Principles of Flight
Principles of Flight
This domain explains how forces, stability and control affect the aircraft: lift, drag, stalls, load factor, stability and high-speed flight.
Lift and drag. Lift increases with angle of attack up to the critical angle, then decreases. Induced drag decreases with speed; parasite drag increases with speed; total drag is lowest at the speed for best glide or maximum endurance. A stall is an angle-of-attack phenomenon, not a specific airspeed. Load factor increases stall speed: Vs increases by the square root of the load factor.
Stability and control. Static stability is the initial tendency to return to equilibrium; dynamic stability is the subsequent motion. Dihedral gives lateral stability; the fin gives directional stability; the horizontal tail gives longitudinal stability. Adverse yaw is caused by the downgoing aileron producing more drag; coordinate with rudder. Va is the design manoeuvring speed and decreases with weight.
Worked example
In a 60° bank, load factor = 1/cos 60° = 2 g. If the 1 g stall speed is 50 kt, the stall speed at 2 g is 50 × √2 ≈ 71 kt. The aircraft will stall at 71 kt if the critical angle of attack is reached, so steep turns need more speed and power.
Traps
Do not say “the aircraft stalls because it is too slow” - it stalls because the angle of attack is too high. Do not use Va above the manoeuvring speed or at a weight above design. Do not ignore adverse yaw in slow flight.