general loads on aircraft structure

8/18/2019 General Loads on Aircraft Structure

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AIRCRAFT STRUCTURE-I

(ASEG 331)


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Main structural Components and Teir Functions

Conventional aircraft usually consist of fuselage, wings and tail plane. The basic functions of an aircraft'sstructure are to transmit and resist the applied loads; toprovide an aerodynamic shape and to protect passengers,payload, systems, etc. from the environmental conditionsencountered in ight.

!in"#

• Spars

• Stringers

• Ribs• Skin


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Contd.

S!"R#

• $ongitudinal member in the %ing.

• &enerally %ing having T%o spars called ront spar(located at )*+ of %ing chord from leading edge andRear spar (located at -+ of %ing chord from the leadingedge.

• &enerally Spar having / cross0section, because / section

having ma1imum moment of inertia, hence 2igheststrength, for the same %eight.

• Spar %ebs takes Torsional load

(i.e. shear stresses and

spar anges takes bendingloads (i.e. bending stresses.


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Contd.

Stringer:• 3sed for 4ending loads.

• &enerally having 5, $, T, channal and small %ings havingrectangular cross0sections because of easy attachment tothe skin and space and %eight advantage.


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Contd.

RIBS:• The dimensions of ribs are governed by their span0%ise

location in the %ing (i.e. "irfoil shape and by the loadsthey are re6uired to support.

• 3sed for maintain the "irfoil shape through out the %ingsection.

• They also act %ith the skin in resisting the distributedaerodynamic pressure loads.

• They distribute concentration loads (e.g. undercarriage andadditional %ing store loads into the structure.


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FUSELAGE

The fuselage of any aircraft has T89 main functions#:. Carries the payload# passenger cargo.


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There are mainly three types f fuselage stru!tures:

:. TR3SS T>!?#

• This type of structure is still in use in manylight%eight aircraft using %elded steel tubetrusses.

• " bo1 truss fuselage structure can also be built

out of %ood@often covered %ith ply%ood.


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"nt#$.

%. &n!'ue stru!ture: it is possible to make a

skin strong enough to carry all the loads %ithoutthe need for any supporting frame%ork.

Consists of0

• Skin.

• ormers.• 4ulkheads.


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Contd.

(. Simi mn!'ue stru!ture:/n this fuselage structure the skin is used toavoid buckling, it is common for the stress skinto carry about half of the total load carried bythe skin and longerons together.

the typical fuselage structure consists of seriesof hoops, or frames at intervals along the skin,%hich gives the fuselage its cross0sectionalshape, connected by longerons that run the

length of the fuselage.mainly consists of0

• Skin

• 4ulkheadsA ormers (frames

• $ongerons#


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TAIL )LA*ES The tail0plane provides stability in !itch >a%.

• $arge "ircraft having

cross0section same

as %ing structure.

• Small "ircraft having

solid section.


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Imprtan!e f stru!tural weight

• The structure of an airplane must %ithstand theapplied aerodynamic load and interior loads not onlyfor the normal ight but also for e1treme conditionsmay 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 bythe available run%ay.

• " pound more structural %eight is a pound less ofpayload.

• The speciBc range is inversely proportional to theairplane %eight, so in increase in structural %eightraises the fuel consumption and the fuel cost.


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Contd.

• The Brst cost of the airplane is generally found to beproportional to the empty %eight.

• /f the payload and range cannot be reduced, a higherstructural %eight re6uires a larger engine to meet thetakeo= and landing re6uirement, thereby raising thestructural %eight even further.

or all these reason, the aircraft structural design hasal%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 lifeeither %ith safe life or %ith fail safe #esign.

Safe life: safe life means that the stresses in acomponents are so lo% that fatigue failure is not possibleover the life of the airplane.


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Contd.

• Fail safe:+ fail safe means that the structure hasalternate loads paths so that no single failure %illbe e=ected to the aircraft. This can be achievedby 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 mostof the ma1imum load.


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General la#s n Air!raft

• 4efore the structural design of an airplane can bemade, the e1ternal loads acting on the airplane inight, landing and takeo= conditions must bekno%n.

Limit la#: limit loads are the ma1imum loadsanticipated on the airplane during its life time.

The airplane structure shall be capable ofsupporting the limit loads %ithout su=eringdetrimental permanent deformations.

Ultimate r #esign la#s: 3ltimate or design loadsare e6ual to the limit load multiplied by a factor ofsafety. /n general the overall factor of safety is :..


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Contd.

• The board general category of e1ternal loads onconventional aircraft can be broken do%n intosuch 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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Contd.

,eight:

The term %eight is that constant force, proportional toits mass. 8hich tends to dra% every physical bodyto%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 thevelocity of the body, but not in the direction, themotion is called translation and from the basicphysics#

"ccelerating force E D a

rom the basic physics


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Inertia fr!es n rtating rigi# -#ies:

• " common airplane maneuver is a motion

along a curved path in a plane parallel tothe F5 plane of the airplane, and generallyreferred to the pitching plane.

