Principles of Flight
Principles of Flight explains how lift, drag, thrust and weight interact, and how stability, control and load factor affect the aircraft. The exam tests the stall, induced drag, stability, control effectiveness and wake turbulence.
Lift, drag and the stall
- Lift depends on angle of attack, airspeed, air density and wing area. For a given configuration, increasing angle of attack increases lift until the critical angle, after which the wing stalls.
- Induced drag decreases as speed increases; parasite drag increases with the square of speed. Total drag is lowest at the speed where induced and parasite drag are equal.
- A stall is an angle-of-attack phenomenon, not a specific airspeed. Weight, load factor, CG and power affect the stall speed.
- Flaps increase lift and drag and lower the stall speed, but alter pitch behaviour and go-around performance.
Stability, control and load factor
- Static stability is the initial tendency to return to equilibrium; dynamic stability describes the subsequent motion.
- Forward CG increases longitudinal stability but reduces elevator authority; aft CG reduces stability.
- Load factor increases in turns, pull-ups and turbulence. In a level turn, load factor = 1/cos(bank angle).
- Wake turbulence is generated by wingtip vortices; avoid it by flying above and upwind of heavy aircraft and by delaying take-off.
Worked example
An aircraft stalls at 50 kt in level flight. In a 45-degree bank, load factor = 1/cos 45 = 1.41. Stall speed increases by the square root of load factor: 50 x sqrt(1.41) = 50 x 1.19 = about 59.5 kt. If the pilot tries to hold altitude while slowing below about 60 kt in the turn, the wing can stall. The correct response is to reduce bank and angle of attack, not simply pull harder.
Traps that cost marks
Do not memorise a single stall speed for all conditions. Do not confuse induced drag with parasite drag. A high load factor raises stall speed and can exceed the structural limit in turbulence. Do not assume flaps always improve go-around performance. Wake turbulence sinks and drifts; taking off behind a heavy aircraft too soon is hazardous even with a light wind.
Key points
- Lift increases with angle of attack up to the critical angle; the stall is an AoA phenomenon.
- Induced drag decreases with speed; parasite drag increases with the square of speed.
- Load factor in a level turn = 1/cos(bank); stall speed rises with the square root of load factor.
- Forward CG increases stability but reduces elevator authority; aft CG reduces stability.
- Flaps increase lift and drag and lower stall speed but affect go-around performance.
- Avoid wake turbulence by flying above and upwind and by delaying take-off.