How do airplane wings generate lift?
A wing turns the air flow and changes its pressure, so pressure is lower above and higher below, producing an upward force. The popular 'equal transit time' explanation is wrong.
AI-drafted from web search results, then checked by a second, independent AI that read 4 of its 5 cited pages in full and found the thread supported after one error it found was corrected. No person has reviewed it.
The short answer
A wing generates lift by being shaped and tilted so that it curves the airflow around it. This changes the speed and pressure of the air, leaving lower pressure on the upper surface and higher pressure on the lower surface. The net result is an upward force. Another way to describe the same event is that the wing turns the air downward, and the reaction force pushes the wing up. [1] [2] [3]
Pressure and speed
The Smithsonian's How Things Fly site explains the Bernoulli principle: when a fluid speeds up, the pressure it exerts drops. When moving air meets a wing, its path narrows around the object, and because the same amount of air must pass each point, the air speeds up. The site adds that some energy of the random motion of molecules, which is what produces air pressure, is converted into forward stream flow, so pressure falls. A wing is shaped and tilted so that air above moves faster than air below, leaving less pressure on top and more beneath. [3]
The AOPA pilot guide gives a similar account, describing a low-pressure zone atop the wing and relatively higher pressure below, with the difference producing lift. [4]
Turning the flow
NASA's Glenn Research Center says lift occurs when a solid object turns a flow of gas. The flow is turned in one direction and lift is generated in the opposite direction, following Newton's Third Law. Both the upper and lower surfaces contribute to the turning. Cambridge's Professor Holger Babinsky likewise says that what causes lift is introducing a shape into the airflow, which curves the streamlines and introduces pressure changes. [2] [1]
Lift needs both air and motion between the wing and the air. It makes no difference whether the object moves through still air or the air moves past a fixed object, as with a kite. Without air, as in space, wings produce no lift. NASA says the lift equation depends on the square of the velocity, the surface area, the shape of the object, its inclination to the flow, and the properties of the air. [2]
A common myth: equal transit time
A popular explanation says the upper surface is longer, so air over the top must travel farther and faster to reach the trailing edge at the same moment as air below. NASA calls this incorrect. The Wright brothers' wings were thin curved plates with equal distance on the upper and lower surfaces, yet they created plenty of lift. [2]
Babinsky filmed pulses of smoke around an aerofoil and showed that transit times are not equal: the air over the top has already passed the trailing edge before the lower flow arrives. He notes the myth appears in textbooks, on television and in pilot manuals. He also points out that a sail, which has equal distances on each side, produces lift because it is slightly curved, so curvature rather than distance matters. [1]
Some simplified sources, including the AOPA guide, describe the split airflow meeting again at the trailing edge, which is the equal-transit idea. The faster air over the top of the wing is real; the myth is the reason given for it. NASA notes that many published explanations of lift are misleading. [4] [2]
Angle of attack and stalls
The National Park Service notes that curvature, the ratio of wingspan to chord, and the angle of attack all affect lift. A greater angle gives more lift up to a point. Beyond a certain angle the smooth flow over the wing becomes turbulent and a stall occurs, meaning lift is lost. At higher speeds, less angle is needed for the same lift. [5]
Sources
- How wings really work (cam.ac.uk) Found by source search. Not checked by a person.
- Lift | Glenn Research Center | NASA (www1.grc.nasa.gov) Found by source search. Not checked by a person.
- Air in Motion | How Things Fly (howthingsfly.si.edu) Found by source search. Not checked by a person.
- The Four Forces - AOPA (aopa.org) Found by source search. Not checked by a person.
- The Fundamentals of Flight (U.S. National Park Service) (nps.gov) Found by source search. Not checked by a person.
Questions to follow next
- Why do wings stall at high angles of attack?
- How can planes fly upside down?
- How do the Bernoulli and Newton explanations fit together?