392 CHAPTER 13 Airframe Loads
or,sinceL=nW(seeSection13.2)
n=V^2
gR+cosθ (13.15)Atthelowestpointofthepull-out,θ=0,and
n=V^2
gR+ 1 (13.16)
WeseefromeitherEq.(13.15)orEq.(13.16)thatthesmallertheradiusoftheflightpath,thatisthe
moreseverethepull-out,thegreaterthevalueofn.Itisquitepossible,therefore,foraseverepull-out
tooverstresstheaircraftbysubjectingittoloadswhichlieoutsidetheflightenvelopeandwhichmay
evenexceedtheprooforultimateloads.Inpractice,thecontrolsurfacemovementmaybelimitedby
stopsincorporatedinthecontrolcircuit.Thesestopsusuallyoperateonlyaboveacertainspeed,giving
the aircraft adequate maneuverability at lower speeds. For hydraulically operated controls, “artificial
feel”isbuiltintothesystemwherebythestickforceincreasesprogressivelyasthespeedincreases—a
necessaryprecautioninthistypeofsystemsincethepilotismerelyopeningandclosingvalvesinthe
controlcircuitandthereforereceivesnodirectphysicalindicationofcontrolsurfaceforces.
Alternatively, at low speeds, a severe pull-out or pull-up may stall the aircraft. Again safety pre-
cautions are usually incorporated in the form of stall warning devices, since, for modern high-speed
aircraft,astallcanbedisastrous,particularlyatlowaltitudes.
13.3.2 Correctly Banked Turn
Inthismaneuver,theaircraftfliesinahorizontalturnwithnosideslipatconstantspeed.Iftheradiusof
theturnisRandtheangleofbankφ,thentheforcesactingontheaircraftarethoseshowninFig.13.10.
The horizontal component of the lift vector in this case provides the force necessary to produce the
centripetalaccelerationoftheaircrafttowardthecenteroftheturn.Then
Lsinφ=WV^2
gR(13.17)
Fig.13.10
Correctly banked turn.