How Do Airplanes Fly?

What four forces act on an airplane in flight?

Four forces act on an airplane in flight: weight, lift, drag and thrust.^[1][2] How the airplane moves depends on the strength and direction of each.^[1]

When an airplane flies straight and level at a constant speed, lift balances weight and thrust balances drag.^[2] If the forces are not balanced, the airplane accelerates in the direction of the largest force.^[1] The balance shifts whenever the airplane climbs, descends, speeds up, slows down or turns.^[2]

How does a wing create lift?

Lift comes from the airplane's motion through the air, and the wings produce most of it.^[1] As air flows around a wing, its speed differs from place to place, and the pressure changes with it.^[3] Added up over the whole wing, that pressure gives the force from the air; lift is the part of the force at a right angle to the oncoming flow.^[3]

NASA describes the same process a second way: the wing turns the air flowing past it, and by Newton's third law, turning the flow produces a reaction force on the wing.^[3] Both the upper and the lower surface help turn the air.^[3]

People often argue over whether lift comes from a pressure difference (the "Bernoulli" view) or from deflecting air (the "Newton" view).^[3] NASA says both are correct, and either can be used to work out the force.^[3] The Smithsonian National Air and Space Museum weighs them differently: it favors the pressure explanation, with lower pressure above the wing and higher pressure below, and calls the downward-deflected air behind the wing an effect of lift, not its cause.^[4]

Why is the "equal transit time" explanation of lift wrong?

NASA warns that many explanations of lift in encyclopedias, physics textbooks and on websites are misleading and incorrect.^[3] The most popular incorrect one, it says, is the "equal transit time" or "longer path" theory.^[3]

The theory says a wing's upper surface is longer than its lower surface, so air going over the top must speed up to reach the trailing edge at the same time as the air underneath, and the faster air has lower pressure.^[3] According to NASA, the error is in the speed: on a real lifting wing, air over the upper surface moves much faster than equal transit times would require.^[3] Bernoulli's equation still gives the right force, but only when the correct speeds go into it.^[3]

NASA names two more incorrect ideas: one treats the wing as half of a Venturi nozzle, and the other compares lift to a stone skipping across water, ignoring the wing's upper surface.^[3] The real details of lift, it adds, are very complex and do not lend themselves to simplification.^[3]

What do an airplane's engines do?

The engines produce thrust, the force that overcomes drag.^[1] On NASA's example airliner, turbine engines under the wings push the airplane forward; smaller, slower airplanes use propellers.^[5] In a jet engine, hot gas rushes out of the back, and thrust is the reaction that pushes toward the front.^[1]

The engines do not hold the airplane up: NASA states that their job is just to overcome drag, and that the wings do the lifting.^[1] Gliders have no engines at all yet fly; some are towed aloft by a powered aircraft, then cut free.^[1] The wings only need motion: an airplane must be pushed through the air to generate lift.^[5]

How do pilots steer an airplane?

An airplane can rotate in three ways: roll (around its front-to-back axis), pitch (around its side-to-side axis) and yaw (around its vertical axis).^[6] Pilots control these with small hinged sections at the rear of the wings and tail; moving one changes the force that surface produces.^[5]

• Ailerons, on the outer rear edge of each wing, move in opposite directions, decreasing lift on one wing and increasing it on the other, so the airplane rolls.^[6]
• The elevator, the hinged part of the horizontal tail, tilts up or down, which tips the nose up or down.^[5][6]
• The rudder, the hinged part of the vertical tail fin, swings from side to side and pushes the tail left or right.^[5][6]
• Flaps, hinged sections at the rear of the wing near the body, are lowered on takeoff and landing to increase the force the wing produces.^[5]

To turn, a pilot uses the ailerons to tilt the wings in the desired direction, usually together with the rudder.^[6]

Why do speed and air density matter for takeoff?

To take off, an airplane needs thrust greater than drag and lift greater than weight.^[2] Lift and drag appear as soon as the airplane has speed, and because aerodynamic force depends on the square of the velocity, doubling the speed quadruples both.^[7][8]

Aerodynamic force is also proportional to the density of the air.^[7] The FAA's pilot handbook says less dense air reduces engine power, because the engine takes in less air; thrust, because a propeller is less efficient in thin air; and lift, because thin air exerts less force on the wings.^[8] Air is thin at high elevations, at low atmospheric pressure, in high temperatures and in high humidity.^[8]

In an FAA example based on one airplane's flight manual, the takeoff ground run at a pressure altitude of 5,000 feet may be 790 feet at standard temperature but closer to 1,000 feet when the air is 20 °C warmer than standard.^[8]