Airplanes with flat plate wings

| Doug McLaren wrote: | | > To be more precise, I don't belive that aerodynamic lift can exist | > without downwash.
... | You are trolling I suppose?
Not at all.
| You do know of the various ways to simulate a wing of infinite | aspect ratio?
I guess I can think of a few ways to approximate it. Probably the simplest would simply be a wing with a very ver large aspect ratio, though I am aware that there are other ways.
But even after ignoring any effects of the ends of the wings, if a wing is creating lift, it's also creating a downwash.
| No downwash does not need to violate any known physical laws, | Newton's or otherwise. Or do I misunderstand your request?
I think you're misunderstanding Newton's third law, but I'm also aware that 1) this has been discussed here before, and 2) I think you've seen it, and 3) if you weren't convinced then, I'm probably not going to convince you now.
And besides, I wouldn't want byrocat to get this thread closed. That was his last warning, after all.
| If I understand your request then I am happy you feel I have such a | bright future. But I would point out others knew this close to 100 | years ago so I doubt if I am in postition to collect much reward.
I'm not so sure about what they `knew' 100 years ago, but I see little point in debating it here. We could move it over to a more appropriate newsgroup if you wished -- sci.physics or rec.aviation.misc would probably work, with the former being a bit more appropriate.
Ultimately, with a model airplane that's grossly overpowerd, you can do lots of things regarding the airfoil and wings and such and it'll still fly. An extreme example is the `Hydroboat' that we've seen lots of movies of -- I don't think anybody will claim that it's an efficient flier, but it's pretty obvious that it does fly.
--
Doug McLaren, snipped-for-privacy@frenzy.com
-- Whip me. Beat me. Make me install Oracle. --
  Click to see the full signature.
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On Sun, 18 Dec 2005 21:13:12 GMT, snipped-for-privacy@frenzy.com (Doug McLaren) wrote:

Hey Doug-
Ain't it amazing how many folks slept through Physics 101, ya' know Newton, F=MA, and such?
People that don't understand the part of downwash in producing lift should stand in the sand under a hovering UH-60 Black Hawk until they gain enlightenment. A basic education in aerodynamics is only seconds away.
Abel
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Doug McLaren wrote:

