Principles of Flight
Scope
Principles of Flight tests lift, drag, stability, control, stall and high-speed aerodynamics, with emphasis on what changes when speed, mass, bank or Mach changes.
Core rules
- Lift = ½ρV²SCL; for a given mass, CL must rise as speed falls. The critical angle of attack is fixed; stall occurs at the critical AoA, not at a fixed speed.
- Induced drag falls as speed rises; parasite drag rises with V².
- In a level turn, load factor n = 1/cos(bank). Stall speed increases with √n.
- Positive static stability means a disturbance produces a restoring moment. Trim balances a steady control force.
- At high Mach, shock waves and Mach tuck affect lift and control; sweep delays the critical Mach number.
Worked example
An aircraft stalls at 60 kt in level flight. In a 60° bank turn, n = 1/cos 60° = 2. New stall speed = 60 × √2 = 85 kt. Reasoning: the wing must produce twice the lift, and lift varies with V², so the speed for the same CL increases by √2.
Traps
Saying stall speed is fixed; assuming lift always equals weight; confusing induced drag with parasite drag; forgetting that aft CG reduces static margin.