Ben Lanterman

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.

BMatthews

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.

RAPPTOR

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[8D]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

Ben Lanterman

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.

mulligan

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.

Ben Lanterman

Agreed!

BMatthews

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.

Ben Lanterman

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.

adam_one

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.

Ben Lanterman

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

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

Gremlin Castle

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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.

Tim Green

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.

BMatthews

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.

rmh

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

mulligan

Yes, they go together. That was my main point- that it is a system, a complete flow field. In visualizing the phenomenon, though, some often make the mistake of thinking of the downwash as the cause of lift- this has the conotation of impulse forces, not the overall flow/pressure field.

Even in your statement above, you slipped into the tendency of talking only about "pushing the air somewhere" without acknowledging what happens on the LE side, which also must occur and could be similarly characterized as "sucking the air from somewhere".

LouW

Ben, you are obviously fishing for someone to come forward with a momentum comment so that you can demolish them with your superior argument. Well here goes.

First let me be clear on terminology. Typically when an engineer refers to downwash he really means “downwash angle�. When discussing momentum, “downwash� means the mass of air accelerated downward by the passage of the wing. The significance of this difference becomes obvious when considering the frame of reference. When considering the effect of the air on the wing, it makes no difference whether the air is seen as flowing past the wing (wind tunnel) or if the wing is passing through still air. When considering the effect of the wing on the air, it is easier to understand by considering the wing to be moving through still air.

When a force acts on a mass for an interval, the acceleration begins when the velocity is zero and velocity is a maximum at the moment the force is no longer acting. In the case of the air affected by passage of a wing, the force is first applied as the area of low pressure is encountered but downward velocity is initially zero. As the wing continues to move past, the mass is accelerated at varying rates depending on the distribution of reduced pressure above the wing. When the trailing edge is reached, there is no longer any accelerating force but the velocity is at a maximum. What happens at the trailing edge and beyond is no longer significant

Now consider your flat airfoil with reflexed elevons, operating at some angle of attack. Since lift is being developed, there must exist an area of reduced pressure above the area of the wing forward of the elevons. (Indeed one of the primary effects of the reflexed elevons is to move the center of pressure forward in order that the wing be both balanced and stable.) Air above is being accelerated downward toward this area of reduced pressure. It will have achieved some downward velocity by the time it encounters the reflexed area where the pressure is now positive. The air will be somewhat retarded as this area of positive pressure passes but since the wing is developing lift the net reduced pressure will result in a net acceleration of the air downward. After the wing has passed, the air that was initially still will now have a motion downward and slightly forward. For a given weight and span, this “downwash� will be the same whether it is a flying wing with the trailing edge reflexed or if it is a conventional airfoil with a tail.

Since the reflexed portion reduces overall lift at a given angle of attack, the reflexed airfoil is not quite as efficient and must operate at a somewhat higher angle of attack to support the airplane, but that’s the price you pay to achieve both balance and stability without a tail.

The article you referred us to was truly excellent. Section 3.15 Momentum in the air is one of the best explanations of how momentum exchange is involved in the production of lift. The first sentence states:

It concludes with:

Mr. Denker has done a remarkable job of explaining a complex subject.

adam_one

My point is: neither the air nor the wing can suck, they can only push.
The article that Ben has referred to has the following sentence (please read the fourth row under the fig 3.2):
Downwash behind the wing is relatively easy to understand; the whole purpose of the wing is to impart some downward motion to the air.

Also, in the chapter 3.15 Momentum in the Air one can read the following:
...The earth transfers downward momentum to the airplane (by gravity). The airplane transfers downward momentum to the air (by pressure near the wings). The momentum is then transferred from air parcel to air parcel to air parcel. Finally the momentum is transferred back to the earth (by pressure at the surface), completing the cycle...

KenLitko

!!!!! - Troll Warning - !!!!! -> But I thought that this was some actual food for thought.

