A plane's wings are shaped and tilted to turn the oncoming air downward, and the air pushes the wings up in return. The same turned flow leaves lower pressure above the wing and higher pressure below it, and NASA treats the pressure account (Bernoulli) and the air-turning account (Newton) as two correct descriptions of one flow. The engines supply the forward speed, and the wings use it to make lift.

What keeps a plane in the air?

A plane is held up by lift from its wings, which NASA describes as the force that directly opposes the weight of an airplane and holds it in the air. Lift exists only while air flows past the wing ("no motion, no lift," NASA says), so in flight the engines keep producing thrust to overcome drag. When the four forces balance, the plane cruises at a constant velocity.

Why does air move faster over the top of a wing?

In the account Cambridge's Holger Babinsky and MIT's Mark Drela give, a wing curves the passing air, which lowers the pressure above the wing and raises it below, and that lowered pressure speeds up the air arriving over the top. That air is not racing to meet the air underneath, and in Babinsky's wind-tunnel smoke film it reaches the back of the wing first.

How can a plane fly upside down?

A plane flies upside down by meeting the oncoming air at an appropriate angle of attack, which Scientific American says curved, symmetrical and even flat wings can all do. The equal transit theory cannot explain inverted flight, which NASA says happens often at air shows and in air-to-air combat.

Is it true that no one knows how planes fly?

It is true only in a narrow sense. Scientific American reports little, if any, serious disagreement about the equations of lift, which aircraft designers solve with computational fluid dynamics. What aerodynamicists such as MIT's Mark Drela and Cambridge's Holger Babinsky still dispute is the simplest correct way to explain lift in plain words.

How Do Planes Fly? The Air on Top Arrives First

The explanation fits in one drawing. It shows a wing in cross-section, curved on top and nearly flat underneath, with two parcels of air arriving at its front edge side by side. One goes over, one goes under, and the pair are due to meet again at the back edge.

The top path is longer, the drawing says, so that parcel must travel faster to arrive on time, and faster air has lower pressure. The wing is held up by the pressure difference.

Holger Babinsky, a Cambridge aerodynamicist, says the explanation turns up in textbooks, on television and even in aircraft manuals for pilots. When he describes it to school audiences and asks who has heard it, “every time 95% of the audience puts their hand up.” The drawing fails at its first step. Nothing makes the two parcels meet, and in Babinsky’s wind-tunnel film the one over the top arrives early.

How do planes fly?

A plane flies because its wings are shaped and tilted to turn the oncoming air downward, and the air pushes the wings up in return. That turned flow also leaves lower pressure above the wing than below it, which is the same push described another way. The engines supply the forward speed, and the wings use it to make lift.

Lift also needs air flowing past the wing, or in the words of NASA’s guide to lift, “no fluid, no lift” and “no motion, no lift.”

Equal transit is a myth, and a smoke film shows it

The drawing’s idea is known as the equal transit time theory, and NASA’s Beginner’s Guide to Aeronautics calls it “the most popular incorrect theory of lift.” Babinsky filmed pulses of smoke flowing around a wing section in a wind tunnel. When the film is paused, Cambridge reports, the air over the top “has already gone past the end of the wing” while the air underneath is still moving along the lower surface.

Babinsky’s 2003 paper in Physics Education grants that the popular explanation is “common, quick, sounds logical and gives the correct answer,” since the air over the top really is faster and the pressure there lower. Part of the trouble is the route to that answer, which “uses a nonsensical physical argument and misleadingly invokes Bernoulli’s equation.”

Equal transit also fails on the numbers. The real air over the top moves much faster than the theory allows, so in NASA’s words it predicts “much less than the observed lift, because the velocity is too low.”

Flat wings and upside-down planes fly anyway

Plenty of wings fly without the longer top surface equal transit needs. A sail is the same length on both sides and still lifts, because it is curved when rigged. As Babinsky puts it, “it’s the curvature that creates lift, not the distance.” Paper planes, whose wings are flat sheets, fly well enough too, as NASA points out.

What a flat wing needs is an angle. A symmetric wing meeting the air level-on makes no lift, according to NASA’s page on inclination.

Tilt the wing into the oncoming air, raising its angle of attack, and lift rises almost in step with the angle at first. Tilt it too far and the airflow separates from the wing, which then loses lift abruptly. That is a stall.

Equal transit cannot explain upside-down flight at all. Flip a curved wing over and, by the drawing’s own rule, the faster air and low pressure move underneath, which should pull the plane downward, as Scientific American points out.

Yet NASA notes that inverted flight is common at air shows and in air-to-air combat. Curved, symmetrical and even flat wings can all fly inverted, Scientific American explains, “so long as the airfoil meets the oncoming wind at an appropriate angle of attack.”

Bernoulli and Newton describe the same flow

With equal transit gone, the argument left is Bernoulli against Newton: a pressure difference across the wing, or the reaction to air the wing turns downward. NASA’s guide puts both names in quotation marks, because neither man ever tried to explain aerodynamic lift.

Add up the pressure over the wing’s whole surface and you get the aerodynamic force. Add up how the wing turns the flow and, by Newton’s third law, you get it again. “So both ‘Bernoulli’ and ‘Newton’ are correct,” the guide concludes.

	The pressure account (“Bernoulli”)	The turning account (“Newton”)
What it cannot explain alone	Why the air over the top speeds up	Why the pressure above the wing drops
Its broken version	Equal transit	The “skipping stone” theory
NASA’s verdict	Correct	Correct

Verdicts and broken versions are from NASA’s guide, and the missing pieces are from Scientific American.

NASA rejects the skipping stone theory, which counts only the air striking the underside, because both surfaces of a wing help turn the flow. Trading equal transit for air bouncing off the bottom swaps one error for another, as the standard correction to the blue-veins myth also does.

Double the speed and the lift quadruples

Angle is one input to NASA’s lift equation, and because the equation squares the speed, “doubling the velocity will quadruple the lift and drag,” as the guide puts it. Slow flight cuts lift just as sharply, so on the approach, through an airliner’s rounded windows, you can watch flaps slide out of the wing’s back edge and slats out of its front. Both enlarge the wing and curve it more, which NASA’s page on flaps and slats says raises the lift.

The physics is settled, and the one-liner is not

Scientific American’s February 2020 headline, “No One Can Explain Why Planes Stay in the Air,” promises more mystery than the article delivers. It reports “little, if any, serious disagreement” over the equations, which aircraft designers solve with computational fluid dynamics. The controversy lies in the plain-language story.

Doug McLean, a longtime Boeing engineer, breaks lift into four parts: air turned downward, air sped up over the top, low pressure above the wing and high pressure below it. In McLean’s own words, quoted by Scientific American, they “support each other in a reciprocal cause-and-effect relationship, and none would exist without the others.”

Even the best short answers draw objections. Mark Drela, a professor of fluid dynamics at MIT, says parcels of air follow the curved top because if they flew off in a straight line, “there would literally be a vacuum created below them.”

Babinsky replies that if a vacuum were the explanation, it would be “hard to explain why sometimes the flow does nonetheless separate from the surface.” Drela “is correct in everything else,” he adds, but “there is no quick and easy explanation.”

The two parcels in the classroom drawing never had an appointment: the one that goes over the top reaches the back of the wing first. The wing stays up by turning air downward, and what the experts still argue over is how to explain that in a single sentence. Strictly FYI.