Lift

What is lift?

Lift is the force that directly opposes an airplane’s weight and holds it in the air. It acts at right angles (perpendicular) to the direction of the airflow.^[1] Most of an airliner’s lift comes from its wings, although every part of the plane contributes a little.^[1]

How a wing makes lift

Lift appears when a moving flow of air is turned by a solid object.^[1] A wing moving through the air deflects the flow downward. By Newton’s third law, the air pushes back on the wing in the opposite direction, which is upward.^[2] Both the top and the bottom surfaces of the wing help turn the flow.^[1]

You can describe the same event with pressure. As the air’s speed changes around the wing, so does its pressure: faster flow means lower pressure (Bernoulli’s principle).^[5] Added up over the whole wing, those pressures give the same lift force. The “Newton” and “Bernoulli” explanations are both correct. They are two views of one physical process, which in full detail is very complex.^[3]

Two things lift can’t do without

• Air (or another fluid). No fluid, no lift.
• Motion. The wing and the air must move relative to each other. No motion, no lift.^[1]

The lift equation (optional)

Engineers calculate lift with a compact formula:^[6]

L = C_L × ½ ρ V² × A

• L is the lift force.
• ρ (rho) is the density of the air.
• V is the speed of the air relative to the wing.
• A is the wing area.
• C_L, the lift coefficient, packs in everything else: the wing’s shape, its tilt to the flow, and effects of the air’s viscosity and compressibility.^[6]

Three things follow straight from the formula:

• Speed matters most. Lift grows with the square of velocity: double the speed and lift becomes four times larger.^[6]
• Air density matters. Thinner air gives less lift at the same speed.^[6]
• The coefficient is usually measured, not calculated. Only for simple shapes and small tilts can it be worked out mathematically. In general it is found by experiment, for example in wind tunnels.^[6]

Angle of attack and the stall

The angle between a wing’s chord line (its reference line) and the direction it’s flying is called the angle of attack.^[7] For small angles, up to about 10 degrees either way, lift rises almost in step with the angle.^[7] A symmetric wing makes no lift at all when the angle is zero; tilt it and lift appears.^[7]

Tilt the wing too far and something dramatic happens. The thin layer of air hugging the wing’s surface (the boundary layer) separates from it, and the wing abruptly loses lift. This is called a stall.^[7] The exact stall angle is very hard to predict mathematically, so engineers find it with wind-tunnel tests.^[7]

A popular explanation that’s wrong

Many books say that air over the curved top of a wing has farther to go, so it must speed up to “meet” the air from underneath at the back edge. This equal transit time explanation is incorrect. It assumes the air above and below the wing reaches the trailing edge at the same moment, which isn’t what happens.^[3]

Lift and the four forces

In steady flight, lift balances weight while the engines’ thrust balances drag. Engines overcome drag, not weight, which is why gliders can fly with no engine at all.^[4]