For the uninitiated.. I support the argument that momentum is involved in lift... but net momentum is zero... momentum doesn't contribute an inertial force to lift. Lift if the result of the pressure distribution around the wing.


The food for thought...

1) If a wing's passage makes the air move downward and slightly forward, by the momentum argument, the wing should move upward and slightly backward. Action... reaction. Thrust is not involved and we need no energy input into the system.

2) If the air above a wing is accelerated downward due to the low pressure, it follows that the air below a wing is accelerated away from the wing due to the high pressure. It follows then that because there is no lift at a wing tip (we ran out of wing!)... there is no pressure to accelerate air... and wingtip vortices do not exist.

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Commentary...

Lift requires energy input. Wings do not work without using up energy. A complete explanation will not be found without discussing thrust or energy input.

Homework... discuss why a propellor works most efficiently only at the design speed and altitude. Be sure to discuss pressure, momentum and the shape of the streamtube entering and exiting the propellor disc. Flow through a propellor is unsteady, but will suffice to make a nice discussion of the pre-mentioned flow phenomena. Relate this to the flow around a wing.

Extra credit... discuss the flow ahead of a wing, around a wing, and behind a wing as a wing passes through the air. Recall that we are talking about low-speed aerodynamics... the flow is incompressible. Recall also that because the flow is subsonic... the wing affects the air in front of it.

Bonus, super extra credit for explaining why this (incompressibility) is very important and how it relates to momentum and pressure...

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OK... done for now :-)

Ken - www.litkoaero.com

adam_one

That condition can be easily simulated/observed if you start blowing air towards a motionless wing, the wing might move both upward and backward depending on the direction of the incoming flow.
But, if you assume that it's the wing that's moving, then you have to have thrust to compensate the backward force (wing drag).

The wing tip vortices do exist because the air from the high pressure under the wing moves to the low pressure above the wing at the wing tips instead of being deflected downward as it does at the other parts of the wing.

KenLitko

OK... vortices exist... So we have upwash next to and behind the wing? Can you quantify how that affects lift? After all... if we are moving mass upwards it follows that it carries momentum upwards.... which would contribute negatively to lift.

The following is copied from http://www.grc.nasa.gov/WWW/K-12/airplane/downwash.html

But that doesn't make sense!!! If the downward flowing air is increased (the downwash), shouldn't the lift increase??? Action-reaction. And actually the upwash in front of the wing was decreased... at least with respect to the frame of reference of the flow direction.... and we still have less lift.

If downward moving air decreases lift... does that mean that upward moving air increases lift?

Doesn't the downward deflected air increase the amount of lift???

Still trolling...

New commentary...

With wingtip vortices we have flow circulating in the flow direction... downwash in the middle behind a wing and upwash outside of the wing tips. There is a change in the local angle of attack that in effect gives us less upwash and more downwash... but we have less lift.

The act of air moving from the bottom of a wing to the top of a wing consumes energy... the amount of energy consumed in this process is drag. Essentially it's lift in the wrong direction.

There is a tremendous amount of downwash behind short wings (low AR) and not so much behind long wings (high AR).

So for wings that produce less lift per span, we have more downwash???

Ken - www.litkoaero.com

adam_one

One should not mix the wing tip vortices (vortex) with the wing's downwash.
Both occur at the same time due to the same pressure differences but at different locations where the tip vortices have negative effect on lift. The wing tip vortices are the price we pay for having lift as they create induced drag.

However, in order for the plane to support its weight, it has to yank down on the air.
The air that has been met by the plane will have a descending motion relative to the rest of the air. The wing tip vortices mark the boundary of this region of descending air.

The total force created by downwash is equal to the mass of the air, times its downward acceleration. This total force created is higher than the lift created, since the total force has also the drag included.

