386 CHAPTER 13 Airframe Loads
13.2 SymmetricManeuverLoads........................................................................
Weshallnowconsiderthecalculationofaircraftloadscorrespondingtotheflightconditionsspecified
by flight envelopes. There are, in fact, an infinite number of flight conditions within the boundary
of the flight envelope although, structurally, those represented by the boundary are the most severe.
Furthermore, it is usually found that the corners A, C, D 1 ,D 2 , E, and F (see Fig. 12.1) are more
critical than points on the boundary between the corners so that, in practice, only the six conditions
correspondingtothesecornerpointsneedtobeinvestigatedforeachflightenvelope.
Insymmetricmaneuvers,weconsiderthemotionoftheaircraftinitiatedbymovementofthecontrol
surfacesintheplaneofsymmetry.Examplesofsuchmaneuversareloops,straightpull-outs,andbunts,
and the calculations involve the determination of lift, drag, and tailplane loads at given flight speeds
andaltitudes.TheeffectsofatmosphericturbulenceandgustsarediscussedinSection13.4.
13.2.1 Level Flight
Althoughsteadylevelflightisnotamaneuverinthestrictsenseoftheword,itisausefulconditionto
investigateinitiallysinceitestablishespointsofloadapplicationandgivessomeideaoftheequilibrium
of an aircraft in the longitudinal plane. The loads acting on an aircraft in steady flight are shown in
Fig.13.6,withthefollowingnotation:
L istheliftactingattheaerodynamiccenterofthewing.
D istheaircraftdrag.
M 0 istheaerodynamicpitchingmomentoftheaircraftlessitshorizontaltail.
P is the horizontal tail load acting at the aerodynamic center of the tail, usually taken to be at
approximatelyone-thirdofthetailplanechord.
W istheaircraftweightactingatitsCG.
T is the engine thrust, assumed here to act parallel to the direction of flight in order to simplify
calculation.Fig.13.6
Aircraft loads in level flight.