Hand Launch Glider Airfoils

Hand launch glider airfoils HLGs are simple flying machines; it’s the flying indoor and the digital stopwatch that complicates things. Those factors separate the theorist from the flyers….and flyers….and the bullshit from the real world. I have spent ! years years in the the real world of the the Hand Launch Launch Glider Glider.. I have won won both indoor and outdoor hand launch at the "eal #ationals$ I have held both indoor and outdoor records many$ many times. I held both indoor and outdoor records for about %! years after I &uit flying hand launch. 'y two sons won the #ationals and still hold (unior  and senior hand launch records they set in the %)*!s. 'y point in all that is; you never  once beat me in any HLG contest. +nly those who beat me can prove I’m wrong. ,d -obot beat me once$ ic/ 0eterson beat me twice$ Lee Hines beat me once$ and 1en Happersett beat me once. ,ach time I learned more than they did$ and came bac/ better   prepared$ so can you. In the real world world of serious serious Indoor Hand Launch Glider Glider competition competition$$  Weight Weight is ight ht is th thee si sing ngle le mo most st im impo port rtan antt fac facto torr det determ ermin inin ing g gl glid ider  er  where where it’s it’s at. at. Weig  performance. 2e 2eight ight is also the second and third most important factors. 2hen a glider glider is over over weight weight there is nothin nothing g that that will will save save it. it. Indoors Indoors there is no no magic strong enough to rescue a poorly built glider. These new digital stopwatches are not impressed by manure$ no matter how deep it’s piled. The ideal weight weight for a conventional hand launch launch glider is about 1 oz per 100- sq. in. of wing area. 2indy 2indy weather outdoor gliders are (ust barely competitive at %. o3. per %!!4s&. in. of wing loading. 5nything heavier and you you might (ust as well be throwing golf balls at the thermals.

+nly after you have convinced your self that weight is by far the most important important factor  in small model performance$ only then can you turn your attention to the minor details. rag rag is the  fourth most important thing to a HLG. rag is a relatively minor item compared to weight. Thou shall never add any drag reduction details that increase flying weight.

6nless of course you need to increase increase weight for a good reason. The only good reason I /now of is to get the model higher. Indoors you should fly a model that uses all of the availab available le ceili ceiling ng height height.. 7ut remember remember that that heavier heavier models models have have a difficult   tas/ in slowing down and ma/ing that first turn at the top. 7etter to be a bit too light rather than a bit too heavy.

!"#$ increases roughly I.  In the climb portion of the flight !"#$ on

the square of the frontal area of the model.

II. In the glide portion of the flight !"#$ increases roughly on the cube of the weight. of the model  These are two very simplified rules that do contain much of our low speed and low "eynolds number /nowledge. 8ou will not win consistantly unless you follow these rules. The fifth most important thing to a HLG is the wing. The wing is the only part of a HLG that actually contributes anything to the end result. ,verything else is e9traneous matter  along for the ride. The stabili3er is (ust that$ it stabili3es the wing and /eeps it operating at the optimum angle of attac/. The rudder provides directional stability in the climb phase$ nothing else. The fuselage’s sole purpose is to hold everything together in their correct positions. The correct positions are as follows: High wing$ Low nose weight$ and Low stabili3er$ and also of course a very low moment of inertia. The important thing to remember is that although the wing is the only functional part of the HLG$ it is number five on the list of  important factors and it’s still (ust a minor factor compared to flying weight. In wing design the most important factors are wing planform$ wing section$ airflow turbulation$ and dihedral. ihedral must include polyhedral brea/ locations and amounts of dihedral$ in relation to the lateral areas and vertical locations of such areas of the rudder and of the fuselage. There is no magic gee haw you can glue on and suddenly become a contest winner. There simply is no magic.

In ! years I have built well over %!!! gliders$ no more than  or < were identical. I test in the real world. I /eep records. I /eep that which is of value and discard that which is of no value. The wing section I have currently settled on is a simple flat plate with an e9panding logarithmic spiral curve on the top surface to delay the stall. I do not e9pect the top surface of the wing to produce much useable lift. In the world of the HLG, about 90% of the lift is produced on the bottom surface of the wing.  2ith a properly undercambered section$ that number is perhaps )=. The main reason for a curve on the top surface of the wing is to control drag at the very high angle of attac/ that is re&uired to fly slow and rac/ up glide time. 8our wing produces lift by displacing air downward$ e&ual to the weight of the glider. The lighter the model$ the lower the re&uired airspeed$ the lower the airspeed$ the lower the drag$ and the lower  the drag the lower the sin/ rate and the better the flight times. #o /idding$ there is no magic here. All wings with their associated airfoils produce eactl! the same amount of lift. 5ll wings produce lift e9actly e&ual to the weight of the model$ its (ust that some do it with less drag.

5 typical flat bottom airfoil produces only about >4= of the lift on the top surface. 5nother >4= is produced by down wash behind the trailing edge$ much of this down wash is a result of the curve on the top surface. 6ndercamber will increase lift drastically  by increasing the down wash drastically$ however undercamber increases drag ev en more drastically at low angles and high speeds. "emember that a properly launched HLG leaves your hand at a speed of over %!! '0H. "eally. If yours don?t$ you are not throwing hard enough We ta%e e&ception to 'ernoulli(s law for hand launch gliders. 7ernoulli’s law really does not have much to do with wing sections. He was a %*th  century -cientists who  published a boo/ in which he proved that the sum of static and dynamic pressures over a streamline shape always remain constant.