Perhaps our problem in getting to a common ground is we simply are not talking about the same physical phenomena? I am going to try to state in fairly rigourous terms what I think you mean by downwash. If I am incorrect in any way in trying to state what you think please correct me as needed.
You have clearly stated that you believe there are two fluid flows, one is tip vortex and the other is down wash. So let me set up a frame of reference to talk about so we are all on exactly the same page.
Consider an airfoil passing thru still air at exactly 5000 feet elevation above mean surface level. Now inscribe a sphere such that the sphere is exactly 5000 feet above mean surface level. This sphere will just touch the trailing edge of the wing as it passes through the air. The sphere will cover the whole earth at 5000 feet altitude. Locally I think you can consider this sphere to be a flat plate. I am going to ignore the occasional mountain that sticks through the sphere as you do not want to fly into the side of a mountain in general.
Now tip vortex induces an outboard circulation of air in the vertical direction and an equal downward circulation behind the end portion of the wing. This vortex is moving real fast. So fast that it is not uncommon for the rate of spin to cause adiabatic cooling in the center of the vortex sufficient to condense water vapor into water droplets. This is the condensation trail you can often see behind the wing tip. With respect to the surface of the sphere I described above there is an exactly equal mass of air moving vertical and outboard across the sphere to the mass of air moving downward across the sphere behind the wing. This vortex does not produce any lift at all as you are moving exactly equal masses up and down across the surface of the sphere. Yet it took a bunch of energy to produce the vortex. The energy the wing gave up to the air to produce the vortex appears as drag on the wing. As no lift was produced and work was done drag is the only available option.
Behind the wing there is also downwash of some magnitude. All of the mass of air in this downwash is permanently injected BELOW the surface of the reference sphere. You are saying that this permanent injection of a significant mass of air below the reference sphere causes lift based on Newton's action/reaction law. It would seem that the air must be permanently injected below the surface of the sphere, otherwise it would simply be part of the inner portion of the tip vortex.
Things to consider. If I shrink the sphere enough there will come a time when the air is no longer injected below its surface. For instance at one inch above mean ground level I think it is safe to say that none of this downwash is felt. So I presume at some altitude of the sphere between one inch and 5000 feet there exists a sphere surface that the air no longer penetrates. As an experiment that any of us could do we could stand off the end of a runway as planes approach to land. It is pretty easy to get to points where the plane passes overhead at an altitude of no more then 500 feet. I have done this on numerous occasions myself. I notice that even under large commercial planes which weigh half a million pounds I can not detect the slightest downward air flow when I am standing on the ground. I recently did exactly this at an airport in KY and watched the leaves of trees right next to me. I suppose these trees were 50 feet tall. The wind was dead calm. I not only could not feel any downwash from the wings I could not see the tree leaves stir in the slightest either before or at any time after the plane passed directly overhead. So it starts to sound a lot like the downwash induced by the wing does not travel very far below the surface of the sphere that just touches the trailing edge of the wing.
Please correct the above to your understanding if I am not correct in stating what you believe is physical reality.
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| > | Doug McLaren wrote: | > | | > | > To be more precise, I don't belive that aerodynamic lift can exist | > | > without downwash. | > ... | > | You are trolling I suppose? | > | > Not at all. ...
| Perhaps our problem in getting to a common ground is we simply are not | talking about the same physical phenomena? I am going to try to state | in fairly rigourous terms what I think you mean by downwash. If I am | incorrect in any way in trying to state what you think please correct | me as needed. | | You have clearly stated that you believe there are two fluid flows, one | is tip vortex and the other is down wash.
No, I haven't. What I have clearly stated is right up there, and I didn't trim it out so you can refer to it if desired.
What I mean by downwash is that if a wing is creating lift in a given direction that air must be deflected in an opposite direction. If the wing is flying level, creating a lift vector that points up, the air being deflected downwards is often called downwash.
| Behind the wing there is also downwash of some magnitude. All of the | mass of air in this downwash is permanently injected BELOW the surface
Permanently? Air moves. It's a fluid.
| of the reference sphere. You are saying that this permanent injection | of a significant mass of air below the reference sphere causes lift | based on Newton's action/reaction law.
No, I'm saying that if there's a force pushing the wing up (often called lift) then there's a equal and opposite force pushing something down, and when that force is applied to the air around a wing, the end result is often called downwash. I don't care if the air movement takes the form of vortexes or whatever, but ultimately air is pushed/accelerated downwards.
Though the air doesn't go down far, as the air molecules bounce into other air molecules and eventually come to rest again. But ultimately aerodynamics, fluid dynamics and indeed thermodynamics in general aren't about tracking individual molecules -- they're about looking at the average behaviors of lots of molecules.
This page has a nice picture of a jet coming out of some clouds, about half-way down --
http://www.allstar.fiu.edu/aero/airflylvl3.htm
(Even if you feel that the page is full of nonsense, you can look at the pretty pictures without being corrupted, right?)
and you can definately see the effects of the downwash. You can also see that it doesn't punch a hole in the cloud all the way down, as the airflow gets too spread out, and the plane has already left.
Note that a helicopter in the same place could eventually `cut' a hole in the cloud, as it would not be moving on as it creates lift. (You do agree that propellers are just wings that are spun around, producing lift, right?)
| I notice that even under large commercial planes which weigh half a | million pounds I can not detect the slightest downward air flow when | I am standing on the ground.
If the 747 flew 15 feet over your head, you'd feel it -- though it certainly wouldn't crush you, and more of what you'd feel would be the turbulence and such caused by it.
If it's 5000 feet up, the weight of the plane is spread out over many millions of square feet, and so only a very tiny part of that would be pushing on you -- and the atmosphere is pushing up on you too, so you won't feel it.
Really, what would happen is that the atmospheric pressure would increase very slightly under (well, under and behind) the plane. The increase and then decrease would be rather gradual (because it would be so spread out), and so I doubt the human body could detect it unless the plane was right overhead, and a leaves on a tree won't move either (as the pressure on both sides of the leaf will still be the same.) But I'll bet it could be measured with sufficiently sensitive detectors.
A somewhat similar situation happens when swimming. If you dive 30 feet down, there's now tons of water above you, but you're not pushed down further because there's water below you pushing up. (The pressure will nearly double at that depth, however, and so that could eventually crush you if your body couldn't adjust it's internal pressure (fortunately, it can.) But this pressure change (1 atm to 2 atm) is still millions of times higher than what would be caused by a 747 flying thousands of feet above you.
| next to me. I suppose these trees were 50 feet tall. The wind was | dead calm. I not only could not feel any downwash from the wings I | could not see the tree leaves stir in the slightest either before or at | any time after the plane passed directly overhead. So it starts to | sound a lot like the downwash induced by the wing does not travel very | far below the surface of the sphere that just touches the trailing edge | of the wing.
Either way, the Earth is ultimately supporting (on the average) the entire weight of the plane, both when it's landing and when it's flying.
If you want an analogy, take a 1 lb bird in a 10 lb bird cage. If the bird is perched, a scale will read 11 lbs for both. If the bird is hovering in the cage, what will the scale read? 10 lbs? 11 lbs? Something in between?
(Of course, the answer is it depends. If the cage is solid like if it were made of glass, the weight will average out to 11 lbs. If it's an open air cage like most captive birds live in, one that's made of wire, then the average measured weight will be slightly more than 10 lbs. And if the cage is merely a theoretical construct with no bars at all (but it still weighs 10 lbs), then the weight would be 10 lbs.
The Earth and it's atmosphere would generally correspond to the totally enclosed glass cage.
| Please correct the above to your understanding if I am not correct in | stating what you believe is physical reality.
This stuff really isn't on-topic at all. Would you like to move it to sci.physics? (I suspect that the regulars there would rip you to pieces, but you might enjoy that.)
To make this somewhat on-topic, do you not feel the downwash of a R/C helicopter or 3D plane when it's hovering just above you? In that case, the propeller is just another wing -- the only difference is that it's going around in circles rather than standing still, which also makes it easier to detect as the effect isn't moving.
And to talk about flat wings and propellers, often ceiling fans and the like have flat blades (wings) -- since the air velocities involved aren't very fast, any lack of efficiency isn't very signifigant, and so it's just cheaper to do it that way. But in a R/C model prop, you'll find that the propeller does have an airfoil shape -- this is because it's more efficient than a flat blade, though a flat blade would work too -- just not as well.
--
Doug McLaren, snipped-for-privacy@frenzy.com I find your lack of faith disturbing.

