AIRCRAFT STRUCTURE-I (ASEG 331)
Main structural Components and Teir Functions Conventional aircraft usually consist of fuselage, wings and tail plane. The basic functions of an aircraft's aircraft's structure are to transmit transmit and resist resist the applied loads; loads; to provide an aerodynamic shape and to protect passengers, payload, systems, systems, etc. from the environmental environmental conditions encountered in ight.
!in"# • Spars • Stringers • Ribs • Skin
Main structural Components and Teir Functions Conventional aircraft usually consist of fuselage, wings and tail plane. The basic functions of an aircraft's aircraft's structure are to transmit transmit and resist resist the applied loads; loads; to provide an aerodynamic shape and to protect passengers, payload, systems, systems, etc. from the environmental environmental conditions encountered in ight.
!in"# • Spars • Stringers • Ribs • Skin
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S!"R# • $ongitudinal member in the %ing. • &enerally &ene rally %ing having T%o spars called ront ront spar (located at )*+ of %ing chord from leading edge and Rear spar (located at -+ of %ing chord from the leading edge. • &enerally Spar having / cross0section, because / section having ma1imum moment of inertia, hence 2ighest strength, for the same %eight. • Spar %ebs takes Torsional load (i.e. shear stresses and spar anges takes bending loads (i.e. bending stresses.
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Stringer: • •
3sed for 4ending loads. &enerally having 5, $, T, channal and small %ings having rectangular cross0sections because of easy attachment to the skin and space and %eight advantage.
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RIBS: •
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The dimensions of ribs are governed by their span0%ise location in the %ing (i.e. "irfoil shape and by the loads they are re6uired to support. 3sed for maintain the "irfoil shape through out the %ing section. They also act %ith the skin in resisting the distributed aerodynamic pressure loads. They distribute concentration loads (e.g. undercarriage and additional %ing store loads into the structure.
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Skin: • •
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The outer cover of the %ing structure is skin. The primary function of the %ing skin is to form an impermeable surface for supporting the aerodynamic pressure distribution from %hich the lifting capacity of the %ing is desired. Skin is e7cient for resisting shear and tensile loads. Skin buckles under comparatively lo% compressive loads. Stringers are attached to the skin and ribs thereby dividing the skin into panels and increasing the buckling stresses .
FUSELAGE The fuselage of any aircraft has T89 main functions# :. Carries the payload# passenger cargo. <. /t forms the main structural links in the complete assembly that is the aircraft. The fuselage often carries the engines and undercarriage. /t also responsible for providing a safe environment so that the cre% and passenger can survive. The fuselage is considered to be made in three sections# •. The nose section. •. The centre section. •. The aft section. The three sections carries di=erent loads depending on the role of the aircraft.
There are mainly three types f fuselage stru!tures:
:. TR3SS T>!?# • This type of structure is still in use in many light%eight aircraft using %elded steel tube trusses. • " bo1 truss fuselage structure can also be built out of %ood@often covered %ith ply%ood.
"nt#$. %. &n!'ue stru!ture: it is possible to make a skin strong enough to carry all the loads %ithout the need for any supporting frame%ork. Consists of0 • Skin. • ormers. • 4ulkheads.
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(. Simi mn!'ue stru!ture: /n this fuselage structure the skin is used to avoid buckling, it is common for the stress skin to carry about half of the total load carried by the skin and longerons together. the typical fuselage structure consists of series of hoops, or frames at intervals along the skin, %hich gives the fuselage its cross0sectional shape, connected by longerons that run the length of the fuselage. mainly consists of0 • • •
Skin 4ulkheadsA ormers (frames $ongerons#
TAIL )LA*ES The tail0plane provides stability in !itch >a%. •
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$arge "ircraft having cross0section same as %ing structure. Small "ircraft having solid section.
Imprtan!e f stru!tural weight •
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The structure of an airplane must %ithstand the applied aerodynamic load and interior loads not only for the normal ight but also for e1treme conditions may be encountered very rarely. The essential character of an aircraft structure is light %eight, because %eight plays such an important role in the performance and economics of an airplane. The importance of empty %eight should be clear from the limitations placed on ma1imum takeo= %eight by the available run%ay. " pound more structural %eight is a pound less of payload. The speciBc range is inversely proportional to the airplane %eight, so in increase in structural %eight raises the fuel consumption and the fuel cost.
