Seagull....help!

[seagull]

Well, too tired with too much on my plate to get into this too much, (and I admit I really just mostly scanned where this thread has gone without reading it carefully), but it appears to me that none of what you wrote changes the basic issue, which is the downward momentum of the air is the RESULT of lift, and not the CAUSE. Nothing you have written demonstrates that it is more than just representational of the lift force, and the wing tip vorticiies are still the culprit of induced drag, the rearward tilting of the lift force and, as you say "shoving the air forward".

In the end, I come away wondering what point you are trying to make?

[B767Driver]

Originally Posted by seagull

In the end, I come away wondering what point you are trying to make?

I started by questioning a book's authors who were adamant that a portion of lift is created by the vertical velocity vector of the wing downwash.

Realms,

What you are saying is that this is accurate, since the wing is imparting a downward net momentum. Is my understanding correct?

[tgrayson]

Originally Posted by Realms09

the net momentum idea you are talking about has to do with induced drag.

No, no, no. I'm talking about net vertical momentums, not momentum change.

[tgrayson]

Originally Posted by Realms09

By the analysis presented in my post with the pictures, we see that the wing, even if it is 2-D, shoves the air downward and gives it downward momentum.

The objection I think you should have made to my original post is this:

It's true that there is no net downward momentum in the vertical plane, but there is a net change in vertical momentum, and it's a change in momentum that indicates the presence of forces.

So part of the confusion on the subject appears to be referring in one instance to the values of momentum and in another instance changes in momentum.

The only point about bringing up the net momentum = 0 concept is to illustrate that net downward momentum cannot be causal because it does not exist. This point only has value to the extent that anyone is arguing that it's a net downward momentum that produces lift. You have not made this argument, but I believe that others have. (Your argument is that there is a net momentum change in the downward direction, which is true.)

However, this momentum change is the equal and opposite reaction of the lift force generated by the airfoil. That fact that there is a momentum change is due to the fact that the air is free to accelerate. The airfoil doesn't particularly care whether this happens or not.

[tgrayson]

Originally Posted by B767Driver

I started by questioning a book's authors who were adamant that a portion of lift is created by the vertical velocity vector of the wing downwash.
Realms,
What you are saying is that this is accurate, since the wing is imparting a downward net momentum. Is my understanding correct?

No, he's not saying that at all. Let me quote him:

Circulation, Newton, and Bernoulli are all ways of saying the same exact thing.

Bernoulli - pressure forces generate the lift force (this is the REAL source of lift).
Newton - the reaction to the lift force shoves the air downward.
Circulation - a mathematical result of modeling the lift.

I added the bolding, but the text is his.

You can always calculate lift by integrating the pressure differences around the airfoil. No need to take into account anything else. Or you can calculate the momentum change of the air to deduce the force that produced it.

[tgrayson]

Originally Posted by seagull

In the end, I come away wondering what point you are trying to make?

In the main, he's objecting to the evidence I offered that the downward momentum is not causal. He's not rejecting the conclusion, only the means of getting there.

[B767Driver]

Originally Posted by tgrayson

It's true that there is no net downward momentum in the vertical plane, but there is a net change in vertical momentum, and it's a change in momentum that indicates the presence of forces.

)

So to conclude my intial inquiry from "Understanding Flight", the downwash plays no part in lift production...but the magnitude of the momentum change is indicative of the lifting force...not causitive.

[tgrayson]

Originally Posted by B767Driver

So to conclude my intial inquiry from "Understanding Flight", the downwash plays no part in lift production...but the magnitude of the momentum change is indicative of the lifting force...not causitive.

<<So to conclude my intial inquiry from "Understanding Flight", the downwash plays no part in lift production...but the magnitude of the momentum change is indicative of the lifting force...not causitive.>>

That's my position (at present), with the caveat that "downwash" isn't the downward momentum we're talking about. Downwash, by definition, is the extra downward momentum produced by wingtip vortices. Our discussion so far is in 2-D flow, so the downward momentum is due solely to the lift force.

Give Realms a chance to respond before forming your conclusions, though. We're still not communicating on one issue and I don't want to speak for him.

[seagull]

B767, my post was meant for Realms, not you, but I withdraw it with apologies, as I missed some of what you wrote.

Tgrayson, thanks, I missed some of what you quoted in my quick read, or it didn't register.

Sorry for any confusion.