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People on the ground don't feel airliner downwash because it's spread out and because they are too close to the gound themselves. The air movement near the ground from an aircraft at altitude is minimal on nothing. The folks who run orchards often have to deal with the risk of frost early and late in the season. As night falls and the sun stops heating the earth's surface, the temperature of the air near the ground will fall and if it's cool enough frost will form on whatever it can, including fruit. Not far above the surface, however, the air temperature has remained essentially what it was during the day, and we have what's called the nocturnal inversion. Orchardists sometimes contract air operators to fly their small aircraft over the orchard on frosty nights, at around 50 or 75 feet, to drive the warmer air down into the trees. If there was no downwash created, there would be a lot of wasted time and money in such pursuits, and a lot of dead fruit. Peter Garrison, a writer with FLYING magazine, once wrote of a trip he made to a Mexican airport. Security being what it often is in such places, the folks he met at the airport took him out to the edge of the runway when they saw a 747 approaching to land. Laying down in the grass, they waited as the jet passed over just before it was to touch down, and the downwash and vortices (which are a result of the lift/downwash pehnomenon) picked up Garrison and his friends and threw them some distance through the grass. Great sport. Can't tell them that there's no downwash below an airplane. I also have a friend who lost an airplane to helicopter downwash when the machine hovered too close and blew the airplane against the hangar. Downwash and vortices are most apparent (and most destructive) when the airplane is heavy and at low airspeed. Aircraft spacing along airways and on approach has to take into account the disturbed air, lest the following pilot loses control.

>They are equalivalent for a symetrical foil.) generated more then

Nothing. Some of the pressure you measured on the bottom was reaction pressure caused by deflecting air downward and creating downwash. Any pressure above ambient would be from reaction.
Dan
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At Sky Harbor airport in Phoenix years ago, you could park at the E end of the main runway, next to the glidepath, to watch the jets land. You could definitely _hear_ the 'birdies'- the swirling eddies made by the tip vortices. Even after the jet was on the ground and taxiing you could sometimes still hear the birdies it left in the air. Kinda cool, and spooky.
Bill(oc)
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Dan_Thomas snipped-for-privacy@yahoo.com wrote:

Yes. tip vortexes are bloody awesome. That is why FAA has a ten mile, if I recall correctly, spacing rule for aircraft flying the same path. A plane following closer could stick a wing in one vortex and find himself in trouble in the blink of an eye just as you say. Many years ago I had a friend who flew lite planes into big airports at times. He said this was a real danger during takeoff following a commercial plane. He had some waiting rule which drove the big airports nuts because he would not take off until the vortices had died. His fear was one might drift back over the runway just as he was going down the runway and he would wind up inverted at ground level. I do not know if he was on target or not. I just recall the story he told. Neat story you tell also.
By the way, check the record. I do not think I have once said that there is no such thing as downwash.

Well, I saw no pressures above ambient whatsoever anyplace on the airfoil. I will admit I did not try to measure the pressures immediately on the leading and trailing edges. These are the so called stagnant areas. On a model they are so small it would be bloody hard to measure them. But it does not matter. The stagnant areas can not be causing lift as they are not pushing up (or down). Also what very small push they have is over such a tiny area that force x area = so close to zero as to be not relevant. I did see a small reduction of pressure on the forward third of the underside of the foil thou. And that is just what you should see.
Thanks for all the comments. It is nice to know more then two of us are still on this thread.

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He might have been a bit overcautious. The vortices move outward and downward, and on takeoff or landing they will move away from the runway unless there's a crosswind, in which case the upwind vortex could be carried across the runway or even held there for some time. The general idea: If taking off behind a departed airliner, get off the ground before the point where he did, and turn away from the runway so as not to climb through his wake. He takes more runway to get airborne but can climb more steeply. His vortices don't start until he raises his nose for liftoff, since there is little lift being generated with the nosewheel on the ground. When landing, approach fairly steeply and land beyond the airliner's touchdown point. I have twice run into the wake left by airplane the same size as mine, and have been rolled 90 degrees before the airplane fell out of the turbulence. Now I stay above his path if I'm fairly close behind. If it's an airliner and I have to cross his wake, I'm a long ways off, maybe 7 or 8 miles.
Another poster says:

No, of course not. Air is heavy (the air in a room, at .078 pounds per cubic foot, can easily outweigh the room's occupants) and has terrific damping qualities. If someone blows at you from six feet away, do you feel much breeze on your face? Not feeling downwash at 500 feet doesn't mean it's not there, it just means the effect didn't reach you. As I said earlier, vortices are formed at the wingtips due to the lift/downwash/pressure differences, as well as along the trailing edge. These vortices absorb most of the downwash energy, turning it into heat. It's not the same effect as a wave travelling across water after being generated by a boat, since water is neither compressible nor will similar vortices form in it. Downwash is real and a result of displacement by the wing. Or propeller. Or rotor. Only a balloon doesn't create downwash. Does Bernoulli account for all the thrust generated by a propeller?
Dan
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Dan_Thomas snipped-for-privacy@yahoo.com wrote:

Absolutely correct. I have tried to address this exact topic with another poster but he consistantly refuses to even come close to a statement such as you just made.
As I said earlier, vortices are formed at the wingtips due to the

Well, I can not quite agree with everything you said here. Correct, the air movement behind a wing are not the same as waves in water. Also correct that the turbulance eventually winds up as heat. No other choices are there? However, air is also an incompressible fluid in any unconfined space and being effected by a way subsonic surface such as a model airplane or even a commercial plane during landing. And water forms exactly the same types of votices at the end of a lifting surface or prop as we see in air. They do not persist nearly as long because of the high viscosity of water vs air. They turn into heat a lot faster.
Downwash is real and a result of