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The Brst cost of the airplane is generally found to be proportional to the empty %eight. • /f the payload and range cannot be reduced, a higher structural %eight re6uires a larger engine to meet the takeo= and landing re6uirement, thereby raising the structural %eight even further. or all these reason, the aircraft structural design has al%ays sought to meet the load re6uirements %ith a least possible %eight. The potentially e=ect of an aircraft structural failure means that the structure must be designed for long life either %ith safe life or %ith fail safe #esign . Safe life: safe life means that the stresses in a components are so lo% that fatigue failure is not possible over the life of the airplane. •
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Fail safe:+ fail safe means that the structure has alternate loads paths so that no single failure %ill be e=ected to the aircraft. This can be achieved by designing so that no one component carries a large part of the load. Therefore, if one part fails, the reminder of the structure can still carry most of the ma1imum load.
General la#s n Air!raft 4efore the structural design of an airplane can be made, the e1ternal loads acting on the airplane in ight, landing and takeo= conditions must be kno%n. Limit la#: limit loads are the ma1imum loads anticipated on the airplane during its life time. The airplane structure shall be capable of supporting the limit loads %ithout su=ering detrimental permanent deformations. Ultimate r #esign la#s: 3ltimate or design loads are e6ual to the limit load multiplied by a factor of safety. /n general the overall factor of safety is :.. •
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The board general category of e1ternal loads on conventional aircraft can be broken do%n into such classiBcations as follo%s# Air la#s: – –
ue to "irplane Daneuvers (under the control of the pilot ue to air gust (not under the control of pilot.
Lan#ing la#s: – –
$anding on land (friction on tyre $anding on %ater.
)wer plant la#s: Thrust. – Tor6ue. –
,eight an# Inertia Fr!es:
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,eight: The term %eight is that constant force, proportional to its mass. 8hich tends to dra% every physical body to%ards the centre of the earth. Inertia Fr!es: Inertia Fr!es fr mtin f pure translatin f rigi# -#y
/f the unbalanced forces acting on a rigid body cause only a change in the magnitude of the velocity of the body, but not in the direction, the motion is called translation and from the basic physics# "ccelerating force E D a rom the basic physics
Inertia fr!es n rtating rigi# -#ies: • " common airplane maneuver is a motion along a curved path in a plane parallel to the F5 plane of the airplane, and generally referred to the pitching plane. • " pull up from steady ight or a pull out from a dive causes an airplane to follo% a curved path.
/f at point " the velocity is increasing along its path, the airplane is being subGected to t%o accelerations# :. at, tangential to the curve at point " and e6ual in magnitude to a t E r a. •
<. an E r H<, an acceleration normal to the ight path at " and directed to%ard the centre of rotation (o. rom the Ie%tonJs la% the e=ective forces due to these accelerations ate#
/f the velocity of the airplane along the path is constant then a t E * and thus the inertia force t E *, leaving only the normal inertia force n. /f the angular acceleration is constant the follo%ing relationships hold#
La# fa!trs The term load factor normally given the symbol KnL can be deBned as the numerical multiplying factor by %hich the forces e6uivalent to the dynamic force system acting during the acceleration of the airplane. or steady ight $ E 8. Io% assume that airplane is accelerated up%ard, sho%s the additional inertia force acting in do%n%ards, or opposite to the direction of acceleration. Thus the total airplane lift $ for the un0accelerated condition must be multiplied by a factor n M to produce static e6uilibrium in the M0 direction.
Since $ E 8, then
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"n airplane can be accelerated along the 10a1is as %ell as the M0a1is.
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igure sho%s an airplane landing on a navy aircraft are being arrested by a cable pull T on the airplane arresting hook. /f the airplane %eight is :<*** lbs, and the airplane is given a constant acceleration of ).g, Bnd the hook pull T, %heel reaction R, and the distance (d bet%een the line of action of the hook pull and the airplane c.g. if the landing velocity is -* D!2.
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9n contact of the airplane %ith the arresting cable the airplane is decelerated to the right the motion is purely translation horiMontally. The inertia force is#
The inertia force acts opposite to the direction of motion, hence to the left. The unkno%ns T and R can no% be solved for by using the static e6uations of e6uilibrium.
To Bnd the distance d, take moment about the airplane c.g.
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"ssume that the transport aircraft as sho%n, has Gust touchdo%n in landing and that a breaking force of )*** lb, on the rear %heel is being applied to bring the airplane to rest. The landing horiMontal velocity is N D!2. neglecting air forces on the airplane and assuming the propeller forces are Mero, %hat are the ground reactions R: and R<. %hat is the landing run distance %ith the constant breaking force.
