Principles of Flight
Principles of Flight tests the physics behind control: lift, drag, stability, load factor and stall behaviour.
Lift and drag
Lift depends on angle of attack, airspeed, air density and wing area. The critical angle of attack is the stall angle; the stall speed is the IAS at which the critical angle is reached in 1 g. Induced drag falls with speed while parasite drag rises, giving L/D max at best glide speed. Flaps increase lift and drag; they lower stall speed but may reduce climb gradient.
Stability and control
Static stability is the initial tendency to return; dynamic stability is the behaviour over time. Dihedral gives lateral stability; the vertical fin gives directional stability; the horizontal tail gives longitudinal stability. An aft CG reduces static margin and makes stall and spin recovery worse. Adverse yaw is countered with rudder. Trim reduces control force, not the need to hold attitude.
Load factor and stall
Load factor n = 1/cos(bank angle) in a level turn. Stall speed increases with the square root of n: VS(n) = VS × √n. At 60° bank, n = 2, so stall speed is 1.41 times the 1 g stall speed.
Worked example
Aircraft stalls at 50 kt in 1 g. In a level 60° bank, n = 1/cos 60° = 2. VS = 50 × √2 = 70.7 kt. If the pilot pulls to hold altitude and ignores the increasing stall speed, the wing can stall at an apparently 'normal' airspeed. Reason with load factor, not bank angle alone.
Traps
Believing stall is caused by low speed rather than excessive AoA; forgetting that weight raises stall speed; pulling harder in a turn; and ignoring the aft-CG effect on recovery.