Well, it is hard enough to understand a wing, let alone a rotating wing. But just let me say this. Many people think Bernoulli is a weak force that is inconsequencial. Much to small to do anything useful. Yet many years ago I was working with some real highly corrosive materials that also were VERY water sensitive. One common way to work with such stuff is in what is called a vaccum line. A vaccum line is simply a low pressure hunk of glass tubing with a variety of fittings on it so you can put vaporized things you are working with into the line and transfer them. This presumes they are volitile which the things I was working with were. Because of corrosion problems I did not want to use a standard mechanical vacuum pump as my forepump. It would have been tough to keep the volitiles out of the pump and if they got in there they eat the pump up and also wind up in the lab air I was breathing. One of the corrosive was gaseous hydrogen chloride. Trapping it would have been a constant headache as long as I was running the line. So I decided to use a real simple solution. I used one of those $5 plastic Bernoulli aspirators you see in just about every Chem Lab in the world. Although most are metal rather then plastic. They are a pure Bernoulli device that will pull a vaccum down to just about the vapor pressure of the lab water running through them. In my case about 30 mm pressure. So just with Bernoulli I have gone from 760 mm down to 30 mm. That is about a 95% reduction in pressure. Not too bad for a "very weak" effect. But scientists do it every day. Then between the vac line and the aspirator I put first a three stage mercury diffusion pump and a high pressure mercury diffusion pump I built myself. I used to be a half decent glass blower. Mercury diffusion pumps are also pure Bernoulli devices. The high pressure pump would take the pressure from 30 mm down to about 1 mm. And the three stage pump took it from 1 mm down to 10**-6 mm in the actual vac line itself. Thus I removed 99.9999999% of the air from my vac line. Or in pressure terms I made a 15 pounds per square inch vacuum. If an airplane wing could do this it would have a lifting force of 2160 pounds per square foot. So do not tell me Bernoulli is a weak effect. The devil is in the details.
I will say there is a heck of a lot more to a prop then simply blowing air. In fact a prop installed backwards will blow about 90% as much air as it blows if installed frontwards. I know because I measured it on a 9x6 prop at 11,000 rpm both ways. But just try to fly your plane with it on backwards and see how it flys. Actually I was surprised it did this well blowing air when backwards as you give up a ton of angle of incidence with the flat bottom foil used on a prop. Not the result I expected and still I am not sure why it did so well. It must be in part because the air speed was a fair bit below the pitch speed of the prop.

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Doug McLaren wrote:

Well I have refered to it and do not have a real clue as to what you think other then downwash behind the wing causes lift. So I am going to try finding out what you think by simply asking yes/no questions.
1.Do you believe tip vortexes are created somehow by wings? Please answer yes or no.
2. If tip vortexes are created by wings do you think they cause lift? Yes means they cause lift. No means they do not cause lift. Please answer yes or no.
3. Does a model airplane wing compress the air somehow someplace? Please answer yes or no.
4. Is the Conanda effect a direct and demanded result of Newton's laws? Please answer yes or no.
5. If I define efficiency of lift production as follows:
Efficiency = Lift produced by experimental airfoil/Lift produced by the best airfoil
And if both experimental airfoil and best airfoil are identical in aspect ratio and planform, the airspeed past the two foils is identical and the angle of incidence is lets just say 4 degrees for both, then:
*Is a flat airfoil going to have an efficiency of much less then 1? Please answer yes or no.
*Is a Clark Y airfoil flown inverted going to have an efficiency of much less then 1? Please answer yes or no.
*If the answer to either of the above two questions about efficiency is no is the efficiency of either much greater then 1? Please answer yes or no.
6. Do you think that the incidence meter you buy at the hobby shop measures the correct angle of incidence for all airfoils? Please answer yes or no.
7. If the answer to question 6 is no do you think the meter measures angle of attack for all airfoils? Please answer yes or no.

I think I said I was standing 500 - thats hundred - not 5000 feet below the plane. If I mistyped I am sorry. I intended to say 500. No downwash. No movement of leaves.

Yes, I understand the conservation of mass.

You know how to copy the thread to a new news group just as well as I do. But thinking that physicists are the sole authoritys on fluid mechanics is a bit of a push. For instance my undergrad physics book had a section on what caused a thrown baseball to curve. The only problem was the book used Bernoulli to explain why a thrown ball can curve. And it just happens to make it curve in exactly the wrong direction! Not a single professor bothered to ever tell us what the book predicted in terms of direction of curve was exactly wrong. I can only surmise that baseball pitchers know more about curve balls the physics professors.
9. On the other hand a physics professor at U of Indiana wrote an article for MA maybe two years ago and he clearly stated that in his view almost all the lift we see in a wing is due to Bernoulli. I am sure you saw his article. Perhaps you feel he was wrong? If you feel he was wrong to say Bernoulli causes almost all the lift we observe please answer yes or if he was correct please answer no.
10. In a link provided very kindly by another poster:
http://www.av8n.com/how/htm/airfoils.html
Please refer to figure 3.2. Do you believe this diagram reasonably accurately displays the actual air flow patterns around an airfoil. I will specify that this diagram is for an airfoil which has an infinite aspect ratio (which is a detail the author of the web site probably felt was unneeded detail which would confuse the casual reader). I will also specify that this diagram is for one specific reynolds number and is not intended to be accurate in every detail for every single reynolds number(which is also detail the author did not feel was needed). Further I will specify that the airflow past the foil is well under the speed of sound which I honestly do not recall if the author addressed in any way.
Would this airfoil be producing lift with the airflows as shown? Please answer yes or no.
By the way, I have seen the pic of the plane coming out of the clouds before as I am sure most people on this group have also. All I see is a very striking picture of tip vortexes. Sorry but I see no evidence of downwash in this picture at all. But it still is a really neat picture. The pic I am referring to is Figure 5 at the web site:
http://www.aa.washington.edu/faculty/eberhardt/lift.htm
It is going to take me a while to learn what you actually think by asking yes no questions. But it seems the only way forward.

