Principles of Flight
Principles of Flight tests aerodynamics: the four forces, lift and drag, angle of attack, stability, control, load factor, stalls, and propeller effects. Expect conceptual questions and simple numerical load-factor problems.
Core concepts
- Lift: increases with angle of attack up to the critical angle, then stalls. Stall is an angle-of-attack phenomenon, not a speed.
- Drag: parasite drag rises with speed squared; induced drag rises as speed decreases. Best glide occurs near maximum lift-to-drag ratio.
- Stability: longitudinal stability depends on CG position and tailplane; lateral stability on dihedral; directional stability on fin and keel area.
- Load factor: in a level turn, n = 1/cos(bank). Stall speed increases by the square root of n.
- Propeller effects: torque, P-factor, slipstream, and gyroscopic precession vary with power and attitude.
Worked example
An aircraft stalls at 50 kt in level flight. In a 60° banked level turn, load factor n = 1/cos 60° = 2. The stall speed becomes 50 × √2 = 70.7 kt, about 71 kt. The reasoning: the wing must produce twice the lift to support the aircraft, so it reaches the critical angle of attack at a higher speed. The same principle applies in abrupt pull-ups and turbulence.
Traps
Do not say an aircraft stalls because it is too slow; it stalls because the angle of attack is too high. Do not ignore load factor in turns, and do not assume best glide is minimum speed. A climb does not necessarily require a higher angle of attack than level flight at the same speed.