Principles of Flight
This domain tests how lift, drag, thrust and weight interact, and how control and stability are produced.
Lift and drag
- Lift equals CL x half rho V squared S. It depends on density, true airspeed squared and wing area, and on angle of attack through CL.
- The critical angle of attack is where the wing stalls; it is the same at any weight, speed, attitude or bank angle. A stall is an angle-of-attack event, not a speed event.
- Induced drag falls with increasing speed and rises sharply at low speed; parasite drag rises with speed squared. Total drag is minimum at the speed for best glide, where CL is maximum for the drag.
- Ground effect reduces induced drag and downwash within about one wingspan of the surface, so the aircraft floats on landing.
Load factor and stalling
- Load factor n equals lift divided by weight. Stall speed increases with the square root of the load factor.
- In a 60 degree banked level turn n = 2 and the stall speed rises by about 41 percent.
Stability and control
- Static stability is the initial tendency to return to equilibrium; dynamic stability is how that tendency develops over time.
- Dihedral gives lateral stability; the fin gives directional stability; a forward CG improves longitudinal stability.
- Adverse yaw comes from the downgoing aileron's higher induced drag; differential ailerons and rudder correct it.
Worked example
Aircraft stalls at 50 kt in level flight. In a 45 degree banked level turn, n = 1/cos 45 = 1.41. New stall speed = 50 x square root of 1.41 = 50 x 1.19 = 59.5 kt. The aircraft still stalls at the same angle of attack.
Traps
Believing a lower speed causes a stall rather than an excessive angle of attack; forgetting that density altitude raises TAS and thus stall speed for a given IAS.