Lift and drag
Overview
Lift and drag tests the lift curve, the drag polar and the speed for best L/D. You should be able to read C_L–α and drag curves and explain what happens at C_Lmax.
Core rules
- C_L increases nearly linearly with AoA until the critical AoA (typically 15–18°), where C_Lmax occurs and the wing stalls.
- Total drag C_D = C_D0 + kC_L². Parasite drag varies with configuration; induced drag varies with C_L².
- L/D is maximum when C_Di = C_D0. That occurs at C_L = √(C_D0/k), and (L/D)max = 1/(2√(kC_D0)).
- Best glide angle is flown at (L/D)max; best glide range follows from that speed adjusted for wind and weight.
- Flaps increase C_Lmax and C_D0, lowering stall speed but increasing drag.
Worked example
C_D0 = 0.020, k = 0.050. C_L at best L/D = √(0.020/0.050) = √0.4 = 0.632. (L/D)max = 1/(2√(0.050×0.020)) = 1/(2×0.0316) = 15.8. Any other C_L gives a lower ratio.
Traps
- C_Lmax is fixed for a configuration, but the speed at which it occurs depends on weight and density.
- Minimum drag speed is not the same as minimum power speed or best-range speed.
- Induced drag falls with speed, so total drag has a minimum at one specific speed.