Principles of Flight
Lift, drag and the stall
Lift depends on air density, true airspeed squared, wing area and lift coefficient; angle of attack, not speed alone, determines the stall. The critical angle of attack is fixed for a given configuration, so an aircraft can stall at any speed, attitude or power setting. Know the lift curve, CLmax, the effect of flaps and slats on CLmax and stall speed, and the drag curve: induced drag dominates at low speed and parasite drag at high speed.
Load factor and stability
In a level turn, load factor n = 1/cos(bank); stall speed increases with the square root of n. Know the relationships between load factor, stall speed, manoeuvring speed and design limits. Static stability concerns the initial tendency to return to equilibrium; dynamic stability concerns the subsequent motion. Understand the effect of centre of gravity on stability and control, the movement of the centre of pressure with angle of attack, and the hazards of the Mach-critical region, shock stall, sweepback and wake turbulence.
Worked example
An aircraft stalls at 60 kt in level flight. In a 60° banked turn, n = 1/cos60° = 2, so the stall speed becomes 60 × √2 = 85 kt. The stall speed rises in any pull-up or steep turn and is not fixed by the ASI.
Rule of thumb
Stall speed multiplies by √n. In a 60° bank the load factor is 2; at 75° it is about 3.9.