388 CHAPTER 13 Airframe Loads
required to balance the aircraft weight, the aircraft experiences an upward acceleration normal to its
flightpath.Thisnormalaccelerationcombinedwiththeaircraft’sspeedinthediveresultsinthecurved
flightpathshowninFig.13.7.Asthedragloadbuildsupwithanincreaseofincidence,theforward
speedoftheaircraftfallssincethethrustisassumedtoremainconstantduringthemaneuver.Itisusual,
as we observed in the discussion of the flight envelope, to describe the maneuvers of an aircraft in
terms of a maneuvering load factorn. For steady level flightn=1, giving 1gflight, although in fact
theaccelerationiszero.Whatisimpliedinthismethodofdescriptionisthattheinertiaforceonthe
aircraftinthelevelflightconditionis1.0timesitsweight.Itfollowsthattheverticalinertiaforceonan
aircraftcarryingoutanngmaneuverisnW.Wemay,therefore,replacethedynamicconditionsofthe
acceleratedmotionbyanequivalentsetofstaticconditionsinwhichtheappliedloadsareinequilibrium
withtheinertiaforces.Thus,inFig.13.7,nisthemaneuverloadfactor,whilefisasimilarfactorgiving
thehorizontalinertiaforce.Notethattheactualnormalaccelerationinthisparticularcaseis(n− 1 )g.
Forverticalequilibriumoftheaircraft,wehave,referringtoFig.13.7wheretheaircraftisshown
atthelowestpointofthepull-out
L+P+Tsinγ−nW= 0 (13.12)Forhorizontalequilibrium,
Tcosγ+fW−D= 0 (13.13)andforpitchingmomentequilibriumabouttheaircraft’sCG,
La−Db−Tc−M 0 −Pl= 0 (13.14)Equation(13.14)containsnotermsrepresentingtheeffectofpitchingaccelerationoftheaircraft;this
isassumedtobenegligibleatthisstage.
Again,themethodofsuccessiveapproximationisfoundtobemostconvenientforthesolutionof
Eqs.(13.12,13.13,and13.14).Thereis,however,adifferencetotheproceduredescribedforthesteady
Fig.13.7
Aircraft loads in a pull-out from a dive.