# Alternative Theories of Lift

## Are There Alternative Explanations of Lift?

The explanation of lift favored by this website states that lift is created by an imbalance of pressure against a wing—lower pressure on the top surface and higher pressure on the bottom surface. This "pressure distribution" can be calculated accurately for both subsonic and supersonic flight. But there are other ways of explaining how lift is created.

Downwash is especially noticeable beneath a helicopter's rotating blades.
Credit: United Technologies Sikorsky Aircraft

## The Momentum Explanation of Lift

According to this explanation, the slight downward deflection of the air leaving the trailing edge of a wing is what produces lift. Since for every action there is an equal and opposite reaction, the downward push on the air must result in an upward push on the wing. But this “downwash” results from air pressure differences on the wing—it is an effect of lift, not the cause of it. The wing cannot “feel” the airstream behind it being deflected downward; but it does feel the pressure of air molecules colliding against its surface.

## The Circulation Theory of Lift

This is the oldest and most complex theory of lift. It explains how the difference in air speed over and under the wing results from a net “circulation” of air. Above the wing, the circulatory flow adds to the overall speed of the air; below the wing, it subtracts. But the actual force on the wing results from the difference in air pressure caused by the difference in airflow speeds.

Q:

## Why does air pressure decrease as airflow speeds up?

A:

The Bernoulli principle is a restatement of the conservation of momentum. The energy of a stream of air is shared between pressure energy (due to random molecular collisions) and stream flow energy (the shared component of the molecular motion in the direction of the stream). As the air moves around the wing, it is forced to speed up because of mass conservation. ... more

## Did You Know?

Pressure distribution is the way nature "grabs hold" of things and exerts a force on them.

## Pop Quiz

Which of the following is not an alternative explanation of lift?

A) Equal transit-time
B) Coandă effect
C) Scientific method
D) Path length

The scientific method is not an alternative explanation of lift, but a way to ask and answer scientific questions by making observations and doing experiments. The scientific method includes observing a problem or phenomena, studying previous research, forming a hypothesis, designing and performing an experiment, analyzing resulting data, and drawing a conclusion based on data.