I see some explanations that lift is due to the suction of low pressure on the top surface of the wing and deflecting down of airflow on the bottom which results to upward force lift. I wonder, which among the two surfaces (top and bottom) contributes most of the lift?
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asked Jul 24, 2020 at 9:02
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1$\begingroup$ It is not really a "Suction" effect as such, whilst there is a pressure drop, I've seen it explained this way. Newtons Third Law states that for every action there is an equal and opposite reaction. This means that as the wing interacts with the airflow, it imparts a force upon that airflow directing the airflow downwards towards the ground. In an equal and opposite direction, the airflow now imparts an upward force upon the wing, lifting the aircraft off the ground. I think this is an easier way to describe and understand lift. $\endgroup$– KaddyukJul 24, 2020 at 12:12
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$\begingroup$ I've experimented with removing the top or the bottom. In my experience it is better to keep the top and remove the bottom, so I think the top contributes the most lift. $\endgroup$– quiet flyerJul 24, 2020 at 19:13
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$\begingroup$ Lift is just the vector summation of ALL the pressure pushing on the wing. And the air on the top of the wing is pushing it DOWN. We only get lift up because the air on the bottom of the wing is pushing the wing up harder than the air on the top is pushing it down. So much misconception here. Suction !!???? ! What? do you think the little air molecules have vacuum cleaners? or is it some kind of glue? $\endgroup$– Charles BretanaJul 24, 2020 at 23:45
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1 Answer
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In subsonic flight, the majority of the lift is generated by the lower pressure (compared to ambient) at the upper surface of the airfoil, which produces a net suction. A portion of the lift is also provided by the lower surface that has higher pressure, but is much smaller in comparison.
The graph below shows experimentally derived data on a symmetric airfoil, which breaks down the pressure distribution between upper and lower surfaces. The vertical axis of the graphs shows difference between local static pressure and free-stream static pressure divided by dynamic pressure, which is also called pressure coefficient. Note that a negative number means lower pressure compared to free-stream static pressure, and is suction; a positive number means higher pressure, and is pushing.
As you can see, in the 9deg AOA case, the upper surface suction dwarfs the lower surface push force by observing that the area under the curve is much larger for the former.
Image ref: http://web.mit.edu/13.021/13021_2003/Lifting%20surfaces/lectureC.htm
The next two plots show the stall and post-stall. This airfoil stalls at leading edge, so the entire suction peak is removed once the flow is stalled.
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$\begingroup$ There is NO SUCH THING as negative pressure,(or suction, for that matter). This is just as silly as saying that there is such a thig as Negative heat when the temperature is is a negative number, or that all the people in Austraila are living in a Negative earth cause their latitude is a negative number. All real pressure is positive. It is caused by moving molecules bouncing off the surface, and imparting their momentum to the object they are bouncing off of. The only reason we use negative numbers to measure it is because we are comparing it to atmospheric pressure (14.7 psi) $\endgroup$ Jul 24, 2020 at 23:58
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$\begingroup$ Lift is just the vector summation of ALL the pressure pushing on the wing. And the air on the top of the wing is pushing it DOWN. We only get lift up because the air on the bottom of the wing is pushing the wing up harder than the air on the top is pushing it down. $\endgroup$ Jul 24, 2020 at 23:58
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2$\begingroup$ @CharlesBretana Of course there's no negative pressure. But as you said, there's negative pressure differential. Suction is a phenomenological and industry-wide technical term. If the top wing generates most of the pressure differential for the given area, then it is absolutely technically correct to call it suction. Amended my answer to clarify the plots. $\endgroup$– JZYLJul 25, 2020 at 0:14
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$\begingroup$ But the use of the word suction creates a wrong understanding. The ops question demonstrates this. Nothing is "pulling" on the top surface of the wing. The air on the top is "pushing" down on the wing. Use of the word suction creates the wrong mental image. $\endgroup$ Jul 25, 2020 at 0:36