Principles of Flight
What the exam tests
Principles of flight tests lift and drag, angle of attack, stall, stability, control effectiveness, load factor, turning, Mach effects, swept wings and high-lift devices. The exam expects cause-and-effect reasoning.
Core rules
- Lift increases with angle of attack up to the critical angle, then decreases at the stall.
- Induced drag decreases with speed; parasite drag increases with speed squared.
- Stall speed increases with weight and load factor: new stall speed = old stall speed x square root of load factor.
- In a level turn, load factor = 1 / cos(bank angle). At 45 degrees, load factor is about 1.41.
- Stability is positive when a disturbance produces a restoring moment. Swept wings improve high-Mach behaviour but need high-lift devices at low speed.
Worked example
An aircraft stalls at 60 kt in level flight. In a 45-degree banked level turn, load factor is 1 / cos 45 = 1.41. New stall speed is 60 x square root of 1.41 = 60 x 1.19 = 71.4 kt. A pilot who maintains 65 kt in that turn is below the new stall speed and risks an accelerated stall.
Traps
Do not confuse angle of attack with pitch attitude. Do not assume stall speed is fixed. Do not add back pressure in a steep turn without checking airspeed and load factor. Do not ignore Mach buffet or shock stall at high altitude.