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| Doug McLaren wrote: | > | > No, I haven't. What I have clearly stated is right up there, and I | > didn't trim it out so you can refer to it if desired. | | Well I have refered to it and do not have a real clue as to what you | think other then downwash behind the wing causes lift. So I am going | to try finding out what you think by simply asking yes/no questions.
Allow me to restate my position on the matter (though I did change it a bit to add in the word `wing', so it's more clear) --
I don't believe that a wing can create aerodynamic lift without also creating downwash. (At least without creative definitions of the terms lift or downash.)
You'll notice that I'm saying nothing about vortexes, Bernoulli, the Coanda effect, specific airfoils or anything else in that statement. I'm keeping it simple.
I'm still not quite certain ... do you agree or disagree with my premise? | 1.Do you believe tip vortexes are created somehow by wings? Please | answer yes or no.
I've not really said anything about tip vortexes. But it does look like wings create them, or are at least involved in their creation. Is that your point?
| 2. If tip vortexes are created by wings do you think they cause lift? | Yes means they cause lift. No means they do not cause lift. Please | answer yes or no.
I've never really said anything about what causes lift in this thread, at least not since I made my `I don't believe that aerodynamic lift can exist without downwash' statement.
| 3. Does a model airplane wing compress the air somehow someplace? | Please answer yes or no.
If the model airplane wing is creating lift, it's also creating downwash. Anything else is not something I was discussing.
Aerodynamics of far-subsonic flight is generally based on the the assumption that air isn't compressible (and that therefore the air is not compressed.) This is just an approximation and isn't completely true, but it generally works and gives us reasonably accurate results.
| 4. Is the Conanda effect a direct and demanded result of Newton's | laws? Please answer yes or no.
I've said nothing about the Conanda effect, or the Coanda effect for that matter.
Do I believe that the Coanda effect is real? It seems to be. Do I believe that the Coanda effect obeys Newton's laws? Yes.
Do I believe that Newton's laws require the Coanda effect? I don't think so, but I really haven't given it a lot of thought. Ultimately, the `laws' of aerodynamics are the end result of applying Newton's laws and other similar `fundamental' laws to large numbers of particles, so it wouldn't surprise me if the Coanda effect could be seen as ultimately the result of some application of Newton's laws. I don't see how it's relevant, however.
| 5. If I define efficiency of lift production as follows:
{ `lift divided by drag' was the general definition, which I have no problem with. }
... I don't feel like looking up airfoil data. If you have a point, provide the figures yourself, and use it to come to your point.
| 6. Do you think that the incidence meter you buy at the hobby shop | measures the correct angle of incidence for all airfoils? Please | answer yes or no.
I've never bought an incidence meter. Your question is based on an incorrect premise, and is therefore moot. That, and my fundamental premise doesn't really discuss how a wing creates lift and downwash -- instead, it just says that if you have one, you have the other.
Model airplanes are often grossly overpowered, so eyeballing this stuff often gives very acceptable results. And while I've been flying gliders a lot lately, they're ARFs or kits and so I let the designers worry about the incidence of the wing. They seem to fly well, so I'm happy with them. Perhaps I could tweak out 1.2% better performance by adjusting some things, but I don't care enough to do it.
| 7. If the answer to question 6 is no do you think the meter measures | angle of attack for all airfoils? Please answer yes or no.
Moot. My premise says nothing about angle of attack.
| > If it's 5000 feet up, the weight of the plane is spread out over many | > millions of square feet, and so only a very tiny part of that would be | > pushing on you -- and the atmosphere is pushing up on you too, so you | > won't feel it. | | I think I said I was standing 500 - thats hundred - not 5000 feet below | the plane. If I mistyped I am sorry. I intended to say 500. No | downwash. No movement of leaves.
Right, you did say 500 feet. But even so, the weight of the plane is spread out over what probably amounts to millions of square feet.
Also note that the plane is probably flying 300+ mph -- that's 440 feet per second. It's only approximately 500 feet away for a tiny fraction of a second. One second later, it's 666 feet away (assuming it's flying level, and it was directly overhead.)
If somebody blows a fan on you from across the room for one second, you're not likely to even feel it. If they left it on for many seconds, you might eventually feel it.
A helicopter involves a wing moving through the air, and yet not leaving the scene of the crime if it's hovering. Allow me to give you two yes/no questions --
1) do you believe that a hovering helicopter's blades are are fundamentally just wings/airfoils? (If you think that they somehow follow different aerodynamic rules, please be explicit about how you think they differ.)
2) do you believe that a hovering helicopter creates downwash?
| > Either way, the Earth is ultimately supporting (on the average) the | > entire weight of the plane, both when it's landing and when it's | > flying. | | Yes, I understand the conservation of mass.
It's not so much a matter of conservation of mass, but that the plane is pushing down with certain amount of weight and it has to be supported somehow, or it'll fall down. If the air is supporting it, then something has to be supporting the air, and that would have to be the Earth.
In large amounts, air is very heavy. The weight of a plane, even a 747 or so, is pretty insignificant compared to the air around it to 500 feet or so. The mass that the plane adds to a square of air 1000 feet on each side is a very small percentage of the total mass, so it's not likely to be detected by a human, but I imagine that the right equipment could detect it easily enough.
| > This stuff really isn't on-topic at all. Would you like to move it to | > sci.physics? (I suspect that the regulars there would rip you to | > pieces, but you might enjoy that.) | | You know how to copy the thread to a new news group just as well as I | do. But thinking that physicists are the sole authoritys on fluid | mechanics is a bit of a push.
I'm not saying that at all. I'm saying that it's the most appropriate Usenet group I can find for a discussion of this sort. There aren't any aerodynamics groups that I can find, and there's no rec.aviation.technology group that I can see.
I'll crosspost this post to sci.physics, and I'll set followups to go only there. If you wish to continue this discussion with me on Usenet, let's do it there rather than here.
| 10. In a link provided very kindly by another poster: | | http://www.av8n.com/how/htm/airfoils.html | | Please refer to figure 3.2. Do you believe this diagram reasonably | accurately displays the actual air flow patterns around an airfoil.
I'm not so sure about the upwash part, but the downwash part looks good. It's hard to tell from the picture, but I'm assuming that the air well in front of the wing was horizontal.
| I will specify that this diagram is for an airfoil which has an | infinite aspect ratio (which is a detail the author of the web site | probably felt was unneeded detail which would confuse the casual | reader).
If infinite bothers you, just pick a wing that's a mile long. Or ten miles long. Or whatever. As the aspect ratio gets larger and larger, the relative effects of the ends of the wing will have to get smaller and smaller.
| I will also specify that this diagram is for one specific reynolds | number and is not intended to be accurate in every detail for every | single reynolds number(which is also detail the author did not feel | was needed).
He is obviously simplifying things ... probably because he wanted to just write a few pages on it, rather than a book. Seems a reasonable compromise.
| Further I will specify that the airflow past the foil is well under | the speed of sound which I honestly do not recall if the author | addressed in any way.
His overall web site is about general aviation, so assuming speeds well below that of the speed of sound seems appropriate. Usually if somebody is talking about supersonic aerodynamics, or even aerodynamics near the speed of sound, it's usually clear.
| By the way, I have seen the pic of the plane coming out of the clouds | before as I am sure most people on this group have also. All I see is | a very striking picture of tip vortexes.
Yes, it does show the tip vortexes very well. But you'll notice that there's also a `hole' in the cloud where the plane just flew through ... but I can see where it would be convenient to blame that hole on the vortexes rather than on downwash. It seems reasonable that one wouldn't even be able to visually separate the two phenomena on any normal sized wing, at least not in a carefully controlled environment like a wind tunnel. I guess it wasn't a very convincing example -- I can see that.
| Sorry but I see no evidence | of downwash in this picture at all. But it still is a really neat | picture. The pic I am referring to is Figure 5 at the web site: | | http://www.aa.washington.edu/faculty/eberhardt/lift.htm | | It is going to take me a while to learn what you actually think by | asking yes no questions. But it seems the only way forward.
I've already made my position (at least the one I'm defending in this thread) clear. It seems to me that you're just trying to confuse it with many of your questions, but so be it.
--
Doug McLaren, snipped-for-privacy@frenzy.com
"[It's] time for the human race to enter the solar system." --Quayle
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Doug McLaren wrote:
In general a bunch of very vague stuff that can be interpreted to mean almost anything. I suspect he thinks I may just have him by the short hairs and am about to drag him around the parking lot. Well, I am getting bored so will just cut to the chase.
The problem with claiming lift is caused by downwash is perfectly simple. We have all seen the pictures over and over. All you have to do is look at the smoke streamer pictures of an airfoil in a wind tunnel. I am sure we will all agree that an airfoil in a wind tunnel is producing lift. If you look far ahead of the airfoil ( say 15 chords of the airfoil) you will see the smoke is traveling in a perfectly horizontal fashion. If you look the same distance behind the airfoil it is again traveling in a horizontal fashion. A bit in front of the leading edge you see the smoke streams BELOW the foil bending in an upward direction. Just behind the airfoil that same stream bends down until it is at its entry position. You have not "blown" any air downwards at all. You simply bent the flow up and then bent it back down. The problem with thinking about downwash as causing lift is then you have to admit that upwash caused an exactly equal downwards lift per standard Newton force relationships. F = MA You accelerated a given mass up and accelerated exactly the same mass back down. The wing got nothing out of this deal. Want to refresh your memory go to this link and look at figure 3.7 for a picture of how the air flows:
