Principles of Flight
The Principles of Flight domain tests how the four forces, angle of attack (AoA), speed, density and load factor interact to produce lift, drag, stall and control. Questions often require both a concept and a quick calculation.
Core rules
- Lift is L = ½ρV²S_CL. For a fixed weight and configuration the required CL is set; the pilot obtains it at a particular AoA.
- Critical AoA is a fixed value for a configuration. Stall occurs when critical AoA is exceeded, not at a fixed speed; weight, bank, gust or density altitude change the speed at which that AoA is reached.
- Induced drag decreases as speed increases; parasite drag increases with V². Total drag is minimum at L/Dmax, the same condition as maximum glide ratio.
- In a level turn, load factor n = 1/cos φ. Stall speed rises by √n.
Worked example
Vs1g = 50 KCAS. In a 60° level turn, n = 1/cos60° = 2.0. Accelerated stall speed = 50 × √2 = 70.7 KCAS. The wing stalls at the same critical AoA, but more lift is needed, so the stall occurs at a higher speed. A pilot who maintains 60° bank and pulls to the buffet has no margin at 70.7 KCAS.
Traps
- Treating stall speed as a single fixed number; it is valid only for a stated weight, configuration and load factor.
- Comparing POH stall speeds in IAS with TAS at altitude.
- Assuming an aft CG is always desirable: it lowers stall speed but reduces static margin and can worsen spin recovery.
- Forgetting that ground effect reduces induced drag and changes the takeoff/landing picture.