Principles of Flight
Principles of Flight explains why the aeroplane flies and how it behaves: lift and drag, stability and control, high-lift devices, compressibility and load factor. Questions are usually conceptual, with one or two simple calculations.
Lift and drag
- Lift = ½ ρ V² S CL. Induced drag falls as speed rises; parasite drag rises with the square of speed. Total drag is minimum at the speed for maximum lift/drag ratio, which is also the best glide speed.
- The critical angle of attack determines the stall, not speed alone. Stall speed increases with weight, load factor and altitude (in TAS).
Stability and control
- Static stability is the initial tendency to return to equilibrium; dynamic stability is how that tendency behaves over time.
- Dihedral gives lateral stability, the fin gives directional stability, and the horizontal stabiliser gives longitudinal stability; an aft centre of gravity reduces all three margins.
- Ground effect reduces induced drag and shifts the centre of pressure when close to the surface.
High speed and high lift
- Above the critical Mach number, shock waves form, causing Mach tuck, buffet and loss of control effectiveness; sweepback delays these effects.
- Flaps increase lift and drag and lower the stall speed; spoilers reduce lift and increase drag.
Load factor
- In a level turn, n = 1/cos φ, and stall speed increases by the square root of n. Wake turbulence vortices sink and drift with the wind, so stay above and upwind of a preceding heavy aircraft.
Worked example
A level turn at 45° bank: n = 1/cos 45° = 1.41. If the 1 g stall speed is 60 kt, the new stall speed is 60 × √1.41 = 60 × 1.19 = 71 kt. At 60° bank, n = 2 and the stall speed becomes 85 kt. That is why an accelerated stall can arrive with little warning in a steep turn.
Traps
- Believing the stall is reached at a fixed speed rather than at the critical angle of attack.
- Comparing an indicated airspeed at altitude with a sea-level TAS.
- Forgetting that manoeuvring speed Va decreases as weight decreases.