How airplanes fly: lift and a common myth

The four forces on an airplane

Four forces act on an airplane in flight: lift holding it up, weight pulling it down, thrust pushing it forward and drag holding it back. Engines produce thrust; they don't lift the plane directly. As the engines push the airplane forward, the wings slice through the air, and that motion is turned into lift. In steady flight at constant height and speed, lift balances weight and thrust balances drag. So asking how an airplane stays up is really asking how a wing produces lift.

The popular equal transit time story

The usual explanation goes like this: the top of the wing is curved and longer than the bottom, air that splits at the front must meet again at the back at the same moment, so the air on top has to move faster, and faster air has lower pressure, which pushes the wing up. It sounds convincing, but it's wrong. There is no physical reason the split air has to reunite at the trailing edge. Smoke in a wind tunnel shows that air over the top reaches the trailing edge well before the air underneath. And if you calculate the speed difference this theory predicts, you get far too little lift to hold up an airliner.

Evidence that breaks the myth

You can see the equal transit idea fail in everyday examples. If it were true, a wing whose top surface isn't longer couldn't produce lift, but that's simply not what happens. A wing's curved shape helps it make lift efficiently; it isn't a requirement for lift.

• Symmetric wings, identical top and bottom, still produce lift
• Aerobatic planes fly level while upside down
• Paper airplanes with flat wings fly just fine
• In wind tunnels, air over the top reaches the trailing edge first

What really happens: turning air downward

A wing meets the air tilted slightly nose-up, and because of that angle and its shape, air leaving the wing is deflected downward. When the wing pushes air down, action and reaction mean the air pushes the wing up. Look at the same thing in terms of pressure and the curved flow creates a region of lower than surrounding pressure above the wing and slightly higher pressure below. That pressure difference, added up over the whole wing area, is lift. The faster air over the top is something that appears along with the lower pressure, as a result of it.

Bernoulli versus Newton is the wrong question

You often see arguments over whether lift should be explained by Bernoulli's principle or Newton's laws of motion. They aren't competing explanations. Turning air downward and having a pressure difference across the wing are the same phenomenon seen from two directions, and done properly, both give the same answer. Bernoulli's principle itself is correct: within a flow, where speed is higher, pressure is lower. What's wrong is the invented reason for why the top air speeds up, the claim that it has to arrive at the same time. The speed and pressure pattern is set by the wing's shape and the angle at which it meets the air.

Angle of attack and stall

The angle between the wing and the oncoming air is called the angle of attack. Increasing it turns more air downward and increases lift, but only up to a point. Beyond a certain angle, the air flowing over the top breaks away from the wing surface and swirls, and lift drops suddenly. That's a stall. A stall has nothing to do with the engine stopping; it's about the angle between wing and air. That's why pilots keep in mind that the angle of attack grows as the plane slows, and why aircraft carry systems that warn when a stall is getting close.

What determines lift

The amount of lift depends on a few factors. Speed counts squared, so with everything else equal, doubling the speed gives roughly four times the lift. That's why at hot or high-altitude airports, where the air is thin, a plane needs more speed to get the same lift and takes a longer run to take off. Planes take off and land into the wind whenever possible for the same reason: it keeps their speed through the air up while lowering their speed over the ground.

• Air density: thinner air means less lift
• Speed: lift grows with the square of the speed through the air
• Wing area: a larger wing makes more lift
• Wing shape and angle of attack: how efficiently the wing turns air at a given speed

Why the wings change shape for takeoff and landing

During takeoff and landing the plane is slow, and the wing in its cruise shape can't make enough lift. So flaps at the back of the wing and slats at the front extend to give the wing more curve and area. From a window seat you can watch the back of the wing stretch out and bend down during takeoff and landing; that's what you're seeing. Once the plane is high and fast, they retract again to cut drag. At the roughly 10 km where airliners cruise, air density is only about a third of that at sea level, but high speed makes up for it, and the thin air also lowers drag and saves fuel. In short, an airplane flies by turning air downward with its wings and riding the reaction, and you can safely forget the equal transit story.