Principles of Flight
Principles of Flight explains why the aeroplane behaves as it does - lift, drag, stability and control. Questions often need a short calculation plus a qualitative judgement about handling.
Lift and drag
- Lift = CL × ½ρV²S. CL rises with angle of attack up to a critical angle of about 15-16°, then the wing stalls. Induced drag falls with speed and with aspect ratio; parasite drag rises with V².
- Total drag is minimum at VMD, where induced equals parasite drag - the speed for maximum endurance (jet) and close to maximum range.
- Ground effect reduces induced drag and downwash within about one wingspan of the surface.
Stability and control
- Static stability is positive when a disturbance produces a restoring moment; dynamic stability adds damping over time.
- Longitudinal stability comes from the tailplane and CG position; an aft CG reduces stick force per g and stability.
- Dihedral gives lateral stability; sweepback gives directional stability and delays shock stall. Dutch roll needs a yaw damper.
- Manoeuvring speed VA is the speed above which full deflection of one control can overstress the structure.
High-speed flight
- Critical Mach number is where local airflow first reaches M1.0; shock-induced separation causes buffet and Mach tuck. Sweep and thin aerofoils raise Mcrit.
- Coffin corner is the narrow band between high-speed buffet and low-speed stall at high altitude.
Worked example
In a level turn at 60° bank, load factor n = 1/cos 60° = 2. Stall speed rises with √n, so a 100 kt stall speed becomes 141 kt. At 45° bank n = 1.41 and the increase is about 19%. At 120 kt in a 60° turn the aeroplane is close to the stall - roll out or add thrust.
Traps
Believing a heavier aeroplane stalls at a different angle of attack (it stalls at the same AoA but a higher speed); confusing VMD with jet maximum-range speed; assuming a T-tail is safer when it can produce a deep stall; and forgetting that a yaw damper does not prevent a spiral dive.