Reflexed Airfoil and Flying Wings - Downwash?  
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Reflexed Airfoil and Flying Wings - Downwash? - 3/17/2005 12:20:18 AM   
Ben Lanterman



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From: St. Charles, MO, USA
Status: offline 		A guy who reads this forum wrote me a note today and I am amazed that no one had thought about it (Well thought out Dave) - certainly I hadn't and I have a half dozen flying wings in the garage. My flying wings are a flat panel with reflexed elevons for pitch. I'll attach a photo of one of the little fast ones during the glide. They are a lot of fun. Some use the little GWS-A size motors or the CDrom brushless versions and a Lipoly battery.

But it seems that the elevons would elinimate most/all/some/none of the downwash of the wing due to angle of attack.

If I sum moments about the CG which is about 15-20% of the wing chord I have to include the the wing lift which is up at 25% and the moment due to reflex that is nose up and around the CG. The reflex moment balances the wing lift moment about the CG. When the moments are balanced and I have lift the airplane flies level.

Then the question is - if by measurements in wind tunnels and other places the downwash is a strong force and is maximum at the trailing edge then shouldn't the downwash produce a big nose down moment? What happens to the downwash? Isn't this an interesting variation on the question?

Thoughts, comments, observations are welcome.



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RE: Reflexed Airfoil and Flying Wings - Downwash? - 3/17/2005 3:07:09 AM   
BMatthews



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Status: offline 		Ya know, I was going to raise this in the the recent thread about pressure vs downwash but I figured there was enough darts and arrows flying around to not get into it. But it would sure be interesting to see a smoke tunnel pic of a reflexed plank style wing section.

But in the meantime if we stick with the idea that lift produces a pressure differential which has the effect of producing downthrust (note how I don't suggest that either is the source of lift.... ) then I think we need to admit that the reflexed airfoils still produce downwash. But perhaps it's not as much and hence the limited lift coefficients of such airfoils and the positive pitch moment.


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RE: Reflexed Airfoil and Flying Wings - Downwash? - 3/17/2005 11:44:01 AM   
RAPPTOR


 

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From: WELLINGTON, FL, USA
Status: offline 		reflex moves the center of pressure/lift forward.. you can fly without elevator.using ALERONS AS elevons.. reflex is your buddy if your wing is swept back alot..flaps down,pitch down,reflex,pitch up..cool huh !!! RD

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RE: Reflexed Airfoil and Flying Wings - Downwash? - 3/17/2005 2:35:59 PM   
Ben Lanterman



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Status: offline 		My foamys are flat panel thin foam with the only reflex in the ailerons and several have been perfect rectangles so there isn't a sweep issue to muddy the waters. I have to think that the more forward negative pressure is cause the vertical velocity of the air above the wing to occur sooner but the acceleration is reduced a great deal and then reversed as the air approaches the trailing edge. That still leaves a down velocity of reduced magnitude.

The flying wing and normal configurations can both produce the same amount of g's with the flying wing needing some more angle of attack - it does sound right. But the wonder in the mind has to be the direction of flow at the trailing edge. At the TE of the wing and right at the surface of the wing it is going up - no doubt. A few percent of the wing chord above the wing and at the TE of the wing the flow is going up. We have to go fairly far above the wing at the trailing edge where the flow isn't going up.

That would argue that the downwash isn't effecting the wing any more in the area of the reflexed ailerons which is the reason the cp moves forward. So most of the lift on the wing is pretty far forward where the downwash velocity is minimal. If lift is a mass times velocity = mass times velocity downwash rocket kind of thing then I don't think it is working right. It would seem that the lift as response to pressure gradients (defined as suction) is the most logical explaination. Most pressure differential, most lift.

You have got to wonder how the air going down pretty far above the wing is managing to get the wing to go up. It would seem that the air pushing up below the wing is the proximate cause of the wing going up.

And like these discussions the air rushing into the low pressure (downwash) above the wing (probably very hot air in this discussion) is not the proximate cause of the lift and is not a reaction to the lifting process but a result of the lifting process. There is a difference between proximate cause and reaction - I believe. Some definitions in physics are in order.

Proximate - Closely related in space, time, or order; very near.

Reaction - An equal and opposite force exerted by a body against a force acting upon it. The force due to the mass and acceleration of the downwash is not directly reacting to or on the wing. Downwash is a result of lift and is equal and opposite to lift but that is all. It is neat and all that but didn't cause the lift.

Take a wing sitting still and reproduce the downwash field from appropriately shaped nozzles. Which way will the forces on the wing go? It will lift down.
Take a wing sitting still and reproduce the pressure field from appropriately shaped nozzles. Which way will the forces on the wing go? It will lift up.

Saying downwash is causing lift is like saying the guy that riveted the wing created the lift. Both are just results in one form or another. But that direct cause? - varying flow velocities creating varying pressures around the wing and whow - lift.


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RE: Reflexed Airfoil and Flying Wings - Downwash? - 3/17/2005 5:02:23 PM   
mulligan



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Status: offline 		Ben, I'm following you around in these threads it seems

Downwash is often thought of as the cause of lift. Rather, it is a result. Everyone (well, ok, let's not go there) knows Bernoulli effects cause accelerated flow and lower pressure over a curved surface. In an airfoil, when this happens (moreso) over the upper surface, it causes a flow field around the ENTIRE airfoil. While most people think about the downwash at the TE of an airfoil, many do not talk about the upwash at the LE. Both are a part of the overall flow field around/over the airfoil. You won't have downwash without an equal amount of upwash at the front (m dot in = m dot out). I wouldn't begin to think of downwash as an independent force on the airfoil- it makes the whole analysis a sequential discussion, when the physical phenomenon is not.

