Lift equation and density
Lift equation: L = CL · ½ρV² · S
| Symbol | Meaning |
|---|---|
| CL | Lift coefficient, set by angle of attack |
| ρ | Air density |
| V | Airspeed |
| S | Wing area |
Speed and AoA:
- Lift rises with the square of airspeed: double speed, four times lift.
- Level flight trades AoA for airspeed: large AoA at low speed gives the same lift as small AoA at high speed.
- Speed up: AoA must come down. Slow down: AoA must go up.
- Speed up without reducing AoA: extra lift climbs or accelerates the aircraft.
Density (ρ):
- At 18,000 ft air is about half sea-level density.
- Same AoA then needs much greater true airspeed for the same lift.
- Falling density: airspeed or AoA must rise to keep lift equal to weight.
Trap: assuming lift rises in step with speed. It rises with the square: double speed, four times lift.
References
- TP 1102 (Flight Training Manual)