• " pull up from steady ight or a pull outfrom a dive causes an airplane to follo% a

curved path.


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• /f at point " the velocity is increasingalong its path, the airplane is being

subGected to t%o accelerations#:. at, tangential to the curve at point " and

e6ual in magnitude to at E r a.


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/f the velocity of the airplane along thepath is constant then at E * and thus the

inertia force t E *, leaving only thenormal inertia force n.

/f the angular acceleration is constant thefollo%ing relationships hold#


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La# fa!trs

The term load factor normally given the symbol KnLcan be deBned as the numerical multiplying factor by

%hich the forces e6uivalent to the dynamic forcesystem acting during the acceleration of the airplane.

or steady ight $ E 8. Io% assume that airplaneis accelerated up%ard, sho%s the additional inertia

force acting in do%n%ards, or opposite to thedirection of acceleration. Thus the total airplane lift $for the un0accelerated condition must be multipliedby a factor nM to produce static e6uilibrium in the M0

direction.

Since $ E 8, then


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Contd.

• "n airplane can be accelerated along the 10a1isas %ell as the M0a1is.


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)r-lem

• igure sho%s an airplane landing on a navy

aircraft are being arrested by a cable pull T onthe airplane arresting hook. /f the airplane %eightis :


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Contd.

• 9n contact of the airplane %ith the arresting cablethe airplane is decelerated to the right the motion

is purely translation horiMontally. The inertia forceis#

• The inertia force acts opposite to the direction ofmotion, hence to the left.

• The unkno%ns T and R can no% be solved for byusing the static e6uations of e6uilibrium.

• To Bnd the distance d, take moment about the

airplane c.g.


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)r-lem

• "ssume that the transport aircraft as sho%n, has

Gust touchdo%n in landing and that a breakingforce of )*** lb, on the rear %heel is beingapplied to bring the airplane to rest. The landinghoriMontal velocity is N D!2. neglecting airforces on the airplane and assuming the propellerforces are Mero, %hat are the ground reactions R:and R


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Contd.

• The airplane being accelerated horiMontally hencethe 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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Contd.

• To Bnd R


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+n /iagram 0el!ity la# fa!tr /iagram1

• The load actor#

• 2ence

• "t higher speeds, nma1 is limited by the

structural design of the airplane. These

considerations are best understood bye1amining by diagram sho%ing load factorversus velocity for a given airplane0 the O0ndiagram.


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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 conditionis represented by point :.

• Io% assume that the angle of attack isincreased to that to obtaining C$ma1, keeping the

velocity constant at O:. The lift increases to itsma1imum value for the given O:, and hence the

load factor nE$A8 reaches its ma1imum valueof nma1 for the given velocity is given by point


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• Io% as O: is increased to a value OQ, then

the ma1imum possible load factor nma1 also

increases, as given by point Q.• 2o%ever nma1 cannot be allo%ed to

increases indeBnitely. 4eyond a certainvalue 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, lineC, is high speed limit. "t velocities greater

than this, the dynamic pressure becomes solarge that again structural damage mayoccur to the airplane.


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• inally, the bottom part of the O0ndiagram, given by curves "? and ?,

corresponds to negative absolute angles ofattack, that is, negative loads factor. Curve

"? deBnes the stall limit.• $ine ? gives the negative limit load

factor, beyond %hich structural damage%ill occur.


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?=ect of guest velocity on O0n iagram

• The acceleration due to the air gust are

not control of the pilot. Since it depends onthe 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 theairplane in a direction normal to the thrust

line (1 0 a1is, a sudden change takesplace in the %ing angle of attack %ith nosudden change in airplane velocity.


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Contd.

• The normal force coe7cient (C5" can be assumed to

vary linearly %ith the angle of attack.

• !oint 4 represent the normal airplane forcecoe7cient C5", necessary to maintain level ight ( $

E 8, %ith a Oelocity O and point C, the value of C5",

after a sharp edge gust of velocity 3, has caused asudden change in angle of attack (, %ithoutchange in O.

• or small angles# E 3AO


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Contd.

• "nd from C5" vs curve,

C5" E m. E m (3AO8here, m E slope of the normal force curve.

The load factor increment due to gust 3 can bee1pressed 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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Contd.

• /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 loadsdue to a )* ftAsec gust acing in any direction.


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Contd.

• /n the belo% diagram a positive gust is notcritical %ithin the restricted velocity of the

airplane, since the guest line intersect the line 4belo% line "4.

• or a negative gust, the gust load factor becomescritical at velocities bet%een , %ith a

ma1imum acceleration as given by point ?.


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!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 horiMontalacceleration of )g (U-.- ftAsec


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). igure sho%s a large transport aircraft %hose gross%eight is :***** lb. The airplane pitching massmoment of inertia /y E Q*,***,*** lb.sec


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general loads on aircraft structure

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