fault with twins and yo u put that warn ing in the text. Th e Waco is a good subject, easy to build , go od flying perform an ce and can be flown as a glide r or co nverted to electric power. Blue Movie wa s very qui ck to build , chea p , alm ost indestruc tible and flew su perbly, the plan is a best se ller.

First Approach You have a nice mod el whi ch you think might be of interest to a n editor . Choose o ne ed itor and se nd him a co up le of ph otos and a description of yo ur model and an SAE. Do not se nd the sa me thing to mor e than o ne ed itor at a time because if they all wa nt it . . . yo u have mad e yo urse lf unpopular with most of them and that is not go ing to help you. If the ed itor is inter ested you have to be read y to se nd him the plan s etc.

The Plan The plan need s to be neat and clear, it must sho w all the information regarding wood sizes and any instruction s, all parts must be sho wn and identifi ed . Ryan B-5 Brougbam plans sold ill tbe USA, aircraft is commercial uersiou ofRyan NYP.

73

FILL THE FRAME. Note that all tbe static pictures

ill this book bave tbe f rame fllled bJI tbe model, II0t like tbis o ne.

Plying sbots will prove that YOllr design flies, tbey

You must reme mber that man y peopl e will bu y the plan with out eve r read ing the article in the magazine so all the inform ation need ed to build the mod el must be on the plan . Unless yo u are a skilled d raug htsman yo ur plan will have to be retrace d to suit the standa rd and form at used by the magazine. It will have to be traced onto Mylar film anyw ay for printing, the pe rson who traces it canno t guess at inform ation that is not includ ed . The plan that yo u send in can be don e in penci l but I prefe r to trace the plan and send in a print , this way you have not lost much if it ge ts lost in the pos t.

A roll of tracing pa per is not too ex pe ns ive and drawing pen s can be ve ry chea p for the disp osabl e typ es, better ones are ex pe ns ive but well worth it o nce yo u ge t into designin g. Some of my ea rly p lans were traced usin g th e old sp ring bow pe ns , wh ich co uld be se t to su it th e lin e be ing dra wn . Now I use 4 pens, .25mm for fine detail and grain marking, .5mm for most of the draw ing, .7mm for main o utlines and l mm for the border of the plan . This is purely for my own satisfaction, as the draw ing w ill be retr aced an yw ay I su gge st th at yo u se ttle for .5mm whi ch is an ideal size of pen .

do II0t bave to be perfect.

Till)' Shrike Commander is a bail)' little beast b u t great filii and cheap, text ioarns of pitfalls.

74

D esigning M odel A i rc raf t

Tbis is better, m)' neighbour's drive gets usedfor most of my static sbots. Letterin g can be don e free hand so lon g as it is legible, you can use a stencil but that is vely slow . Ju st rem ember, the better your plan is, the eas ier it is for the tracer and the less likely he is to mak e mistakes.

Text

Th e pictures must be properly ex posed and in focus, this is not hard to achieve with the average co mpact ca me ra. Wh at is important is to ge t a good ba ckground , it should be as free of any clutt er as possible and sho uld co ntrast with the subject. Con stru ction photos sho uld be tak en ag ai nst a dark background and th e co mp le te d mod el taken aga inst one that co ntras ts w ith the TONE of the mod el. The photo may be printed in black and white so yo u should think in term s of tone, not co lour. I use a neighbour's tarm ac drive for most of my ph otos whil e co ns truc tion photos are tak en w ith the mod el on a dark coloured bedspread. Th e oth er rule for taking p ictures is: "Fill the frame", there is nothing more usele ss than a tiny figur e holding so mething in a vast ex panse of co untryside or a minute mod el with a caption telling you to look at so me fine detail.

Your text mu st be clear and cover all vital points. It sho uld star t with a short int rodu cti on and cover th e con stru ction in a logical way. It sho uld then co ver the flyin g side of the mod el. Get so meo ne to re ad th rou gh yo u r text to se e if they can spot anything that need s cha nging. You kno w wh at yo u mean but w ill other peopl e . The text should be typ ed with double spa cing on on e side of A4 pap er , it should be print ed quite dark if possible so that a co mpute r sca nne r ca n read it. If yo u can't typ e it up you can ofte n find a tam e typi st at work who will Vel/e MOllocollpe in flight.for really good in fligbt photos you need a 300m", do it for a b o x o f telephoto lens, a good pilot m' cameramall, and luck. cho co lates or there are secretarial se rvices who w ill d o it for a s ma ll fee .

Photos Yo u will n e ed a g oo d s e t of ph ot os , ordinary gl o ss co lo ur prints 6" x 4" are quit e acce ptable, alw ays se nd plent y to allow the edi to r to se lect th e one s he wants.


75

\

\

\

i

\ This is not a good background as it is d is tracting.

Where Next?

How lIIall)' th ing s are wrollg with this picture? Noui you k ,IOW what to avoid.

Flying sho ts are helpful b ut very difficult to tak e , if yo u d o not ha ve a go od , powerful tel eph oto lens, mayb e a friend ca n help out. Edi tor's do like to kn ow th at a mo de l does fly. \X/hen I sent my first plan to the USA I se nt an 8mm cine film of the mod el in night (lon g before the da ys of video) . . . it wor ked .

