An airplane's wings make lift, the force that holds the plane up in the air. Lift comes from air pressure, which is a push made by air. A wing helps air push up on the plane. Almost all of an airliner's lift comes from its wings, though any part of the plane can make some lift. A wing's angle of attack, the tilt of the wing to the oncoming air, makes lift by turning the flow of air.

The two famous explanations of lift are both correct. In the Bernoulli view, the wing makes a pressure difference. In the Newton view, the wing turns a flow of air and the air pushes back. Adding up either the pressure or the velocity around the wing gives the same aerodynamic force. People who argue about lift usually fall into two camps, one for pressure and one for the reaction force on a body that deflects a flow of gas. Neither Isaac Newton nor Daniel Bernoulli ever attempted to explain the aerodynamic lift of an object.

There is a third idea, and it is wrong. The equal transit time idea says air going over the top of a wing must meet the air from underneath at the back edge, so the top air must go faster. NASA Glenn calls this the most popular incorrect theory of lift. NASA Glenn warns that many explanations of lift in encyclopedias, on web sites, and even in some textbooks are incorrect and confuse students. The speed the idea predicts is too low: the real speed on top of a lifting wing is much higher than the speed that would give an equal transit time.

The equal transit idea has other problems. Wings that are the same shape on top and bottom still make plenty of lift, and their top and bottom surfaces are the same length. A paper airplane has a flat wing, with a top and a bottom the same length and shape, and it still flies. The idea also cannot explain how airplanes fly upside down, because the longer path would then be on the bottom. What matters is the turning of the air, not the distance it travels.

Air pressure under a lifting wing is greater than the pressure above it, and the difference pushes the wing up. Lift can be explained in two ways that describe the same force: the difference in air pressure above and below the wing, and the wing pushing air downward. Lift is made when a wing turns the air: the air is bent in one direction and the wing is pushed in the opposite direction, as Newton's third law says. A wing pushes air downward, and the more downwash a wing makes, the harder it pushes the mass of air down.

In steady, level flight at a constant speed, thrust equals drag and lift equals weight. The weight a plane must lift includes the plane, the fuel, the passengers and the cargo. A plane has to move for its wings to make lift, and a plane that is not moving through the air makes no lift.

No fluid means no lift. In space, where there is almost no air, wings make no lift. The atmosphere gets thinner with height: air pressure and air density both go down as altitude goes up. Aerodynamic forces depend directly on the density of the air. Earth's atmosphere is a thin layer. If Earth were the size of a basketball, a tightly held pillowcase would be as thick as the atmosphere.

A jet engine takes in a lot of air and adds only a little fuel. In a typical jet engine, 100 pounds of air per second is mixed with only 2 pounds of fuel per second, so most of the hot exhaust comes from the surrounding air. The turbine works like a windmill: the hot exhaust spins its blades, and a central shaft uses that energy to turn the compressor at the front. Most modern airliners use turbofan engines because of their high thrust and good fuel efficiency.

Otto Lilienthal of Germany designed a glider that could fly a person. He wrote a book on aerodynamics, published in 1889, that the Wright brothers used as the basis for their designs. NASA says the first aircraft to achieve complete active control was the Wright brothers' 1902 glider.

The Wrights needed steady winds of 15 to 20 mph, hills for launching, soft areas for landing and privacy for their tests. They found them at Kitty Hawk, North Carolina, which Weather Bureau records showed was the sixth-windiest station in the country.

The Wright brothers took turns piloting four flights on December 17, 1903. On that day, the first flight reached a top speed of only 6.8 miles per hour. It lasted only 12 seconds and went 120 feet. The longest flight that day went 852 feet in 59 seconds.

Words to know

• lift: holds planes up
• pressure: air's push
• downwash: air pushed down
• turbine: spinning engine part