[B767Driver]

Originally Posted by seagull

B767, my post was meant for Realms, not you, but I withdraw it with apologies, as I missed some of what you wrote.

Tgrayson, thanks, I missed some of what you quoted in my quick read, or it didn't register.

Sorry for any confusion.

No prob...I understood your context was to him. Although, fascinated, I'm just hanging on by a thread in this discussion and it was good to reiterate what it is we're trying to figure out.

[Realms09]

Originally Posted by tgrayson

The only point about bringing up the net momentum = 0 concept is to illustrate that net downward momentum cannot be causal because it does not exist. This point only has value to the extent that anyone is arguing that it's a net downward momentum that produces lift. You have not made this argument, but I believe that others have. (Your argument is that there is a net momentum change in the downward direction, which is true.)

A brief history of my motivation (Seagull, the point I was trying to make):

I was not aware that anyone tried to make the argument, because to me it is nonsensical. In my head I probably translated "net downward momentum" into "net change in downward momentum" as I was reading because that's the only thing that makes sense to me when talking about forces (which is the very point tgrayson was making). This led to concern that tgrayson was saying there was no change in momentum in the 2-D case (tgrayson wasn't), so I set out to show, in detail, that there was in fact a change in momentum. However, we really agreed all along. Tgrayson is correct, I'm correct, we're all correct. At least now our correctness is available in extreme detail.

[B767Driver]

Cool.

So now I'll go back to my original post. Where are the "Understanding Flight" authors getting away with downwash being causal to lift production?

Do they have supporters? Is there theory to support their claim? (Tgrayson & Realms just proved otherwise.) Are they just shooting from the hip because it sounds good?

If they have no basis to justify their claims...then they are doing a lot of damage, IMO.

[Realms09]

I haven't read the book, and can't say I'm inclined to now! I did read their synopsis on their web page. Given the background of the authors my intuition is that they are likely trying to say what we are saying, but through simplification and sloppy terminology make an incorrect mess out of it, much like the pilot textbooks they claim to improve upon.

[tgrayson]

Originally Posted by Realms09

A brief history of my motivation (Seagull, the point I was trying to make):

Your posts were very valuable. I had not really noticed that there were two issues of momentum being tossed about until you forced me to distinguish between them. I'll be able to do a better job discussing it in the future.

What do you know about the Coanda Effect? In the past, my Googling didn't turn up much authoritative about it. My fluid dynamics book only has one mention of it towards the end of the book. There seems to be some debate about whether it's a viscosity issue or surface tension. Perhaps those are related concepts.

In aviation, I believe there is some application with boundary layer control and lift augmentation systems, but again there are only one or two references to it in all my books.

John Denker, who wrote the online Aerodynamics book "See How it Flies", dismisses the Coanda Effect, but doesn't provide a whole lot of explanation as to why. He includes in his references a paper by a noted authority on the subject. I acquired a copy of the paper, but found it unenlightening.

[Realms09]

In the three aerodynamics/fluid mechanics textbooks I have in my personal posession I do not recall ever coming across the "Coanda effect" term. I just checked their indices and it is not listed. The engineering library here probably has the answer, so I might look there when I have a substantial chunk of spare time.

[tgrayson]

Originally Posted by Realms09

In the three aerodynamics/fluid mechanics textbooks I have in my personal posession I do not recall ever coming across the "Coanda effect" term. I just checked their indices and it is not listed. The engineering library here probably has the answer, so I might look there when I have a substantial chunk of spare time.

That itself is interesting. Thank you.

[seagull]

I share Tgraysons thoughts on this thread. There was some miscomm earlier on, but in the end, it served to clarify the issue really nicely! Thanks to all for a really great read! Now we have another thread we can point people to.

Originally Posted by Realms09

A brief history of my motivation (Seagull, the point I was trying to make):

I was not aware that anyone tried to make the argument, because to me it is nonsensical. In my head I probably translated "net downward momentum" into "net change in downward momentum" as I was reading because that's the only thing that makes sense to me when talking about forces (which is the very point tgrayson was making). This led to concern that tgrayson was saying there was no change in momentum in the 2-D case (tgrayson wasn't), so I set out to show, in detail, that there was in fact a change in momentum. However, we really agreed all along. Tgrayson is correct, I'm correct, we're all correct. At least now our correctness is available in extreme detail.