Principles of Flight - NCAA PPL
This domain tests the physics behind aircraft control: how lift and drag are produced, how angle of attack governs the stall, how load factor changes in turns, and how energy trades between speed and height. The NCAA PPL paper expects relationships you can apply, not just definitions - for instance, predicting how stall speed changes with bank angle or weight.
Lift and the critical angle of attack
Lift = CL × ½ρV²S. In a given configuration, CL depends only on angle of attack. The critical angle (roughly 15–18° on most light trainers) is fixed: the wing stalls at that angle whatever the speed, weight, bank or altitude. Added weight raises the speed needed to reach that angle, so the 1g stall speed in knots IAS increases by the square root of the weight ratio. Density altitude does not change the IAS stall speed, but the TAS at which the stall occurs is higher.
Drag and the best-glide speed
Induced drag falls as speed rises; parasite drag rises with V². Total drag is lowest at the speed for maximum lift/drag ratio, which is the best-glide speed in a glide. Minimum-sink speed is slower than best glide, so the two should never be confused when choosing a forced-landing speed.
Load factor in turns
Load factor n = 1/cos(bank angle). Stall speed rises by √n, because the wing must support more than the aircraft's weight.
Worked example
An aircraft stalls at 45 kt in level flight at maximum weight. In a 60° bank, n = 1/cos60° = 2, so the stall speed becomes 45 × √2 = 45 × 1.41 ≈ 63.6 kt. Reasoning: the wing's lift demand doubles, so it must fly at a higher angle of attack for any given speed; the same critical angle is therefore reached at a higher speed.
Traps that cost marks
- Treating stall speed as a single fixed number rather than a function of weight, load factor and configuration.
- Mixing IAS and TAS when discussing stall speed at altitude.
- Assuming a stall needs low speed: an accelerated stall in a steep turn can occur at high airspeed.
- Confusing best-glide speed with minimum-sink speed.
- Forgetting that reducing load factor (a pushover) lowers the stall speed.