http://www.av8n.com/how/htm/airfoils.html
The only weakness of this diagram is it only shows the flow for less then one chord in front and behind the airfoil. But I am sure if you think about it you will realize that when you get a little bit farther away both flows will be horizontal in any wind tunnel and at exactly the same altitude for both entry and exit. Well, when a plane flys in the atmosphere far from any other surface it is just as they are in the wind tunnel. That is why an aircraft 500 feet over your head does not produce any detectable downwash. It is not deflecting any air from where you are upwards so it can not send any back down to you.
As I have said before in other threads there is no conflict whatsoever between Newton's laws and Bernoulli. Bernoulli did no more then take Newton's laws and apply them to a moving fluid. His equations are a direct derivation from Newton's laws and are required to be correct by Newton's laws. Anyone who has taken first year undergrad physics better know this. Although I will admit they graduate people from college today who do not seem to know anything much about even their major area so who knows?
What an airfoil actually does is divert air up and at the same time accelerate it a whole bunch. If you look carefully at the cited diagram this acceleration is clearly shown by the smoke dots on the top of the airfoil. The dots are lengthened as the air is moving faster. What is not shown clearly is the air is also accelerated under the airfoil just behind the leading edge. But this acceleration is not nearly as dramatic as that over the top. But both accelerations result directly from work done on the air by the passing airfoil.
Why does the air in front of airfoil know that something is approaching causing it to start moving? That is pretty simple. For all practical purposes every fluid has viscosity. I only know of one exception. Viscosity means a force on any one partical is transmitted in a reduced state to the next partical. In other words air is a little sticky. Due to viscosity the air ahead of the airfoil feels the approach of the airfoil and is pushed out of the way. It is this push that accelerates the moving air. The foil is doing work on the air and the work comes out as faster moving air. The air under the leading part of the airfoil also is moving faster as air needs to flow into the space being vacated by the air diverted from well under the leading edge up and over the top of the wing.
Per Bernoulli, anytime a fluid moves, its lateral pressure (the pressure at right angles to the movement) is reduced. A direct result of Newton's laws. No conflict at all.
It now should be clear that the air on top of the airfoil will exert a lower downward pressure on the foil then stagnant air would exert. Also the air below will exert a tiny bit lower upwards pressure on the bottom of the foil then stagnant air as it also has been accelerated a little. I have long since addressed why you can not get any compression of air at the tiny fraction of the speed of sound at which a model airplane flys. This is usually clearly stated in any text about subsonic flight below speeds of 350 or 400 mph. Even at those speeds the compression effects are so small as to hardly warrent consideration. So there is no place on an airfoil that the pressure can exceed the pressure of the surrounding air if is is not moving relative to the airfoil. Thus lift results from the difference in pressure caused by the accelerated air and the consequence of Bernoulli and nothing else.
Ok it is now a perfectly fair question to ask where in the world does all that tip vortex come from that we all know exists. In any airfoil which has ends to it and is producing lift the air under the end of the foil is at higher pressure then the air on top. So the air tends to flow outwards and upwards to get to the low pressure region. Air will always try to flow towards lower pressure.This air curling up and back winds up with a bunch of angular momentum and just keeps on spinning behind the wing. A vortex turns out to be a pretty stable thing relative to most distubances in air. Watch a smoker blow smoke rings sometime and note how long they will stay intact vs just blowing a puff of smoke out into the air. Another way to think of a tip vortex is as a cylinder of air spinning with a low pressure center. And we know the vortexes off a wing tip must have a low pressure center because they cool down so much water vapor often condenses causing a condenstation trail. The fast moving outside would like to collapse into the low pressure region in the center. But conservation of angular momentum prevents this from happening fast. Watch an ice skater doing spins. They start with their arms outstretched and pull their arms in. As they pull their arms in the spin speeds way up. Again conservation of angular momentum.
Just about inevitably someone in a discussion of lift brings up the Coanda effect. I asked McLaren the question about Coanda because his favorite web site on what causes lift makes it out to be a big deal. Well it is not a big deal at all. But first a word about the language of science. Some things that are basic, like Newton, are called laws. When anything is called an effect it is not a law. It is the result of some underlying law. Coanda simply says that a moving fluid will tend to cling to a solid surface until conditions are reached that induce turbulance. The real question is why does this happen? Well recall that any moving fluid exerts a lower lateral pressure then a nonmoving fluid. On an airfoil next to the top surface you have fast moving air according to Figure 3.2 above. As you go farther from the surface the air is moving slower. This slower air exerts a higher lateral pressure then the faster moving air and thus pushes the faster moving air as far away as it can. As the airfoil is present the fast moving air is trapped. Pure Bernoulli as derived from pure Newton. By the way, all the stuff on McLaren's favorite web site about boundry layers somehow being at the root of Coanda is someones dream stuff.
Lift decreases fast as you go towards the back of the airfoil. How come? Again perfectly simple. That overlying air above the fast moving stream next to the foil is slowly producing drag due to viscous effects as is friction with the airfoil. This causes the air to slow as it passes over the top of the airfoil. As it slows the pressure increases just like any number of actual pressure measurements on foils in wind tunnels have shown. The real fast stuff is towards the front and produces lots of Bernoulli and lots of lift at the front. The slower stuff at the back produces ever decreasing amounts of lift. Net result is center of lift is someplace around 30% of the chord measured from the front.
Does there even have to be any downwash behind the airfoil to cause lift? Well, for practical lift perhaps. However, if you slow the air down more and more you will reach a point at some speed where the foil no longer bends much air in front of the leading edge. Think for a moment about a slow moving foil where only the air above say the 80% point on the chord is pushed up in front of the leading edge and flows over the top. If the foil is at a few degrees angle of incidence the air flowing under the foil behind the 80% point of the chord will be pushed down by the bottom of the foil to the trailing edge. When it hits the trailing edge it must flow UP to get back to its original streamline. But that airfoil is still accelerating air over the top and will still produce lift. Not very bloody much I will be the first to admit. But more then zero. And with an upflow behind the wing instead of a downflow. In fact if you look real close at figure 3.7 you will see something interesting in the bottom illustration. The author happened to chose a flow rate and angle of attack such that the flowline passing just under the foil would just touch the bottom trailing edge of the foil if there were no diversion at all of the airflow. Look at it real close. The author correctly shows this flowline bending up to the bottom of the foil, going to the back and exiting the foil with no up wash or down wash whatsoever.
What if you do not want to believe any of the above? Well we have never passed any laws that say you must believe it or anything else in science. Personally I think computers run mostly on magic and were invented by communist devils. Some smoke also as every time it leaks it's smoke it stops working. About all I can suggest is go to any decent college entry level text book on the theory of lift and spend some time reading. I think Richard Von Mises book that I cited in an earlier post would be a fine starting point. It seems to me if it is good enough for Harvard University as a text book it is good enough for me. Brace yourself for some ugly math thou. Entry level for such courses are usually about junior year I think. Maybe senior.
If someone wants to talk about thrust from a prop and how important blowing air is go ahead and start a new thread. This one started on using flat plates for airfoils. So props is a little off topic. Besides, McLaren still can not understand the clear citations I offered to show that a flat plate is very efficient at creating lift and wants to pull drag into the discussion. I never said a flat plate was any good at all relative to other foils when it comes to drag. The topic was LIFT not DRAG.
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The air below bounced it back up, but downwash was created nonetheless. Just as a planing boat shoved water down, which was displaced back up until things were smooth again. The orchard owners who rely on downwash to keep the frost off must be badly mistaken, according to your ideas, as are the small-airplane owners who get nervous around helicopters. Downwash is a product of lift, and Bernoulli. Wind tunnels have their limitations, being so confined, and only the airflow immediately around the foil is of any real interest. The atmosphere isn't a wind tunnel.
Dan
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A better look at real-life downwash can be found here: http://www.grc.nasa.gov/WWW/K-12/airplane/downwash.html Be sure to see the picture at the bottom. The next page: http://www.grc.nasa.gov/WWW/K-12/airplane/shed.html shows how the energy is ultimately dissipated. Not that in both cases the downwash does not contine downward for any great distance, perhaps 50 to 100 feet, before it's damped out. Note, too, that the vortices play a significant role in generating it. Asnd as an earlier reference, http://www.grc.nasa.gov/WWW/K-12/airplane/bernnew.html pointed out, Bernoulli and Newton are not at odds with eash other. Many people make the mistake of insisting only one is true.
Dan
Dan
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Hi all I keep trying to visualize wingtip vortices produced by swept-forward wings, and am having a hard time with it. It's easy enough to visualize how standard and swept-back wings produce the vortices from the natural 'spillage' off the wingtip. But with forward sweep, it *seems* like the flow should be vectored more inward toward the wing root, with more 'spillage' occuring at the wing root than off the tip. The result should be less vortex-induced drag. After much Googling, I haven't found this question addressed in any depth. Not being the brightest bulb on the tree, I thought I'd inquire of the greater pool of wisdom found here. Thanks.
Bill(oc)
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Bill Sheppard wrote:

