Principles of Flight - Lift, Drag, Stability and Load Factor
Lift, drag and the stall
Lift depends on air density, speed squared, wing area and the lift coefficient, which varies with angle of attack. The critical angle of attack is fixed: the wing stalls at that angle regardless of speed, mass or attitude, which is why a heavier aeroplane stalls at a higher speed at the same angle. Induced drag falls and parasite drag rises with speed, so their sum is minimised at the best L/D speed - the speed for maximum range in a glide.
Stability and control
Know static and dynamic stability in pitch, roll and yaw; dihedral contributes lateral stability, while the fin and tailplane provide directional and longitudinal stability. Understand the secondary effects of controls - adverse yaw from aileron drag, yaw producing roll, pitch producing yaw - and the correct use of trim. Flaps increase both lift and drag and lower the stall speed.
Load factor
In a level turn the load factor is 1/cos(bank), so 60° of bank doubles the load and raises the stall speed by the square root of the load factor. This explains why steep turns and turbulence demand more speed.
Common traps
Believing a lighter aeroplane has a lower critical angle of attack; thinking flaps reduce stall speed with no penalty; and forgetting that stall speed rises with the square root of load factor.
Worked example
An aeroplane stalls at 50 kt in level flight. In a 60° bank the load factor is 1/cos 60° = 2, so the stall speed becomes 50 × √2 ≈ 71 kt.