# What happens when the velocity of the lower surface of the airfoil is lower than the velocity of the free stream?

At a certain AOA I noticed that the speed of air in the lower surface of the airfoil is lower than the free stream's speed. What is the consequence of that?

Local pressure will rise above the freestream pressure. Speed and pressure are linked, as expressed in Bernouilli's principle. Also the local temperature will rise, as expressed in the ideal gas law.

Note that at all angles of attack there is one point where the local flow speed will decrease to zero in 2D flow: This is the stagnation point. On wings in three dimensional flow this becomes a stagnation line, and swept stagnation lines still have some residual flow speed, but in all cases the local speed is below the free flow speed at and around the stagnation line.

Since lift is proportional to the pressure difference between the upper and lower side of the wing, this slowing down is welcome. Another beneficial effect is the lower wall friction of the slower flow. However, the thicker an airfoil is, the higher will be the flow speed on both sides, due to the displacement effect of the thicker airfoil. Thinner airfoils are structurally less efficient, so a compromise needs to be found which allows enough internal structure while keeping thickness reasonably low.

• Does this mean that more lift is generated, I mean since the pressure difference widens?
– Jona
Oct 3, 2017 at 22:02
• In a wind tunnel - stationary wing, moving air - the air on the bottom of the airfoil is slowed down. But in reality - still air, moving wing - you can't "slow" air that is not moving. This would seem to mean that the air on the underside of the wing can have the same or lower pressure than the free stream. Is this correct or is my reasoning flawed? Oct 4, 2017 at 3:02
• @TomMcW: It's all relative. The moving wing becomes a stationary wing for an observer sitting inside the airplane. Oct 4, 2017 at 10:16
• I can see that the pressure differential between the top and bottom is the same from either point of view. That’s why we can use wind tunnels to gather data. So it really doesn’t matter; my question is really just academic. But is there any point on a wing in subsonic flight through still air where the pressure is higher than ambient barometric pressure? The diagram on this answer seems to show just a small point at the leading edge which I suspect is a compressability effect right at the stagnation point. Oct 4, 2017 at 18:15
• @TomMcW: Look at the pressure plot in the question to the answer you linked. The "Fig. 2" airfoil has higher pressure than ambient where the area between the pressure coefficient line and the x-axis is shaded in pink. The flow speed is Mach 0.78, but a very similar distribution can be found with incompressible flow. Basically the higher-than-ambient pressure is around the stagnation point (near the nose) and close to the trailing edge. At higher angle of attack the whole bottom can have positive pressure coefficients, meaning pressure higher than ambient. Oct 4, 2017 at 19:00