Yes, the induced drag increases when you have low speed and/or short wingspan, because the wings are then meeting a small amount of air per unit time and yanking it down violently, producing strong wake vortices.
In contrast there is very little induced drag when you have high speed and/or long wingspan, because the wings are then meeting a large amount of air per unit time, pulling it down gently and thereby producing weaker wake vortices.
But the resulting total force created by the downwash will be the same in both cases above assuming the same weight and drag.
The difference is that the mass of the air deflected when the plan is flying fast and/or having long wingspan, is spread through a larger area than when it's flying slow and/or having short wingspan.

Tim Green

The tip vortex caused by air moving from under the wing, past the wing tip, where it gets an upward momentum. This upward momentum at the tips detroys some of the wing's lift, counteracting some of the lift due to the downward momentum of the air created everywhere else along the wing.

I don't see this technically, as drag. I see it as lost lift. But I can understand modeling this lost lift as drag, for the sake of modeling a set of calculations for predicting lift.

Or does the definition of induced drag already take this into account?

Or am I missing something - and the wing's really dragging some air behind it at the tips?


BTW - here's a quote from NASA ...

"Lift occurs when a moving flow of gas is turned by a solid object. The flow is turned in one direction, and the lift is generated in the opposite direction, according to Newton's Third Law of action and reaction. Because air is a gas and the molecules are free to move about, any solid surface can deflect a flow. For an aircraft wing, both the upper and lower surfaces contribute to the flow turning. Neglecting the upper surface's part in turning the flow leads to an incorrect theory of lift."

You can find it here ...

http://www.grc.nasa.gov/WWW/K-12/airplane/lift1.html

And an interesting book, by a couple of guys with plenty of credentials (yes - it's pro-newtonian - what else would I suggest?) ...

http://www.aa.washington.edu/faculty/eberhardt/lift.htm

Ben Lanterman

There are some darn nice thoughts going on, I have been trying to catch a cold and threw in a migraine or two and yeeck. My head aches.

Adam-one remember air doesn't yank on the air above it, otherwise there is hope for you - sorry about that.

Lou and other guys nicely worded and thought out. The only thing missing in the discussions is how the F=ma and momentum changes and all the flow moving up and around the wing in all of the places gets converted into the pure lifting force that is felt on the wing.

I tried to pretend I was in the wing with no concept of what the air was doing. Noticing that I am off the ground I assume a big rope is holding my wing up. Looking around I don't see a rope, nor hands or rubber bands (sorry). But looking at the skin of my wing I see the covering ballooning up on the top and pushing upward on the bottom. Cool I think (it's my story and I can be cool) what ever is making the covering of my wing deflect must be involved in the lift! I stick some little pressure and velocity probes through the wing into the area just a 1/4 inch above the wing and see what I get.

OK I measure maybe just a little above ambient pressure under the wing and just a little higher velocity than the wing is moving.

Above the wing and along the leading edge I measure big velocity increases and a much lower pressure. Toward the trailing edge of the wing I don't measure much pressure and the velocity along the top of the wing surface is flowing nicely parallel to the wing and then is gone aft somewhere.

The last time I see the air on the top or the bottom of the wing it is flying aft just about the same speed as the wing is moving. As I squint and look at a little farther aft I see just as the air left the wing it starts going down fairly rapidly. Neat I think - I wonder what the heck it does that for? Who knows.

Getting back to what I am seeing in my wing I do some calculations and sure enough, when I mess with the pressure integrals it is equal to my wing weight and excess fat body. So I am compelled to say that - the pressures are the direct cause of the lift. Apparently the increase of velocities all ove the place has something to do with the lift and I determine to find out someday how it is related. But I seem to have all that is necessary to determine my wing's lift.

Later on that day I drop into a chat area and someone mentions relationships between downwash and lift, it sounds as if when the downwash goes down the lift goes up. Cool I think, but how the heck does the air going down make the wing go up. When I measured the velocities right next to the wing the air was following the surface of the wing and going backwards really fast. I remember seeing that when the air was aft of the trailing edge it started going down but that is all.

