Lift from pressure and downwash
Every wing makes lift two ways at once; neither alone is the full story (TP 1102):
| Mechanism | Principle | Action |
|---|---|---|
| Pressure differential | Bernoulli | Faster air = lower pressure (PHAK) |
| Deflected airflow | Newton's third law | Wing pushes air down and back; air pushes wing up and forward |
Bernoulli, pressure differential:
| Surface | Airflow | Pressure |
|---|---|---|
| Above | Wing tilted into airflow; air over curved top forced faster than below | Low |
| Below | Air almost stopped at stagnation point near leading edge, then speeds back up toward trailing edge; pressure higher where slowed | High |
- High below + low above = wider differential = more lift.
- Curved upper surface gives much more lift than flat plate; aerofoil evolved from flat plate to curved (TP 1102).
Newton's third law, downwash:
- Air pushed down and back off trailing edge = downwash; equal and opposite reaction pushes wing up and forward.
Trap: "top surface longer, so air must arrive together, hence faster, hence lower pressure" is incomplete; symmetrical airfoils and flat plates, with no longer top surface, still produce lift.
References
- TP 1102 ch.1 Lift
- PHAK ch.4 Principles of Flight