How Do Airplanes Stay in the Air? Lift Explained

An airliner is enormously heavy, yet it rises off a runway and stays up for hours. The force that holds it there is called lift, and it is one of the most often misexplained ideas in everyday science. NASA's Glenn Research Center, which publishes a long-running beginner's guide to aeronautics, says plainly that many explanations of lift found in encyclopedias, textbooks, and websites are misleading or incorrect.

Here is what NASA says lift is, which popular explanation does not work, and what actually happens.

1. What lift is

NASA defines lift as the force that directly opposes the weight of an airplane and holds it in the air. A few details from its definition matter:

• Every part of the airplane produces some lift, but on a normal airliner most of it comes from the wings.
• Lift is a force with a direction. It acts at right angles (perpendicular) to the direction of the airflow.
• Lift is a mechanical force. It comes from a solid object in direct contact with a moving fluid, such as air. It is not like gravity or magnetism, which can act across empty space.

2. Two rules: no air, no lift; no motion, no lift

NASA sums up two requirements.

No fluid, no lift. The wing must be in contact with air (or another fluid). NASA's example: the Space Shuttle did not stay in space because of lift from its wings, but because of orbital mechanics related to its speed. Space is nearly a vacuum, so wings there produce no lift.

No motion, no lift. There has to be a difference in speed between the wing and the air. It does not matter whether the wing moves through still air or air flows past a still wing; either one works. That is why airplanes need speed down a runway before they can take off.

3. The explanation that is wrong: "equal transit time"

The most popular incorrect theory, according to NASA, is the equal transit time or longer path theory. It goes like this:

1. The top of a wing is curved, so its surface is longer than the bottom.
2. Air molecules that split at the front must meet again at the back at the same time.
3. So air over the top must travel faster.
4. Faster air means lower pressure (Bernoulli's equation), so the wing is pushed up.

NASA's verdict: the problem is step 2. Nothing requires the two streams of air to meet at the same moment. In reality, the air over the top of a lifting wing moves much faster than equal transit time would predict. Because the theory starts from the wrong speed, it gives the wrong amount of lift, even though Bernoulli's equation itself is fine.

NASA lists two other common mistakes:

• Treating the wing as half of a Venturi nozzle (a narrowing tube) to estimate the air speed. A wing section is not really half a Venturi nozzle, so this also gives the wrong answer.
• Explaining lift with only the bottom of the wing deflecting air downward, like a stone skipping across water. This ignores the fact that both the top and bottom surfaces turn the airflow.

4. What actually happens: the wing turns the air

NASA's description of how lift is generated: lift occurs when a moving flow of gas is turned by a solid object. The wing turns the airflow in one direction, mostly downward, and lift appears in the opposite direction, following Newton's third law of action and reaction.

Both surfaces of the wing contribute to that turning. Leaving out the upper surface's role is exactly what leads to the skipping-stone mistake.

5. So who is right, Bernoulli or Newton?

Arguments about lift often split into two camps. NASA describes them as:

• The "Bernoulli" camp: lift comes from a pressure difference between the top and bottom of the wing.
• The "Newton" camp: lift is the reaction to the wing deflecting a flow of air.

NASA points out that neither Bernoulli nor Newton ever tried to explain the lift of a wing; their names are just labels.

NASA's answer: both camps are correct. As air flows around a wing, its speed changes from place to place, and Bernoulli's equation connects those speed changes to pressure changes. Adding up the pressure over the whole surface gives the aerodynamic force. Adding up the speed changes instead gives a net turning of the airflow, and Newton's third law turns that into the same force. They are two ways of describing one event.

NASA adds that the full details are genuinely complex. To understand them completely, you need equations that conserve mass, momentum, and energy in the flow at the same time, known as the Euler equations, or the Navier-Stokes equations if you include the air's viscosity.

6. Lift and drag

The total force air puts on a wing has two parts in NASA's terms:

The words "original flow direction" matter. NASA defines both parts relative to the direction the air was moving before it met the wing, not relative to the ground or to the airplane's nose.

7. Key takeaways

• Lift is the force that opposes an airplane's weight, and on airliners it comes mostly from the wings.
• Lift needs both air and motion; with no fluid or no speed difference, there is no lift.
• The popular "equal transit time" explanation is wrong because air over the top moves much faster than it predicts.
• A wing makes lift by turning the airflow, using both its upper and lower surfaces.
• The pressure view (Bernoulli) and the flow-turning view (Newton) are both correct descriptions of the same force.