Basic Aerodynamics and Flight Controls
Basic aerodynamics and flight controls tests lift and drag, the stall, stability, and how the controls change the aircraft's flight path.
Lift, drag and the stall
- Lift = ½ρV²S CL. CL depends on angle of attack and configuration, not on airspeed directly. The critical angle of attack is fixed, so a stall can occur at any speed, weight or attitude.
- Induced drag falls as speed rises; parasite drag rises with V². Best L/D occurs where they are equal and gives the still-air glide and approximate best-range speed.
- In a level turn, load factor n = 1/cos(bank). Stall speed rises with √n, and design manoeuvring speed Va falls as weight falls.
Stability and controls
Positive static stability means a disturbance produces a restoring moment. Pitch stability comes from the tailplane and CG position; an aft CG reduces both stability and elevator authority. Dihedral, sweep and keel area give lateral stability; fin and side area give directional stability. Adverse yaw from ailerons is corrected with rudder. Trim changes stick force, not the manoeuvre or the stall margin.
Worked example
An aircraft stalls at 50 kt at maximum weight in level flight. At 45° bank, n = 1/cos45° = 1.41, so stall speed = 50 × √1.41 = 59.5 kt. At 60° bank, n = 2 and stall speed = 50 × √2 = 70.7 kt. Flown 10% lighter, the 1 g stall speed becomes 50 × √0.9 = 47.4 kt, but the critical angle of attack is unchanged.
Traps
- Thinking a stall requires low airspeed rather than high angle of attack.
- Confusing lift coefficient with lift; adding power does not unstall the wing until angle of attack is reduced.
- Quoting Va as a fixed speed - it is published for maximum weight.
- Confusing pitch attitude with angle of attack.