Principles of Flight
What this paper tests
Principles of flight explains why the aeroplane flies and how it stalls: the four forces, lift generation, drag, stability, load factor and the V-speed family. Questions often give a bank angle or a weight change and ask for the effect.
Lift, drag and the stall
- Lift = ½ρV²SC_L; at a given weight, more weight, greater altitude (lower density) or higher load factor demands more angle of attack or more speed.
- The wing stalls at the critical angle of attack, not at a particular speed; an aeroplane can stall at any speed, weight or attitude, and in any bank.
- Induced drag falls as speed rises; parasite drag rises with the square of speed; total drag is lowest at L/D max, which is the best glide speed and the speed for maximum range.
- V speeds: Vs (stall), Vso (stall in landing configuration), Va (design manoeuvring, at maximum weight), Vx (best angle), Vy (best rate), Vne (never exceed), Vfe/VLE (flap/gear extension), Vno (maximum structural cruise).
Stability and asymmetric effects
- Static stability is the initial tendency to return to equilibrium; dynamic stability describes the resulting oscillation.
- Longitudinal (pitch) stability comes from the tailplane and CG position; lateral (roll) stability from dihedral and sweep; directional (yaw) stability mainly from the fin.
- Left-turning tendencies: engine torque, propeller slipstream, P-factor (asymmetric blade thrust at high angle of attack) and gyroscopic precession.
- Adverse yaw: the descending aileron's extra lift also creates drag, so lead the turn with rudder.
- Ground effect reduces induced drag near the surface; wake vortices are strongest from heavy, clean, slow aircraft.
Worked example
Clean stall speed is 45 kt. In a 60° bank the load factor is 1/cos 60° = 2, so the stall speed rises by √2: 45 × 1.41 ≈ 64 kt. If the aircraft is below maximum weight the stall speed falls, and because Va is defined at maximum weight, the manoeuvring speed also decreases with weight.
Traps that cost marks
- Assuming a low speed is required to stall; exceeding the critical angle of attack is the only requirement.
- Treating Va as constant regardless of weight.
- Confusing Vx with Vy, or best glide (L/D max) with minimum sink speed.
- Ignoring the accelerated stall - an abrupt pull-up at high speed can exceed the limit load and stall the wing.