Principles of Flight
What this domain tests
Principles of Flight tests lift generation, drag, stability, control, stall and high-speed flight. Expect definitions, relationships and simple numerical questions on load factor and stall speed.
Core rules
- Lift = CL x one-half rho V squared S. CL increases with angle of attack until the critical angle, then stall occurs.
- Induced drag decreases with speed and increases with load factor; parasite drag increases with V squared. Total drag is minimum at the speed where they cross.
- Load factor n = 1/cos(bank) in a level turn. Stall speed increases by the square root of n: at 60 degrees bank, n = 2 and stall speed rises by 41%.
- Stability: static stability is the initial tendency; dynamic stability is the subsequent motion. Dihedral gives lateral stability; a fin gives directional stability.
- High speed: at critical Mach, shock waves form; sweep, thin aerofoils and area ruling raise Mcrit.
Worked example
A 45 degree banked level turn: n = 1/cos 45 degrees = 1.41. If the 1g stall speed is 100 kt, the stall speed in the turn is 100 x square root of 1.41, about 119 kt. The margin above the actual stall speed is reduced unless speed or bank is changed.
Traps
Saying lift equals weight in a climb (it does not; lift is less than weight in a steady climb); confusing angle of attack with pitch attitude; assuming a swept wing stalls at the tip first; forgetting that flaps increase CL but also increase drag.
Key habit
Write the lift formula and identify which variable the question changes before calculating.