Structures: V-n Diagrams Lecture 18 ME EN 415 Andrew Ning - PowerPoint PPT Presentation

Structures: V-n Diagrams Lecture 18 ME EN 415 Andrew Ning aning@byu.edu

Safety Factor B777 Wing Bending https://youtu.be/Ai2HmvAXcU0 §25.303 Factor of safety. Unless otherwise specified, a factor of safety of 1.5 must be applied to the prescribed limit load which are considered external loads on the structure.

Air Speed True airspeed (TAS): speed of aircraft relative to air Equivalent airspeed (EAS): airspeed at sea level with equivalent dynamic pressure 1 2 ρ TAS 2 = 1 2 ρ SL EAS 2 r ρ EAS = TAS ρ SL

Design airspeeds (all in EAS) V s : V A : V B : V C : V D :

V-n Diagram n mmax n stall V ( EAS ) V D V C stall

V-n Diagram n mmax n stall V ( EAS ) V D V C stall

V-n Diagram n mmax n stall V ( EAS ) V D V C stall except that n may not be less than 2.5 and need not be greater than 3.8

V-n Diagram n mmax n stall V ( EAS ) V D V C stall

V-n Diagram n mmax n stall V ( EAS ) V D V C stall From an initial condition of stabilized flight at VC/MC, the airplane is upset, flown for 20 seconds along a flight path 7.5° below the initial path, and then pulled up at a load factor of 1.5g (0.5g acceleration increment). The speed increase occurring in this maneuver may be calculated if reliable or conservative aerodynamic data is used.

V-n Diagram n mmax n stall V ( EAS ) V D V C stall

V-n Diagram n mmax n stall V ( EAS ) V D V C stall

V-n Diagram gusts n V ( EAS ) V D V C V B

Reference Gust Velocities 10.43 ft/s 20.86 ft/s 60,000 ft altitude V D V B — V C 44 ft/s 15,000 ft 22 ft/s 56 ft/s 28 ft/s 0 60 ft/s 30 ft/s

Gust Equations 2 W/S reduced mass µ = cC L, α g ρ ¯ K g = 0 . 88 µ gust factor 5 . 3 + µ n g = 1 ± K g U ref ( EAS ) C L, α (using standard metric units) 1 . 63( W/S )

V-n Diagram intersection gusts n 1 V ( EAS ) V D V C V B

V-n Diagram Critical Points positive high positive low angle of attack n angle of attack V ( EAS ) negative high negative low angle of attack angle of attack

compression compression tension tension positive high angle of attack positive low angle of attack

tension tension compression compression negative high angle of attack negative low angle of attack