But if these guys were right then the air going down must somehow get all redirected or something to push the air back under the wing so it will hold the wing up. So I think all that downwash must be making a big positive region of air under the wing. Somehow it must come down over the wing, back of the wing and make pressures under the wing and hold it up.

Then I fly through a cloud. Looking around I see the little streamlines flowing around the wing and think neat - you can actually see where the air is going. Can't see the pressures though, just the direction the air is taking over and under the wing.

Then the next cloud I fly though I see as my wing goes through it the cloud is rushing toward the wing but it never touches the wing. Just as it gets close the air coming over the wing going real fast pushes it to the rear. Hummm - I think, that air coming down into what must be a low pressure never gets to touch the wing. How could it possibly influence the wing? Doesn't seem likely but those guys said it is causing the lift by Newton and F=ma. F=ma -- OK -- but the clouds that I am watching don't ever hit the wing on the top. And they certainly can' get around and hit the wing on the bottom, Where the hey is that F hitting.

Yea I see the clouds coming down to the top of the wing and they are being accelerated so no doubt the air is too, but they don't hit the wing. Could it be that the wing is somehow making the air flow down - sure the air is flowing down into those big negative pressures I measured earlier in the day. That is pretty cool. My wing goes through the air and causes air above it to be "sucked" down an as I keep going I can look aft of the wing and see where the air is still moving down and pushing the clouds down and aft of the wing down.

This is neat I think. But what does that have to do with the fact my wing has pressure on it and is flying? I think a really long time on this. Could it be the same amount of ma that the cloud and air above me has is equal to the ma that my wing and I have? Darn it works out that way.... cool again. But just because they are equal what is the downwash ma pushing on. Since the pressure on top and bottom of the wing seemed to be lifting the wing how does the cloud ma get around to pushing on the wing.

The big kids keep saying that it's action-reaction, Newton, but I look around my wing and I just don't see that cloud hitting the wing. I see velocity fields that were proportonal to the pressures thay made and I saw the wing make the air above it flow downward but if I look around for something to be pushing on the wing I am having a hard time finding something. For the F=ma Newton thing to work that downwash must be pushing on something, maybe if I look hard enough I will find it. A place on the wing where the clouds in the downwash actually touch the wing.

No, I couldn't find it anywhere, it kept being blown off the wing before it ever touched the wing. Could it be that the big negative pressure is just "sucking" the air down as I go by and it always happens that the downwash is equal to the lift??

Oh I forgot to check, I bet if when the wing sucks the air down that the air sucks the wing up, sure that is how it works, then F-ma. I bet it is just like some, what, hummmm, oh yeah, like suspenders holding the wing up. All I have to believe is that the air can suck up the wing. Then the air F=ma coming down is equal to the wing F=ma going up. I don't have to worry about the messy pressures and questions like how does the downwash get around to pushing on the wing.

Shoot I bet I can make some good cases for the earth pushing some F=ma too, after all the downwash eventually gets down there and if it does then probably the wing F=ma is equal to the Earth F=ma. After all, all we need is for the numbers to be the same and no doubt that makes them directly related, not just a cause and effect that don't exchange forces because air can't suck. No sir, that little problem went away just as soon as I believe that the wing is sucked up by the downwash.

After awhile I'm sure I will believe it, just a little more convincing is all that is needed.

RAPPTOR

My Feedback: (41)

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 >
THIS IS ALL THAT NEEDS TO BE SAID.. [X(] THE CENTER OR PRESSURE MOVES!! FLY IT INSTEAD OF WRITING IT!!! MAN OH MAN RD

adam_one

After all those interesting observations it seems that you've missed an important detail:
In order for the wing to stay up there, it has to send air down.
And by the time you see the air going down (downwash) the wing has already gone, yes, that's a dynamic process.
You don't need to be sorry Ben, that's undeniable facts and they're as simple as they sound.