Principles of Flight: Lift, Drag and Stability
Lift and the four forces
Understand the four forces and how they balance in steady flight. Lift depends on air density, velocity squared, wing area and lift coefficient; the lift coefficient varies with angle of attack and reaches a maximum at the critical angle. Know that a stall is an angle-of-attack phenomenon, not a speed phenomenon, and that it can occur at any speed and any attitude.
Drag
Know the two components: parasite drag (form, skin friction, interference) which rises with the square of speed, and induced drag which falls with speed and rises with lift coefficient. Total drag is lowest at the speed where the two are equal - the minimum drag speed. Understand the drag curve and the region of reverse command behind the back side of the curve.
Stability and control
Know static and dynamic stability in each axis, and how the CG position affects longitudinal stability. Understand why dihedral gives lateral stability and how the fin provides directional stability. Know the effect of flaps on lift, drag and stall speed.
Traps
- Confusing the critical angle of attack with a specific speed.
- Forgetting that induced drag increases with mass at a given speed.
- Mixing up static stability (tendency to return) with dynamic stability (the manner of return).
Worked example
If speed doubles, parasite drag quadruples, but induced drag falls by roughly a factor of four. Total drag therefore depends on which component dominates at that speed. Near the minimum drag speed, a small speed change has little effect on total drag; far from it, the change is large.