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I find in many books that a wing flies in its own downdraft.

Does wing really fly in own downdraft and what does that mean?

Video Source 18:15 - 19:30

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user707264
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2 Answers2

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First of all a clarification: what you call downdraft is more correctly termed downwash.

Wing fly in own downdraft?

No

The flow field around a wing might be complicated but in general it can be seen that, around a lifting wing, air:

  1. goes upwards in front of the wing;
  2. accelerates on the upper surface of the wing; and
  3. decelerates on the belly;
  4. goes downwards on the back of the wing.

If you could isolate those movements, you would see the air basically rotating around the airfoils of the wing.

So the wing isn't really flying in its downwash but it is actually producing a downwash behind itself. This is also stated in the video you linked.

So there's really no way for a wing to fly in its own downwash? Yes there is. This happens all the time for example in a helicopter rotor. Each blade generates a downwash it's going to collide with as soon as it has done a complete rotation.

sophit
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  • author in video tell that downward motion happened even before wing(listen at 18:47) ... I see that downward motion is above,below and behind wing – user707264 Dec 11 '23 at 21:51
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    I'm pretty sure that he just wanted to point out that the wing modifies the airflow already well in front of it and that the net effect is a downward movement (downwash). In front of the wing the airflow is pushed upward, as also visible many times in the wind tunnel tests in the video. – sophit Dec 11 '23 at 22:23
  • From max camber downstream, air has downward component, so we can say 3/4 of wing travel in own downwash – user707264 Dec 12 '23 at 19:22
  • No because the airflow begins to go upwards well in front of the wing, so it's not really ¼ vs ¾. The net downwash is behind the wing, not on the wing. – sophit Dec 12 '23 at 21:02
  • Well, if you look at the Pistolesi theorem, the last quarter of wing chord is already in downdraft. But the first three quarters are in updraft. – Peter Kämpf Dec 13 '23 at 17:03
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A wing creates lift by pushing down on the surrounding air (some may debate whether pushing is the best word but let’s go with that for now). This unavoidably makes the surrounding air move downwards, so much of the wing will be moving through air that is moving somewhat downward. So it’s essentially true that a wing does fly in its own downwash. That said, the wing is continuously in movement and so it’s always just on the edge of escaping from the downwash behind it, if it helps to visualise things that way.

Frog
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