Sunday, 21 August 2016

How a Wing Produces Lift (FAA Edition)

It can be a challenge to explain how lift is created while doing all of the following:

1. Use Bernoulli's Theorem (which is what the FAA likes).
2. Avoid telling obvious falsehoods (which is what flight instructors like).
3. Be vaguely comprehensible.

Below is one explanation which meets all three criteria.

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Because of the shape of the wing and the Angle of Attack, air flows faster over the top of the wing than over the bottom of the wing. In this sentence is hidden all the complexity and mathy nonsense that scares people away from looking into how lift is created, I'll re-state it. Because of the shape of the wing and the Angle of Attack, air flows faster over the top of the wing than over the bottom of the wing.
Now let's talk about types of pressure. Total pressure, is always equal to dynamic pressure plus static pressure (in subsonic ideal flow) ...which is illustrated by the below equation.
Now dynamic pressure is proportional to velocity, so if velocity goes up, dynamic pressure goes up. As we stated above, air flows faster over the top of the wing than over the bottom of the wing which means there is greater dynamic pressure on the top of the wing than on the bottom


Because total pressure must stay constant, the higher dynamic pressure on the top means static pressure must be lower on the top. Conversely, the lower dynamic pressure on the bottom means that static pressure is higher on the bottom.
Dynamic pressure only acts in one direction, the direction of the arrows above and below the wing, which means it does not exert any force on the wing. This means that only Static Pressure exerts a force on the wing.
So only static pressure exerts a force on the wing, and dynamic pressure is higher on the top of the wing than on the bottom of the wing ...which means that static pressure is higher on the bottom of the wing than on the top of the wing. This is lift. Lift is the difference in static pressures between the top and bottom of the wing.

For a more complete explanation watch this lecture


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