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I have a question regarding the production mechanism of induced drag.

I understand the downwash aft of the trailing edge (steaming from wingtip vortices) causes the induced drag by changing the relative flow (blue line) into the effective relative airflow (red line), and thus by bending the lift vector rearward (L to Leff).

enter image description here

My question here is: How does the downwash, that has already passed behind the trailing edge of the airfoil, influence the airflow upstream, causing the entire relative airflow to bend upward as in the picture.

Or is it just an imaginary concept to explain the induced drag?

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    $\begingroup$ See Why does the vortex created by wing affects its own angle of attack? $\endgroup$ – aeroalias Feb 26 '17 at 9:35
  • $\begingroup$ "The influence of the downwash actually extends ahead of the wing. The air ahead begins to be pulled down in response to the flow behind the wing, in proportion to the downwash velocity, even before the wing arrives". (Source.) The why is viscosity, and the initiator is the downwash of the tip vortices. $\endgroup$ – mins Feb 27 '17 at 7:52
  • $\begingroup$ @mins: Your source is incorrect: Ahead of the wing the flow moves up. This can be explained by vorticity (the bound vortex adds an up component there) and by physics: The suction on top of the airfoil sucks in more air. See this answer for a better explanation. I recommend to doubt everything you read on the Interwebs (including my answers). $\endgroup$ – Peter Kämpf Feb 27 '17 at 21:29
  • $\begingroup$ @PeterKämpf: With due respect, I'm not convinced because it seems to me we are mixing images of 2D sections of airfoil without tip vortices downwash effect and 3D figures with it. So let's put the two together. In a) the 2D section with clearly a up-down path like in your picture. In b) 3D actual flow with the inner downwash of the tip vortices: I see clearly a descending pattern. Image from Aircraft Flight (Barnard, Philpott) $\endgroup$ – mins Feb 27 '17 at 22:58
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How does the downwash, that has already passed behind the trailing edge of the airfoil, influence the airflow upstream, causing the entire relative airflow to bend upward as in the picture.

The downwash is behind the trailing edge, but the acceleration of the air downwards happens above and below the wing. The highest acceleration is around the quarter-chord where the centre of pressure is.

In subsonic flow, the pressure changes do propagate upstream due to how the various flow equations work. The sharp trailing edge is indeed an important feature of efficient wings that affects how the flow behaves around the wing.

Or is it just an imaginary concept to explain the induced drag?

It isn't. But there are many ways to explain it. I prefer using the conservation of momentum and energy (that I explained here).

Note that for supersonic wing, the mechanism changes to wave drag. The conservation argument still holds and explains why there can't be lift without drag, but now that the pressure changes do not propagate upstream the pressure field looks differently.

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