AE 452 Aeronautical Engineering Design IIAir Loads
Prof. Dr. Serkan Γzgen
Dept. Aerospace Engineering
February 2017
Level turn
3
β’ The greatest air loads on an airplane usually come from thegeneration of lift during high-g maneuvers.
π = πΏ π, load factor and π =1
πππ π
β’ 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.
β’ 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.
β’ Factor of safety = 1.5 nultimate=1.5*nlimit.
Level turn
5
β’ Stall limit for the maximum load factor (instantaneous turn):
πΏ = ππππ₯π,πΏ = 1 2πβππ π‘πππ2 πΆπΏ,πππ₯π
ππππ₯ = 1 2πβππ π‘πππ
2 πΆπΏ,πππ₯
π π
β’ 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.
β’ Modern fighters have a corner speed around 300-350 knots.
Level turn
6
β’ Corresponding turn rate:
π =π π2 β 1
πββ’ Corresponding turn radius:
π =πβ2
π π2 β 1
V-n diagram
8
β’ Corner velocity: the slowest speed at which the maximum loadfactor can be reached without stalling.
β’ Dive speed: represents the maximum dynamic pressure, qβ. The point representing maximum qβ and nlimit is important forstructural design. Exceeding Vdive may result in phenomena likewing divergence, control surface reversal, etc.
ππππ£π = 1.5 β ππππ’ππ π or ππππ£π = 1.2 β ππππ₯ for subsonic airplanes
ππππ£π = ππππ₯ + 0.2 for supersonic airplanes
Sustained turn
β’ An aircraft will probably not be able to maintain speed andaltitude while turning at the maximum instantaneous turn rate.
β’ 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.
π = π·, πΏ = ππ β π =π
π
πΏ
π·
β’ Load factor in a sustained turn increases when T/W and L/D increases.
9
Pull-up maneuver
β’ At π‘ = 0 π = 0 :
πΉπ = πΏ βπ = π π β 1 = ππβ2
π =
π
π
πβ2
π
β’ Solving for turn radius:
π =πβ2
π(πβ1)
β’ Solving for turn rate:
π =πβ
π =
π(πβ1)
πβ
12
Pull-down maneuver
β’ At π‘ = 0 π = 0 :
πΉπ = πΏ +π = π π + 1 = ππβ2
π =
π
π
πβ2
π
β’ Solving for turn radius:
π =πβ2
π(π+1)
β’ Solving for turn rate:
π =πβ
π =
π(π+1)
πβ
13
Gust loads
14
β’ 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
β’ When an airplane encounters a gust, the effect is to increasethe angle of attack:
ΞπΌ = π‘ππβ1π
πββ
π
πβ
βπΏ = πβπ πΆπΏπΌβπΌ =1
2πβπβππΆπΏπΌπ
βπ =βπΏ
π=
πβπβπΆπΏπΌπ
2 π π load factor due to a gust
increases for aircraft with low wing loading!
Gust loads
16
β’ The assumption that an airplane instantly encounters a gustand this gust instantly effects the airplane is unrealistic.
β’ Gusts follow cosine-like intensity increase allowing aircraftmore time to react. This reduces the acceleration experiencedby the airplane.
Gust loads
17
β’ Gust velocity:
π = πΎπππ , πΎ: gust alleviation factor
πΎ =0.88π
5.3+π, subsonic flight
πΎ =π1.03
6.95+π1.03, supersonic flight
π =2 π π
πβπ ππΆπΏπΌ, mass ratio
πππ = 30 ππ‘/π , standard vertical gust, produces n=3g loadfactor, suitable for CS23 airplanes.