Why a flat plate still flies
A common explanation for lift says air over the top of a curved wing has farther to travel, so it moves faster and pressure drops. It's tidy, and it's wrong. If it were true, a flat plate — with identical path lengths top and bottom — could never lift. Paper aeroplanes would be impossible.
They fly anyway. The reason is angle of attack: tilt a surface into the flow and it turns the oncoming air downward. By Newton's third law, pushing air down pushes the plate up.
Where the old story breaks
The "equal transit time" idea assumes two parcels of air that split at the leading edge must meet again at the trailing edge. They don't. The air over the top actually arrives sooner, not just faster — an outcome that only makes sense once you bring in circulation.
Lift is the reaction to a change in the momentum of the air. Anything that deflects flow downward generates it.
That's the thread I'll pull on in the next post: how circulation ties the pressure picture and the momentum picture together into one consistent account.