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The downwash is caused due to higher pressure below the wing than above it (for a positive lift generating wing that is). What I am not able to understand is how would the flow be in a gross sense? Will the flow move bottom to up around the wing tips and move top to bottom along leading and trailing edge? I thought of this because it is the only way I am able to justify that downwash induces a negative velocity component which decreases the angle of attack that the wing sees at the leading edge.

But can’t the air also move bottom to up along leading or trailing edge? To me, at leading edge it seems unlikely based on intuition because of the air upstream constantly trying to direct the air ahead but at trailing edge it seems more likely.

So what is the correct visualisation of downwash over a finite wing?

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Downwash is a misleading concept when discussing airfoil lift. Instead, just focus on the pressure distribution around the airfoil.

If I understand your confusion correctly, you are picturing, correctly, that there is an overall low pressure on top and an overall high pressure on the bottom, then why isn't air flowing from high pressure to low pressure?

This will indeed be the case, if there is no airflow. What you are missing is that, the static pressure distribution is caused directly by the airflow around the airfoil. The distribution is steady-state, because the combination of dynamic pressure and static pressure resists further changes; it's at an equilibrium.

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  • $\begingroup$ I am quite confused about the use of static pressure in this picture. Can you kindly more on the questions below? 'The static pressure distribution is caused directly by the airflow around the airfoil. $\endgroup$ – ModCon Feb 25 at 18:23
  • $\begingroup$ @Niket I'm not sure I understand what you're confused about. The "pressure" from your original question is equivalent to static pressure. My answer just says that there's also dynamic pressure that resists changes to the flow at steady-state. $\endgroup$ – JZYL Feb 25 at 18:56
  • $\begingroup$ I am sorry but I am still not able to get it. Is the static pressure distribution something that is always there and then when the airplane is passing through that region , this also comes with it some dynamic pressure. Now if I understand you correctly, the initial static pressure distribution and the newly introduced dynamics pressure distribution get into an equilibrium. Since all this has happened and the air is in equilibrium, we don't have much (if at all any) air that moves top to bottom or vice versa on the wing. Is this true? $\endgroup$ – ModCon Feb 26 at 13:27
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The downwash is caused due to higher pressure below the wing than above it

No, not really. Actually, the wing sucks and pushes air down so it leaves the trailing edge in a different direction compared to the flow direction ahead of the wing. Air that flows over and below the wing gets a downward impulse that leaves it moving downwards behind the wing. That movement is downwash.

The low pressure area over the forward upper wing does indeed suck air upwards into it. The flow ahead of the leading edge is sucked upwards and once the lower part of it has arrived on the bottom of the wing, it will not flow back – that would require a complete change in flow direction and costs too much energy!

High and low pressure alone will not cause a change in movement, only a pressure gradient will. The stronger that gradient, the more sudden the change in movement. A complete reversal of the direction of flow would require a very high gradient which can only be found in places like the inside of engines but not around a wing.

Note that a wing has to move through air in order to create those pressure gradients, and their strength increases with the square of velocity. Therefore, as JZYL explains, you need to see the flow from high to low pressure in combination with the general movement of the wing.

For more details, please read this answer.

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  • $\begingroup$ Could you please answer these follop-up questions? 1. So, is it right to say that the phenomenon of air being pushed downward is dowmwash? 2. High and low pressure alone will not cause a change in movement. Why is it so? $\endgroup$ – ModCon Feb 25 at 18:11

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