Talk:Downwash

Just Wrong

This page has information that is totally wrong. Should be corrected. — Preceding unsigned comment added by 192.31.106.36 (talk) 22:05, 8 March 2013 (UTC)

Yes - I agree, lift isn't caused by downwash but rather by the low P above a wing and high P under a wing. My understanding is that these pressures combine to cause tip vorticies at the wing tips and these vorticies have a downward component, called the downwash. ie for a 2D scenario, lift is still generated without downwash — Preceding unsigned comment added by 122.58.85.200 (talk) 01:05, 19 May 2013 (UTC)

The cited refs disagree with that, but if you want to add alternative theories then you will need to cite refs to support it, everything here is based on verifiable sources. - Ahunt (talk) 10:51, 19 May 2013 (UTC)

I've done my best to include both theories with information from the Bernoulli's principle article. Maybe someone else could complete it better than i have. Federicoaolivieri (talk) 21:20, 9 November 2013 (UTC)

I reviewed it and it actually looks okay there! - Ahunt (talk) 02:50, 10 November 2013 (UTC)

Downwash

A drawing would be helpful in explaining if there is one. --Natasha2006 18:37, 30 March 2007 (UTC)

There are factual errors for the Downwash section.

Basically,

[1] A bound vortex cannot induce downwash on itself. Otherwise the wing and bound vortex would accelerate to the speed of light without any external power source.

[2] The downwash is caused by the UNBOUND, or free vorticity, shed from the wing.

The trailing streamwise vorticity rolls up into a pair of counter-rotating vortices.

The net effect of the trailing vorticies is to induce a bubble of descending air which happens to contain the the wing (bound vortex) and the trailing vortices. This bubble of downward moving air is called the downwash.

A wing in level flight is actually flying upward through the downwash.

The power required to overcome the induced drag is = The downwash velocity at the wing multiplied by the lift of the wing. —Preceding unsigned comment added by 122.104.164.14 (talk) 12:26, 7 January 2009 (UTC)

I agree! I have made some major changes. See my diff. Dolphin (t) 12:34, 25 April 2022 (UTC)

Another way to visualize Downwash

An alternative way to think about the the bound and unbound vorticity of the wing, is to imagine the wing as one part of a long smoke ring being tugged in the direction of flight. The free trailing part of the smoke ring curls up and descends through the air. The trailing smoke will remain visible as a descending counter-rotating vortex pair. It will be a persistent shape because it is contained within the downwash bubble, as is the wing itself. —Preceding unsigned comment added by 122.104.164.14 (talk) 12:26, 7 January 2009 (UTC)

More info?

Any chance of someone expanding this page? Other things to talk about:

1. Effect of induced downwash on lift

2. Relevance of downwash - eg. multiple planes landing at airports

3. Connecting induced downwash to induced drag.

4. References...

Unfortunately I lack the writing skills and knowledge to do this on my own!

Any takers?

Jez 006 (talk) 15:53, 21 December 2009 (UTC)

NASA's comments on downwash

On 25 April I made a significant change to this article. My edit summary includes “Attempting to bust the myth that downwash is the cause of, or directly related to, lift on an airfoil.” See my diff.

Mr swordfish has reverted my edits. His edit summary includes “lift via flow turning is not a "myth" at least not according to NASA.” See his diff.

My opening comment is to note that Mr Swordfish and I are inadvertently talking about different things. I am referring to downwash. Mr Swordfish is referring to flow turning. These are two different phenomena and this is where the myth lies. In my experience, many people imagine that the flow turning that causes aerodynamic lift is called downwash. It isn’t. In reliable published sources downwash is precisely defined. Numerically it is equal to the lift coefficient divided by pi divided by the aspect ratio of the wing.

Mr Swordfish alluded to the NASA website currently cited at Reference No. 5. Let’s look closely at that website:

1. NASA’s second sentence, in large and bold font, correctly says “Downwash causes a local induced angle of attack which reduces lift.” Note that it says downwash reduces lift; it doesn’t cause lift or even enhance lift; downwash reduces it.
2. NASA correctly says “For a three dimensional wing, there is an additional effect on lift, called downwash, …” NASA is correctly acknowledging that in 2-dimensional flow, or flow around a wing of infinite span, there is no downwash. Even though a wing of infinite span can generate lift, it does so without the influence of downwash because downwash is only associated with three dimensional wings. Many reliable published sources confirm that around a wing of infinite span there is no downwash because the wing’s aspect ratio is infinite.
3. NASA correctly says “The effective angle of attack of the wing is decreased by the flow induced by the downwash, giving an additional, downstream-facing, component to the aerodynamic force acting over the entire wing. The downstream component of the force is called induced drag … …” NASA correctly associates downwash and induced drag; not downwash and lift. Clancy reinforces this in Aerodynamics, sections 5.14 through 5.19 inclusive.
4. Just in case anyone is disposed to place too much reliance on this website, NASA concedes “The analysis for the reduction in the lift coefficient is fairly tedious and relies on some theoretical ideas which are beyond the scope of the Beginner’s Guide.” This analysis is not too tedious for Anderson, Clancy and other reliable published sources and they provide a comprehensive explanation of what downwash is, and by deduction, what it isn’t.

