ae 452 aeronautical engineering design ii
TRANSCRIPT
AE 452 Aeronautical Engineering Design IIAir Loads
Prof. Dr. Serkan รzgen
Dept. Aerospace Engineering
February 2017
Level turn
3
โข The greatest air loads on an airplane usually come from thegeneration of lift during high-g maneuvers.
๐ = ๐ฟ ๐, load factor and ๐ =1
๐๐๐ ๐
โข The largest load the airplane is expected to encounter is called the limit load and the corresponding load factor is called the limit load factor.
โข Ultimate load factor or the design load factor is the limit loadmultiplied by a factor of safety to account for material andworkmanship quality, design errors, uncertainty, etc.
โข Factor of safety = 1.5 nultimate=1.5*nlimit.
Level turn
5
โข Stall limit for the maximum load factor (instantaneous turn):
๐ฟ = ๐๐๐๐ฅ๐,๐ฟ = 1 2๐โ๐๐ ๐ก๐๐๐2 ๐ถ๐ฟ,๐๐๐ฅ๐
๐๐๐๐ฅ = 1 2๐โ๐๐ ๐ก๐๐๐
2 ๐ถ๐ฟ,๐๐๐ฅ
๐ ๐
โข The speed at which the maximum lift is equal to the allowablestructural load factor is the corner speed and provides themaximum turn rate for a given altitude.
โข Modern fighters have a corner speed around 300-350 knots.
Level turn
6
โข Corresponding turn rate:
๐ =๐ ๐2 โ 1
๐โโข Corresponding turn radius:
๐ =๐โ2
๐ ๐2 โ 1
V-n diagram
8
โข Corner velocity: the slowest speed at which the maximum loadfactor can be reached without stalling.
โข Dive speed: represents the maximum dynamic pressure, qโ. The point representing maximum qโ and nlimit is important forstructural design. Exceeding Vdive may result in phenomena likewing divergence, control surface reversal, etc.
๐๐๐๐ฃ๐ = 1.5 โ ๐๐๐๐ข๐๐ ๐ or ๐๐๐๐ฃ๐ = 1.2 โ ๐๐๐๐ฅ for subsonic airplanes
๐๐๐๐ฃ๐ = ๐๐๐๐ฅ + 0.2 for supersonic airplanes
Sustained turn
โข An aircraft will probably not be able to maintain speed andaltitude while turning at the maximum instantaneous turn rate.
โข Sustained turn rate is usually specified in terms of themaximum load factor at a given flight condition that the aircraftcan sustain, e.g. 4-5g at M=0.9 at 30000 ft.
๐ = ๐ท, ๐ฟ = ๐๐ โ ๐ =๐
๐
๐ฟ
๐ท
โข Load factor in a sustained turn increases when T/W and L/D increases.
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Pull-up maneuver
โข At ๐ก = 0 ๐ = 0 :
๐น๐ = ๐ฟ โ๐ = ๐ ๐ โ 1 = ๐๐โ2
๐ =
๐
๐
๐โ2
๐
โข Solving for turn radius:
๐ =๐โ2
๐(๐โ1)
โข Solving for turn rate:
๐ =๐โ
๐ =
๐(๐โ1)
๐โ
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Pull-down maneuver
โข At ๐ก = 0 ๐ = 0 :
๐น๐ = ๐ฟ +๐ = ๐ ๐ + 1 = ๐๐โ2
๐ =
๐
๐
๐โ2
๐
โข Solving for turn radius:
๐ =๐โ2
๐(๐+1)
โข Solving for turn rate:
๐ =๐โ
๐ =
๐(๐+1)
๐โ
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Gust loads
14
โข The loads experienced when the airplane encounters a stronggust (when flying close to a thunderstorm or during clear air
turbulence encounter) may exceed the maneuver loads.
Gust loads
15
โข When an airplane encounters a gust, the effect is to increasethe angle of attack:
ฮ๐ผ = ๐ก๐๐โ1๐
๐โโ
๐
๐โ
โ๐ฟ = ๐โ๐ ๐ถ๐ฟ๐ผโ๐ผ =1
2๐โ๐โ๐๐ถ๐ฟ๐ผ๐
โ๐ =โ๐ฟ
๐=
๐โ๐โ๐ถ๐ฟ๐ผ๐
2 ๐ ๐ load factor due to a gust
increases for aircraft with low wing loading!
Gust loads
16
โข The assumption that an airplane instantly encounters a gustand this gust instantly effects the airplane is unrealistic.
โข Gusts follow cosine-like intensity increase allowing aircraftmore time to react. This reduces the acceleration experiencedby the airplane.
Gust loads
17
โข Gust velocity:
๐ = ๐พ๐๐๐ , ๐พ: gust alleviation factor
๐พ =0.88๐
5.3+๐, subsonic flight
๐พ =๐1.03
6.95+๐1.03, supersonic flight
๐ =2 ๐ ๐
๐โ๐ ๐๐ถ๐ฟ๐ผ, mass ratio
๐๐๐ = 30 ๐๐ก/๐ , standard vertical gust, produces n=3g loadfactor, suitable for CS23 airplanes.