@or years our teachers$ none of whom ever read the boo/ and didn’t understand the  physics$ have been teaching their un&uestioning students that the air over the top of the wing has to travel further that the air over the bottom of the wing. Therefore the air over  the top of the wing has a lower static pressure and produces all the lift we need. 'y teachers also in(ected the well /now fact that the air molecules that separated at the leading edge had to really speed up in order to re(oin the same air molecules again at the trailing edge. This is all sort of true under certain conditions. The sad news was that I for one believed them$ …. ,ven when I /new that there was something wrong and things did not add up correctly. The air molecules that separate at the leading edge of the section never ever meet again at the trailing edge. There are at least A! good reasons for this but one ma(or reason is the span ward flow of those air molecules on the top of the wing. The very best reason for them not meeting again at the trailing edge is simply that there is no good reason why they should. Wind Tunnel data collected within a confined airflow type tunnel is almost totally useless.

 #ot totally useless$ but almost. Hand launch gliders fly at a very low "eynolds number  and at an angle of attac/ that is unbelievable to the old school of aerodynamics. In the climb portion HLGs operate at near a B% degree angle and in the glide portion we float along in the range of "#$ to "# degrees.  5nd thats for real. 2ing planform does matter in that it plays a ma(or role in controlling span ward airflow. The wing platform developed by on @oote in the late %)>!s seems to be about the best. This is the platform used on his old timer gas model$ the 2esterner and all the  )and *aunch $liders that have ever e&ceeded 1+,0 in dead air. on @oote claimed that this wing platform resulted in a model that would glide slower and thermal better. I suspect the Cthermal betterD part come about because he also reduced rudder area about != at the same time he went to the new wing platform. Incidentally$ that beautiful %E>

 by AE> elliptical platform that loo/s so great on a -pitfire is (ust about the worst you can choose for any glider type model. 2hat I have decided is that the only part of the wing that is really operating up to my e9pectations is the portion with the trailing edge perpendicular to the intended airflow. The trailing edge must be perpendicular to the airflow to minimi3e the spanwise flow of  the air that must produce the lift we need. 2ing planform is another sub(ect altogether  and is dependent up on the intended use and re&uired angle of attac/. The section I use is (ust a flat plate with some curve on top to delay the stall. igure 1 is the basic section. The top surface is a = thic/ e9panding logarithmic spiral curve with a very sharp leading edge. #ote there is no 0hillips entry and no leading edge radius. The = may seem thin but it is a fact of life that as you fly at lower and lower "eynolds numbers you must also reduce wing loading and wing section thic/ness. #o room for the  bull. The section shown in figure , is the actual section I use under most conditions. +ver  the years I have shifted the high point of the section from %<= to as far aft as F!= of the chord. I thin/ the A= location is best for most conditions. The true e9panding logarithmic spiral has its high point at >!.!=; I fudge this location by treating the distance from the leading edge to the desired high point as the total wing chord. 8ou can do that with this curve and it wor/s out perfectly every time.

The most noticeable part of this section is also one of the least important details of the section. @rom the high point to the trailing edge is a straight line. This has no measurable affect on the glide times when tested with identical gliders at identical weights. The big reason for doing this is that it eliminates wood from the wing   and as a side effect$ this section provides a better recovery at the top. 2e no longer have to waste altitude with the classic HLG "oll +ut at the top; we can now use a simple slip out  recovery at the very top with no loss of altitude. The wood that is eliminated from the wing is very substantial$ far in e9cess of the material needed for the entire tail assembly. The model weighs less and flys better. igure  shows a similar section that does not glide very well. I thin/ the reason is the lac/ of sufficient curvature right behind the leading edge. This is where many$ many would be glider flyers have gone wrong and ended up with good flying gliders with no hang time and poor flight times. #ow indulge me for a moment. Ta/e a sharp pencil and straight edge and draw a line from the leading edge to the high point of the section. Go ahead and do this on figures A and > and note the differences. @igure > does not wor/$ never seems to get up on the step.

That straight line you (ust drew measures out to be (ust about a %F4degree angle of attac/  to the bottom of the wing. 2hen we get the wing to operate at a %F4degree angle of attac/  we will have a perfectly good airfoil from the leading edge to the high point of our wing

section. 5nd$ a couple of degrees either way won’t hurt much. 2hat happens behind the high point doesn’t seem to matter much. &ur onl! ob'ecti(e is to displace air e)ual to the weight of the glider and with the least possible drag.

5 flat plate at %A to %< degrees angle of attac/ pushes a lot of air downward and creates a lot of drag on the top surface$ I thin/ the curvature on the top surface does alleviate the drag problem. 'y own totally untested$ unproven theory is that low "eynolds number wing sections$ all tend to retain most of their laminar flow characteristics until the air is past the trailing edge of the wing. Laminar flow sections sure are beautiful in the wind tunnel. I am certain that the sharp leading edge starts the turbulation on the top surface of any airfoil at a high angle of attac/. +n these very low "eynolds number sections I suspect that a flowing burble of air forms on the top surface and has the ability to change si3e$ shape$ and location to delay the stall point. To ta/e advantage of this section you also need to /e ep your wing tips light and thin li/e a trailing edge$ /eep your leading edges light and sharp. 2ith any flying model you always must /eep the e9tremities very light to reduce the moment of intia. 8our HLGs should be made to fly$ not to survive a crash. The crashes are you own fault. 2hen a model survives a crash you should ree9amine you design. 5re you building airplanes or tan/s   #ow that you suspect that I may be a bit of a nut who probably believes the world is also flat$ let me say that I have traveled around the world$ and I have seen the curvature of the earth from F$!!! feet. The world may not be perfectly round but it’s not flat either. There are no sure things and you have no good reason to assume that I’m right about everything. 'ut let the digital stopwatch be the final /udge

*urt +te(ens odel -esearch Labs /#/9,  revised !<4>!4!A