Principles of Flight
Lift and drag
Principles of flight rewards knowing the four forces and how they change. Lift L = one half rho V squared S C_L, and C_L rises with angle of attack until the critical angle, beyond which the wing stalls. Drag is parasite drag, increasing with V squared, plus induced drag, decreasing with V squared and proportional to C_L squared divided by aspect ratio. Total drag is minimum at best L/D, where parasite and induced drag are equal.
Stability and control
Static stability needs a restoring moment; the horizontal tail provides longitudinal stability. Dihedral gives lateral stability and the fin gives directional stability. Ailerons roll, elevator pitches, rudder yaws; consider adverse yaw, aileron drag, slipstream and torque effects. Keep the relationship between angle of attack, speed and load factor clear, because stall depends on angle of attack, not on speed alone.
High speed and turns
As Mach number rises, shock waves form, drag rises at the critical Mach number, and Mach tuck or upset can occur; sweep and thin aerofoils delay this. In a level turn the load factor n = 1 / cos bank angle, and stall speed rises with the square root of n. The coffin corner lies between high-speed buffet and low-speed stall.
Worked example
An aircraft stalls at 50 kt in 1 g. At 60 degrees of bank the load factor is 2, so stall speed becomes 50 × square root of 2 = 71 kt. Rule of thumb: 45 degrees gives about 1.41 g, 60 degrees gives 2 g.