Flight
Four forces, and the one that has to be explained carefully.
Flight requires managing four forces. Lift acts upward, weight downward, thrust forward and drag backward. Steady level flight is lift equal to weight and thrust equal to drag.
How a wing lifts
A wing deflects air downward. By Newton's third law, pushing air down pushes the wing up. This happens through two mechanisms working together: the angle of attack deflects oncoming air, and the curved upper surface causes flow to follow the camber and leave the trailing edge angled downward.
The common classroom explanation — that air must travel further over the top and therefore moves faster, generating lower pressure — is wrong. There is no principle requiring the two flows to arrive together, and they do not. Bernoulli's principle does describe the pressure difference correctly, but the equal-transit-time story that usually accompanies it does not. Symmetrical wings and inverted flight both work, which the faulty version cannot account for.
Control
| Surface | Axis | Motion |
|---|---|---|
| Ailerons (wings) | Longitudinal | Roll |
| Elevators (tail) | Lateral | Pitch |
| Rudder (fin) | Vertical | Yaw |
Stalling
Beyond a critical angle of attack, airflow separates from the upper surface and lift collapses. A stall is about angle, not speed — an aircraft can stall at any speed and any attitude if that angle is exceeded. Recovery means reducing the angle of attack, which usually means lowering the nose.