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The airplane being accelerated horiMontally hence the inertia force through the airplane c.g. acts to%ards the front of the airplane. rom the e6uilibrium e6uations#
$anding run#
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To Bnd R<, take moment about point "#
+n /iagram 0el!ity la# fa!tr /iagram1 •
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The load actor#
2ence "t higher speeds, n ma1 is limited by the structural design of the airplane. These considerations are best understood by e1amining by diagram sho%ing load factor versus velocity for a given airplane0 the O0n diagram.
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Consider an airplane is ying at velocity O :, "ssume that the airplane is at an angle of attack such that C$P C $ma1. This ight condition is represented by point :. Io% assume that the angle of attack is increased to that to obtaining C $ma1, keeping the velocity constant at O :. The lift increases to its ma1imum value for the given O :, and hence the load factor nE$A8 reaches its ma1imum value of nma1 for the given velocity is given by point <. /f the angle of attack is increased further, the %ing stalls and the load factor drops. Therefore, point ) is stall region of the O0n diagram.
Io% as O: is increased to a value O Q, then the ma1imum possible load factor n ma1 also increases, as given by point Q. • 2o%ever nma1 cannot be allo%ed to increases indeBnitely. 4eyond a certain value of load value, deBned as the limit load factor as sho%n by the horiMontal line 4C. Structural damage may occur to the aircraft. • The right hand side of the O0n diagram, line C, is high speed limit. "t velocities greater than this, the dynamic pressure becomes so large that again structural damage may occur to the airplane. •
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inally, the bottom part of the O0n diagram, given by curves "? and ?, corresponds to negative absolute angles of attack, that is, negative loads factor. Curve "? deBnes the stall limit . $ine ? gives the negative limit load factor, beyond %hich structural damage %ill occur.
?=ect of guest velocity on O0n iagram The acceleration due to the air gust are not control of the pilot. Since it depends on the direction and velocity of the air guest. • &enerally the ma1imum velocity of the air gust is )* ftAsec. GUST L2A/ FA"T2R: • 8hen a sharp edge gust strikes the airplane in a direction normal to the thrust line (1 0 a1is, a sudden change takes place in the %ing angle of attack %ith no sudden change in airplane velocity. •
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The normal force coe7cient (C 5" can be assumed to vary linearly %ith the angle of attack.
!oint 4 represent the normal airplane force coe7cient C5", necessary to maintain level ight ( $ E 8, %ith a Oelocity O and point C, the value of C 5", after a sharp edge gust of velocity 3, has caused a sudden change in angle of attack (, %ithout change in O. or small angles# E 3AO
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"nd from C5" vs curve, C5" E m. E m (3AO
8here, m E slope of the normal force curve. The load factor increment due to gust 3 can be e1pressed as#
8here, 3 E gust velocity (ma1. )* ftAsec. E &ust correction factor. O E /ndicated air speed in D!2. 8 E gross %eight of the airplane.
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/f 3 E )* ftAsec and KmL is slope per unit degree.
Therefore the load factor KnL, %hen airplane is ying in horiMontal attitude e6uals#
The airplane shall %ithstand any applied loads due to a )* ftAsec gust acing in any direction.
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/n the belo% diagram a positive gust is not critical %ithin the restricted velocity of the airplane, since the guest line intersect the line 4 belo% line "4. or a negative gust, the gust load factor becomes critical at velocities bet%een , %ith a ma1imum acceleration as given by point ?.
!R94$?DS :. "n airplane e6uipped %ith oat is catapulted into the air from a Iavy cruiser as illustrated in igure. the catapulting force ! gives the airplane a constant horiMontal acceleration of )g (U-.- ftAsec<. The gross %eight of airplane is U*** lb. and the catapult track is ) ft. long. ind the catapulting force ! and the reactions R : and R< from the catapult car. The engine thrust is U** lb. %hat is the airplane velocity at the end of track runV
<. The airplane in igure, :Q*** lb. it is ying horiMontally at a
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velocity of ** D!2 (W)) ftAsec %hen the pilot pulls it up%ard into a curved path %ith a radius of curvature of <** ft. assume the engine thrust and airplane drag e6ual, opposite and collinear %ith each other, Bnd# "cceleration of airplane in 5 direction. 8ing lift ($ and tail (T forces. "irplane load factor.
). igure sho%s a large transport aircraft %hose gross %eight is :***** lb. The airplane pitching mass moment of inertia /y E Q*,***,*** lb.sec<.in. The plane is making a level landing %ith nose %heel slightly o= ground. The reaction on the rear %heel is ):U,*** lb inclined at such an angle to give a drag component of :**,*** lb and a vertical component of )**,*** lb. ind# • The inertia forces on the airplane. • The resultant load on the pilot %hose %eight is :N* lb. and %hose location is as sho%n in igure.
