Principles of Flight
This domain explains why an aircraft flies and how it behaves: lift, drag, thrust, weight, stability, control, load factor, turns and high-speed effects. Expect qualitative reasoning plus simple calculations.
Lift and drag
- Lift depends on angle of attack, dynamic pressure and wing area. At a given weight, a stall occurs at the critical angle of attack, not at a fixed speed.
- Induced drag falls with speed; parasite drag rises with the square of speed. Total drag is lowest at best L/D speed.
- Flaps increase lift and drag and lower the stall speed.
Stability and control
- Static stability is the initial tendency to return to equilibrium. Aft CG reduces longitudinal static stability.
- Dihedral gives lateral stability; fin and rudder give directional stability.
- Adverse yaw is countered with rudder; ailerons alone are not enough.
Load factor and turns
- Load factor n = 1/cos(bank angle). Stall speed increases with √n.
- A level 60° bank gives n = 2; the stall speed is 1.41 times the 1 g value.
Worked example
An aircraft stalls at 50 kt in level flight. In a 60° bank, n = 1/cos60° = 2.0. The new stall speed is 50 × √2 ≈ 71 kt. If the pilot tries to hold altitude by pulling, the stall margin disappears.
Traps
- Saying an aircraft stalls because it is too slow; it stalls because the critical angle is exceeded.
- Forgetting that aft CG can make stall recovery harder.
- Confusing induced and parasite drag.
- Assuming load factor increases in a descending turn; it depends on turn rate and bank.
- Ignoring Mach effects such as shock stall and Mach tuck.