Let me finish with a quick example. Imagine a model of a wing in a wind tunnel. Large end plates at the tips of the model ensure the flow is 2-dimensional and the aspect ratio of the wing is effectively infinite. Imagine the model is inclined to the floor of the wind tunnel at about 10 degrees so the angle of attack is 10 degrees. As the air approaches the model it is curved upwards at about 10 degrees, and as it departs the model it is curved downwards at about 10 degrees. Lift is generated because the flow has been turned through 20 degrees.

Now the end plates are removed from the model and the flow becomes 3-dimensional, including trailing vortices and induced drag. Let’s imagine the trailing vortices are associated with downwash of 1 degree. Now the air approaches the model at only 9 degrees, but the Kutta condition tells us that it still leaves curving downwards at 10 degrees. Lift is generated but less than before because the rotation of the flow field by 1 degree has reduced the angle of attack from 10 degrees to 9, so the flow is turned through only 19 degrees instead of 20. Now there is significant induced drag that is directly associated with 1 degree of downwash. Anderson in Fundamentals of Aerodynamics, equation 5.1, Figure 5.4, Section 5.1 etc. calls the downwash the “induced angle of attack” and it is always negative because it causes a reduction in the geometric angle of attack. Dolphin (t) 00:57, 26 April 2022 (UTC)

There are certainly many myths regarding aerodynamic lift. But the notion that conservation of momentum or Newton's 3rd law is involved is not one of them. Conversely, one of the most pervasive myths out there is that somehow the air can go whizzing by the wing imparting a force on the wing without the air being deflected in the opposite direction. This violates Newton's third law and the law of conservation of momentum.

I think the source of this misunderstanding is the mathematical model of 2-D potential flow, which is part of the standard undergraduate curriculum in aerodynamics. Solutions to 2-D potential flow do not include any net deflection of the airflow. They also don't predict stall, or the existence of eddys (although the irrotational nature of the airflow is an assumption not a consequence of 2-D potential flow.) It's a very useful model, but not dispositive and fails to predict all the behavior of a real-world wing. My mathemetician's view of 2-D potential flow is that it describes an infinite amount of air being deflected through an infinitesimal angle (in the limit as the wing span goes to infinity).

That said, I understand that it gets a bit complicated when considering all the mathematics behind the theory. For a wing generating lift, it must deflect "the air" downward at a rate of -dp/dt = L where L is the lift and dp/dt is the rate of change of the momentum. I'm sure you recall the several-weeks long discussion over at Lift(force) regarding what was meant by "the air". The answer is that for a control volume that's tall and thin, this formula holds (more precisely, -dp/dt converges to L in the limit as the width -> 0 and the height -> infinity). Now, I don't think it would be appropriate to go into that much detail in this fairly simple article. But it is consistent with the current wording of the article:

Lift on an airfoil is an example of the application of Newton's third law of motion – the force required to create the downwash is equal in magnitude and opposite in direction to the lift force on the airfoil.

This statement is simple, easy to understand, and supported by the sources. Yes, it's not the full story, and many pages could be written to elaborate on the technical details. But I think we owe it to our readers to keep it simple and understandable.

While it is true that "...the momentum of the air in the upwash is equal to the momentum of the air in the downwash..." for 2-D potential flow or wings of infinite span, it is not the case for real world 3-d wings. I think we're really much better off not trying to address infinite wings, mathematical theories of lift, or even the "source of lift on the wing" in this simple two-paragraph article. I strongly favor keeping the current version. Mr. Swordfish (talk) 18:39, 27 April 2022 (UTC)

Most of what you have written here is off-topic so I won’t respond to individual paragraphs or sentences.

You have drawn attention to the statement “the force required to create the downwash is equal in magnitude and opposite in direction to the lift force on the airfoil” and you have written “This statement is ... ... supported by the sources.” I disagree and I challenge you to find and identify a reliable published source that supports it. The problem is the word downwash (which is the title of this article.) If the sentence were to say “the force required to deflect the air in the downwards direction is equal in magnitude and opposite in direction to the lift ...” then I will agree with it, and agree that you will find sources that support it.

There is a case that can be made to cover the popular, dictionary meaning of downwash in this article. It is a non-rigorous application of the word and isn’t supported by reliable sources of the calibre of Anderson, Clancy etc. Consequently I don’t support the popular meaning of downwash being mixed with rigorous, scientific explanations such as Newton’s laws of motion, Kutta-Joukowsky Theorem, horseshoe vortex etc. Where these rigorous, scientific explanations are employed it should be to support explanation of the word downwash with the meaning it is given in reliable published sources of the calibre of Anderson, Clancy etc.

I will give some thought to drafting a new version in my sandbox. Dolphin (t) 06:19, 28 April 2022 (UTC)

Changing the language to “the force required to deflect the air in the downwards direction is equal in magnitude and opposite in direction to the lift ...” is fine by me, and avoids conflating the popular, dictionary meaning of downwash with the technical definition found in aerodynamics texts. I'll make the change to the article. Mr. Swordfish (talk) 19:16, 28 April 2022 (UTC)