RE: Reflexed Airfoil and Flying Wings - Downwash?  
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RE: Reflexed Airfoil and Flying Wings - Downwash? - 3/22/2005 9:44:50 PM   
adam_one


 

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Status: offline 		Bruce,
Very interesting observations, I agree with most of it but when you try to choose which comes first... In my opinion they occur simultaneously…


< Message edited by adam_one -- 3/22/2005 9:51:12 PM >

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RE: Reflexed Airfoil and Flying Wings - Downwash? - 3/22/2005 9:52:35 PM   
LouW



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Status: offline 		Bully for you, Bruce, I believe you have it. Now if we could just get Ken to actually read a post before launching into a reply we might come to some agreement.

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RE: Reflexed Airfoil and Flying Wings - Downwash? - 3/22/2005 9:52:25 PM   
Ben Lanterman



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Status: offline 		Good thinking Bruce except -

"* C- ........ The reversal acceleration forces are coupled to the wing via the low pressure regions along the upper surface.

So my thoughts are that while the wing generates the acceleration of the air that produces the downwash and thus the lifting effect it's the pressure differential that actually controls, forms and directs this acceleration and it is the pressure that provides the "glue" to transfer the momentum from the accelerated air to the wing's surface.

OK.... I'm spent now..... I may come back later and edit some of the mixed up portions but hopefully the concept comes through clearly. "

Oh how little energy you have when you get old :-)

That the air is deflected as it flows around the upper curve is true. It's contribution is to create a low pressure area due to velocity increase, not due to turning the flow direction. Keep in mind in our shortcuts we forget that the upper wings static pressure is still high enough to push the air enough to cause it to conform to the wing's upper surface. But that push is down on the wing. Any pressure field above the wing (although we call it low) is still pushing down on the wing.

As luck would have it pressures on the lower surface are much larger and the differential is a net up force on the bottom of the wing. But like I say, the fact that the flow is following the curved surface is just still pressure forces and it is down.

The flow conditions above the wing cannot couple the wing to the downwash. There is no mechanism that allows air to pull.




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RE: Reflexed Airfoil and Flying Wings - Downwash? - 3/22/2005 10:22:42 PM   
Ben Lanterman



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Status: offline 		Let me add a crude picture. The lines aren't to scale just general. The red is the pressure field and is relative to the ambient freestream pressure so it shows up as up on top and up on lower surfaces in this example

Note there is a pressure force down and aft on the upper surface and up and aft on the lower surface. The sum of all the forces on the wing is an acceleration up on the wing and aft from drag, let's ignore it for the moment.

The upper surface flow has been turned by the wing but only because the ambient freestream pressure is pushing it down to the wing's surface.

At the same time as the above happens the air above the wing is responding to the low pressures and is being shoved downward by the freestream pressures.

You end up with downwash equal to wing lift, but the actual wing lifting mechanism is wing upper low and lower high pressures. The actual downwash mechanism is upper wing low pressure and ambient freestream pressure.

The tie between the two is a low pressure area. That doesn't tie them to each other vectorily, needed for action for reaction.

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< Message edited by Ben Lanterman -- 3/22/2005 10:26:46 PM >


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RE: Reflexed Airfoil and Flying Wings - Downwash? - 3/22/2005 10:31:54 PM   
dick Hanson



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Status: offline 		.True or false?
It is all a matter of creating a pressure imbalance which is equal to the support required.
what happens as it leaves the airframe -is someone elses problem.
Nature tries to rebalance itself -so depending on the unbalance induced - the retreating air (wake/ downwash/ whatever .) just spins or ripples depending on the amount and direction set up by the imbalance created.
Once it leaves the area -who cares.

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RE: Reflexed Airfoil and Flying Wings - Downwash? - 3/22/2005 10:37:49 PM   
Ben Lanterman



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Status: offline 		That is pretty much it Dick, until someone swears it is something it isn't :-)


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RE: Reflexed Airfoil and Flying Wings - Downwash? - 3/22/2005 10:48:08 PM   
adam_one


 

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quote:

True or false?
It is all a matter of creating a pressure imbalance which is equal to the support required.
what happens as it leaves the airframe -is someone elses problem.
Nature tries to rebalance itself -so depending on the unbalance induced - the retreating air (wake/ downwash/ whatever .) just spins or ripples depending on the amount and direction set up by the imbalance created.
Once it leaves the area -who cares.

