Principles of Flight
Why the aircraft flies
Principles of flight questions test the physics of lift, drag, thrust and weight, and how the aircraft behaves as a result. Understand the four forces, the lift equation (lift depends on density, speed squared, wing area and lift coefficient), and the drag curve: parasite drag rises with speed squared, induced drag falls as speed increases, and total drag has a minimum at the best glide speed.
Angle of attack is the key variable: the stall occurs at a critical angle of attack regardless of speed, mass or attitude. Know the stall speed relationship (stall speed rises with mass and load factor, and with forward CG), the meaning of load factor and the V-n diagram, and the effect of flaps and slats on lift and drag. Stability: static versus dynamic, longitudinal, lateral and directional, and how CG position affects it.
Traps: equating stall with low speed (it is an angle-of-attack phenomenon); confusing best glide speed with minimum sink; and forgetting that load factor increases stall speed by the square root of the load factor.
Worked example
At 60° bank, load factor = 1 / cos 60° = 2. Stall speed increases by √2 ≈ 1.41. If the 1 g stall speed is 50 kt, the accelerated stall speed at 60° bank is about 71 kt.