Here's another way to think of it...
It is the pressure difference between the upper and lower surface of the wing which is responsible for the lifting force. This pressure difference causes the air to "push" up on the wing. In accordance with Newton's 3rd law (action=reaction), the wing will push down on the air. In a way, the downwash is the result of the lifting force.
Some questions that may be asked:
1. What causes the pressure difference in the first place?
The pressure difference is caused by the difference in velocity between the air molecules on the lower surface of the wing, and those on the upper surface. Bernouilli's theorem shows the relationship between velocity and pressure.
2. Why does the air travel faster on the upper surface?
This can become pretty complicated, but the crucial reasons behind this have to do with the shape of the wing itself and to the existence of viscosity.
If you were to expose a non-rotating cylinder to a stream of air, the pressure pattern above/below the cylinder would be identical, and thus, there would be no net lifting force. If you were to rotate the cylinder clockwise (assuming the stream of air is coming from left to right), the air would travel faster above the cylinder than below it, and this difference in velocity would cause a difference in pressure, and thus, a net lifting force would be produced.
The air travels faster above the cylinder when it is rotating because of the existence of viscosity between the air molecules and the cylinder. In a way, the cylinder is pulling on the air as it rotates, causing it to travel relatively faster above it than below it.
The important point here is this: Due to the shape of the wing, it can generate a "circulatory flow" like the rotating cylinder example above, without actually having to rotate.
3. How does a wing generate this circulatory flow?
The circulation is able to be produced because of the wing's shape. An important reciple is to have a streamlined leading edge, and a sharp trailing edge. The sharp trailing edge causes the air to "separate" from the wing at the trailing edge, and this separation occurs, once again, because of viscosity.
Imagine what would happen if the leading edge were very sharp. The airflow would not be able to remain "attached" to the wing as it flows over it, and the result would be a large amount of drag. This will destroy the pressure differences between the upper and lower surfaces, and this will destroy any lifting force.
This is similar to how a stall occurs. Once you exceed a certain AOA, the airflow can no longer maintain its smooth, attached flow, and it seperates at the leading edge, causing a complete brakedown of airflow and pressure distributions.
So, overall we see:
- Due to the shape of the wing and the airflow, the air travels faster above the wing than below.
- This causes a pressure difference in accordance with Bernoulli's theorem (higher pressure below, relatively lower pressure above the wing).
- The pressure differences causes a net force which acts upwards. The higher pressure below the wing is pushing up on the airplane.
- In accordance with Newtons 3rd law, the reaction is that the airplane pushes down on the air (downwash).
Now I personally don't fully understand how the shape of the wing truly causes the circulatory flow, nor do I fully understand why the air has to seperate at the trailing edge, but from my research, they are extremely important to the production of lift.