You have real decent intuition Bill. Nice to know at least a few people around this place are capable of independant thought.
The easy way to think about airflow over a wing is to start with an infinitely long, straight wing and then change one thing at a time. In a long wing the air has no choice but to simply flow straight over or under the wing and leave the back at some angle depending on how much air the wing bent up in front. Now if we sweep this infinitely long wing forward what happens? Still no real option but to flow straight over or under the wing. If the wing angled any significant amount of air towards the center it would cause a huge airflow at the fuse and I think we know that does not happen.
Ok, now lets chop off the end of the wing at an angle so that it matches the angle of the fuse and make the wing finite in length. From past analyses and all kinds of actual measurements we know that the fast moving air on top exhibits a lower lateral pressure then the slower moving air on the bottom of the wing. So air will tend to flow off the end, up over the end and curl around towards the top of the wing. But I think common sense thinking also tells you that if you now swept the wing back towards straight this tip vortex would tend to get worse. And it does.
Sweep forward is simply one way of reducing tip vortex and thus induced drag. But swept forward also reduces yaw stability so you need a bigger vertical stab. It also acts as anhedral in upright flight so you need to build in a bit more dihedral to make it however forgiving you wish it to be. And probably worst of all it moves the point of first stall in a wing without built in twist from the tip to the root. Stalling first at the root tends to cause the nose to pitch up with the stall instead of down so recovery can be a bear. For these reasons you do not see any planes with wings severely swept forward to reduce tip vortex and thus induced drag.
Besides there are other ways to reduce the vortex without making all kinds of other problems for yourself. The after market swept up tips on the MD11 and 727 are examples. Some STOL light planes have put flat plates on the end of the wings. The USSR MIGs had air dams built right into the wing.
By the way, there is one application where tip votex is desired. Take a look at the planes used as crop dusters. Things like the Ag Wagon have short stubby big chord wings. ie very low aspect ratios. This maximizes the tip vortex so, if you fly a half wingspan above the crop you are spraying, the tip vortex smashes the spray right down into the crop. This minimizes droplet evaporation and wind drift both of which are critical concerns in the business.
Personally I think your bulb is burning just fine.
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To bm459: Thank you sir for the insights on forward sweep. The reason i was curious is because of a little scratchbuilt 30 inch span RC plane that flies incredibly well with none of the deficits you mentioned. It has no problem with adverse yaw and no problem with roll stability despite having zero dihedral. Also it is utterly benign in stall, with no tendancy to pitch up or drop a wing. This may be due to the airfoil being semi-symetrical at the root, transitioning to flat-bottom at the tip.. thus moving the point of first stall forward(?). The plane is a real sweety and (almost)daily driver, weather permitting. It goes like stink and slows to land like a thistle. Some pics of it are shown here- http://community-2.webtv.net/oldcoot/TurboTipsyand /
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Bill Sheppard wrote:

