the equal transit time story is wrong; wings push air down and pressure difference does the rest
A plane stays up because its wings are angled and shaped to deflect the oncoming air downward. By Newton's third law, the downward push on the air produces an equal and opposite upward push on the wing.
The wing does not need to hit the air like a paddle. As the wing moves, the flow bends around it. The upper surface curves the air down over the top; the lower surface, especially when the wing is tilted up at a positive angle of attack, pushes air down directly. The air leaves the wing with a downward component of velocity. Changing the air's momentum requires a force, and the wing provides it. The air provides the reaction force on the wing. That reaction force is distributed over the wing's surface as a pressure difference: lower pressure on top, higher pressure on the bottom. The pressure difference is not an alternative explanation; it is the same force viewed locally.
Consider a 1,000 kg airplane flying at 70 m/s. Its weight is about 9,800 N. In one second, the wing sweeps through a large tube of air. For illustration, suppose the wing effectively influences an area of about 50 m². The mass of air passing through that area each second is density times speed times area: 1.2 kg/m³ × 70 m/s × 50 m² = 4,200 kg. To produce 9,800 N of upward force, the wing must give this air a downward velocity change of 9,800 / 4,200 ≈ 2.3 m/s. So a small downward deflection of a very large mass of air is enough to support the plane. This also explains why speed matters: at higher speed, more air flows past each second, so the same lift requires an even smaller deflection.
The old "equal transit time" story claims air over the top must travel farther in the same time, so it goes faster and creates lower pressure. That is false: the air over the top does not take the same time as the air below, and a flat plate tilted upward also produces lift without any curved upper surface. The real relationship is that a pressure difference exists, but it is caused by the wing bending the flow, not by a magical speed gap. Fast air does not automatically mean low pressure; it depends on whether the streamlines are curving.
The momentum balance is always true in steady flight, but it does not predict when the flow separates. At a high angle of attack, the smooth downward deflection breaks down, the wing stalls, and lift drops sharply. In supersonic flight, shock waves compress the air and set the pressure distribution in a different way, though the wing still pushes air down. And in a gust, the lift changes before the downstream wake settles, so the simple steady argument is only a starting point.
Cram School said air on top takes a longer path, so it moves faster and lifts the plane. Right?
Rep It is the most repeated myth in aviation. Nothing forces the top and bottom flows to reunite behind the wing.
Cram So where does lift actually come from?
Rep From the wing pushing air down. Newton's third law says every push gets an equal push back, so the wing is shoved upward.
Cram Pushing air down? The wing never touches most of that air.
Rep It does not need to. The wing carries a pressure field with it, and the whole flow around it bends to follow the shape.
Cram Then why is the top curved, if not for faster air?
Rep Curve and tilt together steer the flow. Air leaves a lifting wing angled downward, and that downwash is the visible sign of the push.
Cram So a flat board held at an angle would fly too?
Rep Up to a point. Tilt adds deflection, but push the angle too far and the flow breaks away. That is a stall, and lift collapses.
Cram Is the pressure explanation totally wrong then?
Rep Not useless, just backwards. Low pressure on top is real, but it exists because the wing turns the flow, not because air races along the top.
Cram How strong is this push at cruising speed?
Rep In level flight it equals the weight of the whole aircraft. Speed and angle tune it, which is how slow heavy jets stay up.
Cram Give me the one sentence version.
Rep A wing is a device for throwing air downward. The air pushes back, and that reaction is what we call lift.