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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?

mins
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lemonincider
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    See Why does the vortex created by wing affects its own angle of attack? – aeroalias Feb 26 '17 at 09:35
  • "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. – mins Feb 27 '17 at 07:52
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    @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). – Peter Kämpf Feb 27 '17 at 21:29
  • @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) – mins Feb 27 '17 at 22:58
  • The above drawing just appeared on the Quora Site. It fails. There is a tendency to complicate this and use misconceptions. If you are willing to accept that the yop-bottom pressure difference causes the tip-spill thus contributing to (not fully causing) the tip vortex, THEN, you should also be able to understand that these same pressures cause the up-wash ahead of the wing. – Steve Noskowicz Jun 30 '22 at 21:13
  • Following up on Bernoulli's work in the mid 1700s, it was Euler who determined that a Pressure Gradient causes acceleration of fluid from the higher toward the (relatively) lower pressure areas. There is no indication in Bernoulli's notes that he understood any cause and effect relationships (John Anderson). – Steve Noskowicz Jun 30 '22 at 21:14
  • If the under wing pressure is higher than the above wing pressure, then ahead of the leading edge there WILL be upward acceleration and THIS IS OBSERVED in real airflows; NOT amateur drawings! In addition , the above drawing fails because it ignores the up-wash. The up-wash has the effect of an increase in AoA. – Steve Noskowicz Jun 30 '22 at 21:15
  • This is evidenced by the sharper curve in the upper airflow around and above the leading edge, because of this up-wash upward component. Look only at real flows, such as Babinsky's Cambridge video. That above wing sharper curved flow is evidence that the pressure change (drop) is greater than below the wing (increase) and, THEREFORE, a larger contributor to the top-bottom lift causing pressure difference. Try this on for size: Understanding lift Correctly: https://rxesywwbdscllwpn.quora.com/ – Steve Noskowicz Jun 30 '22 at 21:16

1 Answers1

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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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Jan Hudec
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  • The downwash doesn't cause an upward bend. The higher pressure under the wing compared to above it is the source of an upward acceleration both ahead of the wing AND around the tips. A Pressure Gradient is the Cause of an acceleration - from low pressure to relatively higher pressure. Pressure Gradient causes fluid acceleration.(Euler). – Steve Noskowicz Jun 30 '22 at 21:21