I do not doubt you a bit. Just about every type of problem can be designed out of a plane if you know about it ahead of time. Or get lucky and compensate for it by accident. Swept forward does give a neat looking plane to boot.
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From bm459:

Exactly. The 'look' was what was driving the design from the outset. It went thru several iterations before arriving at the present configuration. Originally the plane was tailless, with extended 'tabs' on the rearmost ends of the elevons for increased pitch authority. The plane flew fine at high speed, but still lacked pitch authority at low speed. It also had adverse yaw at low speed. Finally i had to 'bite the bullet' and go to a conventional tail, ailerons and larger fin area, which cured the problems but with a 'looks' penalty. <g Bill(oc)
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Bill Sheppard wrote:

You and I must be like perverse spirits. I love homebrews. They often fly like crap in the first iteration. But start adjusting the various lifting surfaces for angle of incidence and ditto with the power plant and all of a sudden they can fly real sweet. I suspect your guy might have been ok with somewhat bigger tabs on the elevons and just a vertical stab on the tail. I have seen planes with only a vertical stab that flew with no real bad habits. I prefer a plane that keeps going where it is pointed regardless of reasonable throttle position and has no built in self correcting stuff anyhow. Makes them more aerobatic and as easy to fly inverted as right side up. You do have to fly them thou.
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