How do planes stay up?

A plane stays up because its wings bend the oncoming air downward. The wing's shape and its tilt, called the angle of attack, turn the flow, and that turning goes hand in hand with lower pressure above the wing and higher pressure below. The pressure difference and the downward push on the air are two descriptions of one event, and done properly they give the same lift.

What does a wing actually do to the air?

Stick your flat hand out of a car window and tilt the front edge up a little. Your hand gets shoved upward and the air behind it gets deflected down. That's lift in its crudest form.

NASA's Glenn Research Center defines lift as the force that opposes an airplane's weight and holds it in the air, and says it happens when a moving flow of gas is turned by a solid object. For a wing, both the upper and lower surfaces help turn the flow. The wing pushes air down, and by Newton's third law the air pushes the wing up. NASA adds two blunt conditions. No fluid, no lift. And no relative motion, no lift, because lift comes from the difference in velocity between the wing and the air.

In steady, level flight, the FAA's Pilot's Handbook notes that the upward forces balance the downward ones, so lift matches weight. A heavy airliner cruising at constant altitude is continuously turning a large mass of air toward the ground.

Is it pressure or is it Newton?

Explanations online often argue about this. NASA's page on the subject says both "Bernoulli" and "Newton" are correct.

You can find the force on a wing by adding up the pressure acting on every bit of its surface. Or you can add up how the wing changes the air's velocity, which shows a net turning of the flow, and get the force as the reaction to that turning. Same wing, same air, same force. It's like checking a bank transfer from the sender's statement or the recipient's. You are counting the same money.

Holger Babinsky of the University of Cambridge describes the mechanism in terms of curvature. The wing's shape bends the streamlines of air flowing around it, and bending the flow creates pressure differences, lower above the wing and higher below. A thin sail, he points out, lifts once it is slightly curved. In his words, "it's the curvature that creates lift, not the distance."

Air over the wing doesn't have to meet up again

You may have learned this one in school. The top of a wing is more curved, so air going over it travels farther. The air on top has to meet the air underneath at the back edge, so it must go faster, and faster air has lower pressure. Lift.

The last step is fine. Faster flow over the top really does go with lower pressure, and NASA says so. The problem is the middle. NASA lists several ways the story fails.

• Air molecules that start side by side at the front of the wing don't end up next to each other at the back. Air going over the top reaches the trailing edge before air going underneath.
• A symmetric wing, with top and bottom the same length, still produces plenty of lift when tilted.
• If you assume equal transit times and calculate the speed, the predicted lift is much smaller than what real wings produce, because the computed speed is too low.
• It can't explain how planes fly upside down.

You can watch it fail. In 2012 Cambridge released a short wind tunnel film of smoke pulses flowing around a wing, and when you pause it, the smoke over the top is clearly ahead. Writing in Scientific American in 2020, Ed Regis put it simply. There is no physical reason the two parcels of air must arrive together.

The popular demonstration of blowing over a strip of paper so it rises has its own problems. Babinsky points out that if you hang a sheet of paper vertically and blow down one side, it doesn't move, because the pressure on both sides is the same despite the obvious difference in air speed.

Angle of attack is the control knob

The angle of attack is the angle between the wing's chord line, a straight line from its front edge to its back edge, and the direction the plane is moving. NASA notes that lift rises almost in a straight line with angle of attack for small angles, within about plus or minus 10 degrees. A symmetric wing makes no lift at zero angle. A cambered, curved wing makes a little even at zero.

This is why pilots can change lift without changing the wing. The FAA handbook explains that moving the control yoke fore or aft changes the angle of attack. As a plane slows down, the angle of attack has to increase to keep lift equal to weight. It is also how aerobatic planes fly inverted. Tilt the wing so it meets the air at a useful angle, and the flow still gets turned toward the ground.

What is a stall, really?

Not an engine problem. A stall is an aerodynamic event. Keep increasing the angle of attack and eventually the air can no longer follow the wing's upper surface. NASA describes the boundary layer, the thin layer of air next to the surface, separating from the wing, which is why wings lose lift abruptly at high angles. NASA also says predicting the exact stall angle is very difficult mathematically, so engineers rely on wind tunnel tests.

The FAA defines a stall as a rapid decrease in lift caused by airflow separating from the wing after it exceeds the critical angle of attack, and makes two points worth remembering. A stall can happen at any pitch attitude or airspeed, since it depends on angle, not speed. And, correcting a belief the handbook says many pilots hold, the wing doesn't stop producing lift entirely. It just can't make enough to sustain level flight. The handbook calls stalls "one of the most misunderstood areas of aerodynamics."

Press Play and raise the angle until the upper flow separates. The paths show the idea of turning air and losing attached flow, not a computed pressure field or a universal stall angle.

So does anyone actually understand lift?

Scientific American ran that 2020 article under the headline "No One Can Explain Why Planes Stay in the Air," which is catchier than it is fair. The same article says there is little, if any, serious disagreement about the equations or their solutions, and the computational fluid dynamics built on those equations underpins modern aircraft design.

What experts argue about is the plain-language story. Doug McLean, one of the experts in the piece, sees four things happening together that depend on one another. The flow turns downward, the air speeds up over the top, the pressure drops above the wing and rises below it. None of them comes first. It also quotes John D. Anderson, who wrote in 2003 that "There is no simple one-liner answer to this." The equations are settled. The argument is over which one-sentence summary leaves out the least.

Curved tops, bird wings and the Wright brothers

The question underneath all this is why air follows the curved top of a wing instead of shooting straight past it. After that, birds make lift in their own way, and the Wright brothers had to work it out with no textbook. Learnfold can take any of those as a starting question, show you a course plan, and write a fresh linked page for each follow-up, and sharper questions get sharper pages. Aerodynamics attracts confident wrong answers, AI-written ones included, so check its explanations, or come back to the ground with how GPS knows where you are.