In fact it is what I've said before, when the air leaves the TE the work is already done, because the downward/upward forces have already been applied at about 1/4 of the chord.

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< Message edited by adam_one -- 3/22/2005 10:57:13 PM >

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RE: Reflexed Airfoil and Flying Wings - Downwash? - 3/22/2005 11:30:55 PM   
Ben Lanterman



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Status: offline 		You have rediscovered body axis and stability axis systems.

Now work on how the downwash force vector gets into the drawing. You can't.


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RE: Reflexed Airfoil and Flying Wings - Downwash? - 3/22/2005 11:32:25 PM   
Ben Lanterman



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Status: offline 		Lou look at my crude drawing, isn't is a proper accounting for all the basic elements (ignore units and length on vectors - please)?


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RE: Reflexed Airfoil and Flying Wings - Downwash? - 3/22/2005 11:52:05 PM   
KenLitko


 

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quote:

ORIGINAL: LouW

Bully for you, Bruce, I believe you have it. Now if we could just get Ken to actually read a post before launching into a reply we might come to some agreement.


LouW,

I can assure you I have read all the posts.

I just can't get satisfactory answers to my questions... I am still trying to find out what replaces all this downwashed air. Are we leaving holes in the atmosphere above? We are certainly not magically creating air to fill the void.

If, as adam_one explains the "fill in" air comes from above we are stuck with a massive column of air that must be deflected downward. Impossible because we don't have the energy to do that.

If instead we argue that the "fill in" air actually comes from below... we get... bum bum bum... circulation.

No net downward momentum, because we simply have air circulating in our bound, trailing and starting vortex sheet, and we get a lifting force due to the pressure differential created on our wing.

I don't think anybody has argued with me how more downwash gets us less lift. This is a paradox that yourself, adam_one or anyone else has not been able to resolve.

Check out this guy's momentum explanation of lift and spot the flaw in his logic:

http://www.av8n.com/fly/vortex.htm

Hint... in his +1 -1 diagrams he is missing the outer portions of the trailing vortices.... and his symmetry returns.

Ken - www.litkoaero.com

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RE: Reflexed Airfoil and Flying Wings - Downwash? - 3/22/2005 11:57:28 PM   
Crane



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Status: offline 		Ken,

"No net downward momentum, because we simply have air circulating in our bound, trailing and starting vortex sheet, and we get a lifting force due to the pressure differential created on our wing."

Amen

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RE: Reflexed Airfoil and Flying Wings - Downwash? - 3/23/2005 3:45:35 AM   
Tim Green


 

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Ok - are you focused on a helicopter.

Now focus on standing under it while the rotors are going real fast. Are you holding on to your hat. I hope so, because that's some big honking downwash you're experiencing.

Do you care where the "fill-in" air comes from. No. Why on earth would you?

And you know darn well there's a net change of thrust to the chopper, because you've held on to your own airplane models with the props revving, and felt it pull. Just like the chopper I just had you imagine is doing.

But your plane's never pulled without loads of downwash. And the more it pulls, the more downwash you feel from the props. You've felt it on your hands, on your pants, on your face.

And you know, the more air you fell against your body, the faster that plane's going to take off. The harder it pulls against you.

And you know that plane's not going anywhere, unless it's shoving loads of air back at you. You know it, and pretending anything else is self delusion.

This argument has to factor down to common sense. There cannot be anything else.



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RE: Reflexed Airfoil and Flying Wings - Downwash? - 3/23/2005 4:20:23 AM   
Ben Lanterman



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Status: offline 		For the downwash to be producing the lift it must be reacting on the bottom of the heli blade - but it isn't. That you can feel the downwash is obvious. It has been stated that the downwash is equal to the lift and that isn't a trivial number if we want the airplane to fly. If the angle of attack is high enough on the blade it will indeed push some air down by the "fan" action of the bottom of the blade. But at the low blade angles where the heli blade is going to be efficient I doubt it is happening.

Consider what would happen if the pitch of the prop approached 90 degrees. That would be a lot of beating of the air and both upwash and downwash would take place through the prop as the flow is shed out the tips. I have no doubt that a regular prop running on an airplane sitting still is pushing some air in addition to the flow through the prop due to pressure differential.


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RE: Reflexed Airfoil and Flying Wings - Downwash? - 3/23/2005 4:55:25 AM   
BMatthews



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