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In my training as a Certified Flight Instructor, I am required to teach Lift Theory. While simple explanations have sufficed in the past, as I progress in my training and understanding, I want to be able to understand lift in greater detail. During my last lesson, I stumped my Instructor. I asked about the cause of the increased velocity above an airfoil and he could not explain it so I am trying to research it and come to some clear understanding.
I have a hunch described below. Please let me know in semi-layman terms if I am on the right track.
- As Velocity increases, Pressure decreases and vice versa. (Bernoulli)
- The air moving across the top of an airfoil moves faster than the air below the airfoil. Some have said that this is because the air above has a farther distance to travel but this has been proven untrue.
- Higher pressures below the airfoil and lower pressure above the airfoil create a portion of the lifting force of the airfoil.
- Additional lift forces are created by air being directed down by angle of incidence and the shape of the wing (Newton). This occurs from the air running into the bottom of the wing and being forced downward. Lift is also occurring because the air above is moving at a higher velocity than the air below, and when they meet at the trailing edge, the faster air pushes the slower air downward. (Vortices)
- From the Cambridge University video (see References) showing time-lapse video, we see that the air above the airfoil moves faster such that it reaches the trailing edge sooner than air below the airfoil.
- This proves that the air over the airfoil moves faster but what is the cause for that air to move faster?
Because of the angle of incidence and angle of attack to the relative wind, a higher pressure is created upon impact at the separation. This upper airfoil high pressure is considered ‘high’ relative only to the air pressure at other areas of the upper airfoil such as the center of pressure or the trailing edge of the wing. (It is not meant to be considered ‘higher’ pressure than the air under the airfoil.
Also due to angle of incidence and angle of attack, an even lower pressure is created at the trailing edge of the wing.
Attached is a screen shot of the Cambridge video, with my hypothetical pressures jotted in.
Because high pressure air always seeks lower pressure air, the air above the airfoil should get an extra push and accelerate to a velocity faster than that of the air under the airfoil.
I found (see attached) a diagram of Pressure Coefficient distribution across airfoil geometry. This diagram is from an unrelated paper treating a separate subject of shapes of trailing edges. It however is the only graph I could find detailing what pressures occurred where on the airfoil.
References: 1 Cambridge University video https://www.youtube.com/watch?v=UqBmdZ-BNig
2 Morphing Trailing Edges with Shape Memory Alloy Rods. November 2002 URL: https://www.researchgate.net/publication/228531008_Morphing_Trailing_Edges_with_Shape_Memory_Alloy_Rods 1st Silvestro Barbarino 15.46 · Sikorsky Aircraft 2nd Wulf G Dettmer 3rd Michael Ian Friswell 45.16 · Swansea University