374 CHAPTER 12 Airworthiness
Fig.12.1
Flight envelope.
The curves OA and OF correspond to the stalled condition of the aircraft and are obtained from the
well-knownaerodynamicrelationship
Lift=nW=^12 ρV^2 SCL,maxTherefore,forspeedsbelowVA(positivewingincidence)andVF(negativeincidence),themaximum
loads which can be applied to the aircraft are governed byCL,max. As the speed increases, it is pos-
sible to apply the positive and negative limit loads, corresponding ton 1 andn 3 , without stalling the
aircraft so that AC and FE represent maximum operational load factors for the aircraft. Above the
designcruisingspeedVC,thecut-offlinesCD 1 andD 2 Erelievethedesigncasestobecoveredsince
it is not expected that the limit loads will be applied at maximum speed. Values ofn 1 ,n 2 ,andn 3
arespecifiedbytheairworthinessauthoritiesforparticularaircraft;typicalloadfactorsareshownin
Table12.1.
AparticularflightenvelopeisapplicabletoonealtitudeonlybecauseCL,maxisgenerallyreduced
withanincreaseofaltitude,andthespeedofsounddecreaseswithaltitude,therebyreducingthecritical
MachnumberandhencethedesigndivingspeedVD.Flightenvelopesarethereforedrawnforarange
ofaltitudesfromsealeveltotheoperationalceilingoftheaircraft.