The idea of a reflexed airfoil in a flying wing is simply a means to shift the pressure distribution so the natural moment isn't as great, since you don't have a tail moment for pitch control. These airfoils are not as efficient (L/D), but that is offset (presumably) by the drag savings of not having a tail.

< Message edited by mulligan -- 3/17/2005 5:03:34 PM >

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RE: Reflexed Airfoil and Flying Wings - Downwash? - 3/17/2005 5:37:24 PM   
Ben Lanterman



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Status: offline 		Agreed!


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RE: Reflexed Airfoil and Flying Wings - Downwash? - 3/17/2005 6:32:15 PM   
BMatthews



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Status: offline 		I'm guessing that the reflex redirects the air a little but perhaps these airfoils actually fly at an apparently higher than normal angle of attack to achieve the required lift coefficient? That higher angle would counteract the reflex and still produce the proper pressure differential and a corresponding downwash.

Consider that a Clark Y flying inverted is your untimate reflexed airfoil. But it can still fly that way as long as the angle of attack is negative enough.

I know it's only someone's translation but I tried out an inverted airfoil in Foilsim. Minus 4 degrees of camber and a 10 degree angle of attack. I noted that the airflow lines and pressure distribution chart shows a strong low right up by the leading edge and the downwash lines are weak and basically just flow off the rear at about the same line as the angle of attack. So I guess it's accurate to say that the reflex and far forward high point (most plank type airfoils have the max camber located at around 25 to 30%) all conspire to move the lift pressure forward and thus produce a positive pitching moment airfoil. But they do so at the expense of manuevering the air for the most lift.

Ben, you flat foamies are still using a "symetrical" airfoil. If you tried an S curve you'll find you need a bit more reflex I suspect.


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RE: Reflexed Airfoil and Flying Wings - Downwash? - 3/17/2005 7:06:53 PM   
Ben Lanterman



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From: St. Charles, MO, USA
Status: offline 		Probably, I used to have an old Hustler Delta by Weldon Smith (if I remember the name right). I still have plans and wing ribs if the urge ever hits me. It used a .19 and was really big, I think 3 inch or more thick and lots of reflec and forward camber. I guess it might hav been efficient from drag points of view but sure was a lot of work. I believe later on he went to a .15 powered Pylon racer that was just symmetrical airfoils with reflexed trailing edges.

I tried a foamy a couple of years ago with the leading edge curve, etc and it wasn't all that great. Of course the modern electric power plants and Lipoly cells can cover a host of sins.




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RE: Reflexed Airfoil and Flying Wings - Downwash? - 3/17/2005 7:44:10 PM   
adam_one


 

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quote:

Downwash is often thought of as the cause of lift. Rather, it is a result.

You just can't have one without the other, in fact they occur simultaneously, you can't get a pressure difference without pushing the air somewhere else.

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RE: Reflexed Airfoil and Flying Wings - Downwash? - 3/17/2005 7:46:51 PM   
Ben Lanterman



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I just found this again, excellent article - Read it then read the whole site -

http://www.av8n.com/how/htm/airfoils.html


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Ben Lanterman

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RE: Reflexed Airfoil and Flying Wings - Downwash? - 3/17/2005 9:16:37 PM   
Gremlin Castle


 

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Status: offline 		This looks like a pretty well written site based on sound theory(mostly) and real world flight experience. I did not find anything in there that would lead a person seriously astray.
How about plate compression? At first I thought it was best used around picnics when the trash can got filled but now I also think that foam flyers have a real friend in this theory.
quote:

ORIGINAL: Ben Lanterman


I just found this again, excellent article - Read it then read the whole site -

http://www.av8n.com/how/htm/airfoils.html



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RE: Reflexed Airfoil and Flying Wings - Downwash? - 3/17/2005 11:43:47 PM   
Tim Green


 

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quote:

ORIGINAL: Ben Lanterman
Take a wing sitting still and reproduce the downwash field from appropriately shaped nozzles. Which way will the forces on the wing go? It will lift down.


I don't see how this is correct.

If you attach the nozzle of an airhose to a wing, and the nozzle is angled down, it will cause the wing to move up.

Anyone who's held an airhose in their hand has felt the reactive force against their hand as the air jets away from the hand. Point the nozzle down, and it will push your hand up.

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RE: Reflexed Airfoil and Flying Wings - Downwash? - 3/18/2005 12:51:23 AM   
BMatthews



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From: Burnaby, BC, CANADA
Status: offline 		I think the idea of trying to explain this with air nozzles (and vacuum cleaners) to try to split up the factors of lift is misleading in the extreme. The whole thing is not an easy concept to grasp but it is necessary to grasp the whole gestalt or none at all.


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RE: Reflexed Airfoil and Flying Wings - Downwash? - 3/18/2005 2:17:33 AM   
dick Hanson



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Status: offline 		The wing pitches over - (down) but what does this have to do with the price of eggs ?

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RE: Reflexed Airfoil and Flying Wings - Downwash? - 3/18/2005 2:25:12 AM   
mulligan