Contents
In short: the story most of us learned says the top of a wing is longer, so the air over it must go faster to meet the air below at the trailing edge. NASA calls it one of the most widely circulated, incorrect explanations of lift. Nothing makes the two meet: in a wind tunnel the air over the top reaches the back long before the air below, and flat paper wings and upside-down planes lift too. What really happens: the wing's shape and tilt turn the air down, and the air pushes the wing up. Seen as pressure, it's lower above the wing and higher below: the same lift described two ways, so Bernoulli and Newton are both right.
The story most of us learned
NASA Glenn Research Center: "The theory described on this slide is one of the most widely circulated, incorrect explanations. The theory can be labeled the “Longer Path” theory, or the “Equal Transit Time” theory."
What it says: "The air molecules (the little colored balls on the figure) have farther to travel over the top of the airfoil than along the bottom. In order to meet up at the trailing edge, the molecules going over the top of the wing must travel faster than the molecules moving under the wing."
And it's everywhere. Prof. Holger Babinsky of the University of Cambridge: "You find it taught in textbooks, explained on television and even described in aircraft manuals for pilots."
Nothing makes them meet
NASA Glenn: "This part of the theory attempts to provide us with a value for the velocity over the top of the airfoil based on the non-physical assumption that the molecules meet at the aft end." And: "Two molecules near each other at the leading edge will not end up next to each other at the trailing edge".
In a wind tunnel, the top air arrives first
The University of Cambridge, in 2012: "Babinsky filmed pulses of smoke flowing around an aerofoil … When the video is paused, it’s clear that the transit times above and below the wing are not equal: the air moves faster over the top surface and has already gone past the end of the wing by the time the flow below the aerofoil reaches the end of the lower surface."
NASA Glenn says the same: "particles moving over the top arrive at the trailing edge before particles moving under the airfoil."
Flat wings and upside-down planes lift too
NASA Glenn: "Think of a paper airplane. Its airfoil is a flat plate –> top and bottom exactly the same length and shape and yet they fly just fine."
And: "This theory also does not explain how airplanes can fly upside-down which happens often at air shows and in air-to-air combat. The longer surface is then on the bottom!"
What really happens: the wing turns the air
NASA Glenn, "What is Lift?": "Lift occurs when a moving flow of gas is turned by a solid object. The flow is turned in one direction, and the lift is generated in the opposite direction, according to Newton’s Third Law of action and reaction." And: "For an aircraft wing, both the upper and lower surfaces contribute to the flow turning."
What does the turning is the wing's shape and its tilt. NASA Glenn: "The magnitude of the lift generated by the wing depends on the shape of the wing and how it moves through the air." And: "The angle between the chord line of the airfoil and the flight direction is called the angle of attack. Angle of attack has a large effect on the lift generated by a wing."
Babinsky: "What actually causes lift is introducing a shape into the airflow, which curves the streamlines".
Seen as pressure, the same lift
Babinsky describes the result as "lower pressure on the upper surface and higher pressure on the lower surface".
The myth is right that the top air is faster; it just gets the speed wrong. NASA Glenn: "Experiment #1 shows us that the flow over the top of a lifting airfoil does travel faster than the flow beneath the airfoil. But the flow is much faster than the speed required to have the molecules meet up at the trailing edge." And: "The upper flow is faster and from Bernoulli’s equation the pressure is lower. … this part of the theory is correct."
Bernoulli and Newton are both right
NASA Glenn, "Bernoulli and Newton": "From Newton’s third law of motion, a turning action of the flow will result in a re-action (aerodynamic force) on the object. So both “Bernoulli” and “Newton” are correct."
And: "Newton’s laws of motion are statements concerning the conservation of momentum. Bernoulli’s equation is derived by considering conservation of energy. So both of these equations are satisfied in the generation of lift; both are correct."
Takeaway
- The longer-path story is wrong: nothing makes the air over and under the wing meet again, and the top air gets there first.
- Lift comes from turning air: shape and tilt bend the flow down, and the air pushes the wing up.
- Lower pressure above, higher below is the same lift seen another way: Bernoulli and Newton are both right. That's why flat wings and upside-down planes fly too.
Sources
- NASA Glenn Research Center, Beginner's Guide to Aeronautics, Equal Transit Theory Interactive (incorrect lift theory)
- NASA Glenn Research Center, What is Lift?
- NASA Glenn Research Center, Bernoulli and Newton
- NASA Glenn Research Center, Effects of Inclination on Lift Interactive
- University of Cambridge, How wings really work (Prof. Holger Babinsky, Department of Engineering), 25 Jan 2012
The video is animated with code and the script was written with AI assistance; every quote above is checked against its source. Illustrative: the air flow (the streaks, the smoke pulse, the pressure shading and the arrows) is computed from a textbook ideal-flow model, not measured, so it shows the pattern, not a real wing's numbers; the smoke pulse is our drawing of what Babinsky's wind-tunnel smoke shows, not his footage; the planes are generic drawings. The video is narrated in English. Made with Shotloop, my free Mac app for code-made explainers, fixed one shot at a time.