Sending the Material 111 Pack everythi ng up in a good , sto ut, ca rd ba ck ed en velop e. The print ca n be se nt rolled but I usually just put th em in th e e nve lo pe . Se nd it by regis tered o r recorded delivery a nd the n sit back a nd wait. Very often an ed itor will let yo u kno w that he has receiv ed it but ed itors are bu sy peop le so you may not he ar for so me time . If yo u have heard nothing after abo ut a month yo u co uld telephon e to make sure that it has arrived . Once yo ur plan has been accepted for publicati on forget ab out it. You may be lucky and see it publi shed in a matte r of months but you co uld have to wait yea rs. Do not keep ringin g up and as king because it will not ende ar you to the e d itor and yo u ca n't afford to offend them , at lea st, not if yo u want to se ll them more plan s. You can remind them after a yea r has passed but do not over do thin gs, just se nd in more plan s. \'(Th en yo ur pl an is publi sh ed yo u w ill hav e th e ex cite me nt of seeing yo ur wor k in print and sho rtly after that you will ge t a nice at che que in the post.

76

With o ne p lan awaitrng publ ication the urge to design mor e models will grip yo u, and wh y not , there is a big ma rket out there . Once yo u have had two o r three plans publish ed yo u can co ns ide r th e American market. If yo u lo o k through American plan s catalog ues you will find se veral plans from British design ers that ha ve appeared in magazines over here . The reason for this is that there is ver y little trad e in ma gazines across the Atlanti c and even less in the way of plan s, so it is worthwhile for the Amer icans to p ublish these p lans. Yo u ca n offer a plan to an Ame rican magazin e that ha s been publish ed here but yo u sho uld talk to the ed itor of the British ma gazine first. R/ C Mod e l Wo rld buys the copyright but will normally give yo u pe rmission to se nd the plan to th e USA, oth e r ma ga zin es onl y buy First British Ser ial Rights so yo u still own the co py right, but it is o nly polite to tell the edito r an yw ay . You must also tell th e Ameri can editor if the plan has been publish ed o ver here , so me Amer ican magazines ma y onl y want a plan o n an ex clus ive ba sis. Any p lan that yo u offer to a n Ameri can ma gazine ha s to have so me thing a little special ab out it, after all, the y ha ve a lot of design ers ove r there . Slightl y unusu al sca le subjects go down well, out of the rut sports mod e ls or even unorthodox, providing that yo u can p rove that it flies we ll. Get hold of so me American maga zin es a nd study their style an d pr esentation. Make your ap proac h to the ed itor and se e ho w yo u ge t o n.

Looking B a ck Th is book has tried to sh ow yo u the simple, un complicated wa y of designing mod e ls, a subject which ca n, and does, fill bo ok s. You now kn ow the basic mech ani cs of design and e no ug h of the very Simple aerody na mics and struc tures to design successful models. The re are bo ok s wh ich will te ach yo u more about aerodynam ics if you feel the need , there are bo ok s o n de signing scale mod e ls as we ll as sports models . Add these to your co llectio n as they will always be useful referen ce sources but the mo st impor tant thin g that yo u ne ed now is ex pe rience and yo u can o nly ge t that by design ing mod els.

Designing M od el A ircra f t

You need a good pilot and guts to get this close, eoen tuitb a 300mm lens.

Moolldmlcer trick sbot. Tbe model is in fligbt but bow did I get it? Simple, take tbe moon at night tuitb a 500mm mirrorlens, remind tbe film and reload Oil exactly tbe same mark and take )'our pictures as normal; toitb tbe moon ill a clear area

Tbis is m)' design for a scale Rnssiau powered glider, tbe MAK 15. It is C02 potoered free fltgbt aud pubtisbed b)' SAMS, would be a tuinner as all

of shy.

RIC design.

Filling the frame and pill sbmp,f/)'illg pictures like this are bard to get. Top and bottom pictures are Gemini model kits.


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Useful Addresses Aviation Bookshop 656 Holloway Roa d London N193PO Tel: (0171) 272 3630 Ae rofoil sbeets, modelling books and magaz ines, full size aviation books a nd magazines. Also second ba nd books and magazines. Get tbeir catalogue. Transair Pilot Shop 50a Camb ridge Street Lon d on SW1V 4Q Q Or : Fairoaks Airport Surrey Tel:(01276) 858533 for orders and catalogues. Squa re protractor s, books, Videos, av iation novelties a nd fo r tbe full size pilot, all navigation aids, equipme nt, sim ulators etc.

Balsacraft (International) Ltd Norwich Road Industrial Estate \Vatt on Norfolk IP25 60 R Top qua lity toood selected to yo ur specification at tbe best prices, also large range 0/ accessories. Real ualue f or money , one stop mail order, 3 fi rst class stamps/or catalogue.

78

Designing Model Ai rcraft

Notes

Notes

DESIGNING MODEL AIRCRAFT This book covers everything that you need to know about designing models, starting with what drawing instruments you need and where to buy your drawing paper in Chapter One. Everything that you need to know about aerodynamics and structural design is covered in simple terms without any unnecessary complications. Each stage of preparing a design is clearly laid out with suggestions for different types of construction. Methods for developing the more complex shapes of scale models are shown. There are chapters on scaling plans up, flight testing your new design and how to go about getting your plan accepted by a magazine. There are no theories in this book. Every technique and principle has been and is used by the author on a regular basis when designing his own models.

r\ Truplct Puhlicat ion

Designing Model Aircraft

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