CHAPTER 13 Airframe Loads...........................................................................
InChapter11,wediscussedingeneraltermsthetypesofloadtowhichaircraftaresubjectedduring
theiroperationallife. Weshallnowexamineinmoredetailtheloadswhichareproducedbyvarious
maneuversandthemannerinwhichtheyarecalculated.
13.1 AircraftInertiaLoads................................................................................
Themaximumloadsonthecomponentsofanaircraft’sstructuregenerallyoccurwhentheaircraftis
undergoingsomeformofaccelerationordeceleration,suchasinlandings,take-offs,andmaneuvers
withintheflightandgustenvelopes.Thus,beforeastructuralcomponentcanbedesigned,theinertia
loadscorrespondingtotheseaccelerationsanddecelerationsmustbecalculated.Forthesepurposes,we
shallsupposethatanaircraftisarigidbodyandrepresentitbyarigidmass,m,asshowninFig.13.1.
Weshallalso,atthisstage,considermotionintheplaneofthemasswhichwouldcorrespondtopitching
oftheaircraftwithoutrolloryaw.Weshallalsosupposethatthecenterofgravity(CG)ofthemasshas
coordinates ̄x,y ̄referredtoxandyaxeshavinganarbitraryoriginO;themassisrotatingaboutanaxis
throughOperpendiculartothexyplanewithaconstantangularvelocityω.
The acceleration of any point, a distancerfrom O, isω^2 rand is directed toward O. Thus, the
inertiaforceactingontheelement,δm,isω^2 rδminadirectionoppositetotheacceleration,asshown
in Fig. 13.1. The components of this inertia force, parallel to thexandyaxes, areω^2 rδmcosθand
ω^2 rδmsinθ,respectively,or,intermsofxandy,ω^2 xδmandω^2 yδm.Theresultantinertiaforces,Fx
andFy,arethengivenby
Fx=∫
ω^2 xdm=ω^2∫
xdmFy=∫
ω^2 ydm=ω^2∫
ydminwhichwenotethattheangularvelocityωisconstantandmaythereforebetakenoutsidetheintegral
sign.Intheaboveexpressions,
∫
xdmand∫
ydmarethemomentsofthemass,m,abouttheyandx
axes,respectively,sothat
Fx=ω^2 xm ̄ (13.1)Copyright©2010,T.H.G.Megson. PublishedbyElsevierLtd. Allrightsreserved.
DOI:10.1016/B978-1-85617-932-4.00013-0 379