Lift of an airfoil

I visited the NASA visitors center in Cleveland the other day. Really an interesting display. One display that caught my eye was a small
wind tunnel with an airfoil in it.
The observation chamber was plastic so you could see what was going on inside. There was an airfoil mounted on a rotatable rod so you could adjust angle of attack. You could also adjust air speed. I would guess air speed was zero to about 60 mph depending on how you adjusted it. Angle of attack was adjustable to any desired angle.
The air foil had tiny strings attached at various points so you could visualize the flow patterns. The foil also had four pressure taps on the top and four more on the bottom on a chord line in the center of the span. These pressure taps were each hooked to a simple manometer so you could see pressures under different conditions.
The results were exactly as anyone who understands why an airfoil provides lift would expect. First, under no conditions was there the slightest down flow of air off the trailing edge of the foil. Rather, at angles of attack where the flow was smooth air flow straight back off the trailing edge. At angles of attack approaching a stall the air departed the foil above the trailing edge and went straight back.
Second the manometers showed pressures at different points on the chord on both the top and bottom. On top there was a great pressure reduction at the leading pressure tap when angles of attack were positive but below stall. As you went to the back of the chord the pressure reductions dramatically dropped until by the last tap there was very little pressure reduction. On the bottom the front tap showed a small pressure reduction and as you went back on the chord the pressure went rapidly to essentually atmospheric.
This display shows clearly that lift is due to a reduction of pressure on the top of the airfoil relative to the pressure on the bottom of the foil. Further it shows that all the nonsense you have read about lift being due to downflow behind the airfoil is simply wrong. There is no downflow under these particular conditions.
The only thing missing in this demo was what a flat airfoil would have done. This is too bad as if it had been included the public could have seen that airfoil shape has very little to do with lift. At reasonable angles of attack a flat airfoil would have given exactly the same air flows and pressure reductions.
Well, there was not a thing new in this demo that anyone who has studied the science of lift would learn. But the model hobby has historically been so burdened with total nonsense about the topic of lift that is 100% disproved by this demo that I wish AMA would put such a demo in their display. There is nothing like an actual physical display to bring the point home that lift is a result of Bernoulli caused by the viscous behavior of air in front of the leading edge of the wing at subsonic speeds. Those of us who have worked in the science of lift have known this for the last 100 years. But it does seem to be a hard concept for non scientists to grasp for some reason I fail totally to understand. It really is quite simple.
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flyrcalot wrote: ...

...
You are trolling ;-)
Main address: http://www.grc.nasa.gov/WWW/K-12/airplane/lift1.html Quote: "... 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. ..."
NASA, Glenn Research Center: Lift from Flow Turning: http://www.grc.nasa.gov/WWW/K-12/airplane/right2.html Quote: "... So, to change either the speed or the direction of a flow, you must impose a force. And if either the speed or the direction of a flow is changed, a force is generated. ... Lift is a force generated by turning a flow. ..."
NASA, Glenn Research Center: Bernoulli and Newton: http://www.grc.nasa.gov/WWW/K-12/airplane/bernnew.html Citat: "...The integrated velocity variation around the object produces a net turning of the gas flow. From Newton's third law of motion, a turning action of the flow will result in a re-action (aerodynamic force) on the object. So both "Bernoulli" and "Newton" are correct. Integrating the effects of either the pressure or the velocity determines the aerodynamic force on an object..."
-
University of Genoa Faculty of Engineering Hydraulic Institute: Gallery: http://www.diam.unige.it/~irro/gallery.html Citat: "...Trailing vortices and downwash phenomenon of an aircraft in flight are seen clearly in this figure..."
Glenn
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On Mon, 18 Jan 2010 17:36:22 +0100, Glenn wrote:

Thanks for taking the time to dig this up -- I didn't want to rebut without data, and I'm too lazy this morning to find it.
In a wind tunnel you're pretty much required to have no net change to the vertical velocity -- there's walls in the way. So it follows that in a wind tunnel with the wing extending from wall to wall you won't see any change in velocity. Keep that in mind, and the wind tunnel model is probably otherwise pretty good.
Like any problem in engineering or physics it's nice to have different ways to understand what's going on. The nice thing about the "lift comes from air being flung downward" model is that (a) lift doesn't make sense without it, and (b) Prandl's model of induced drag becomes pretty easy to understand, with just high school physics; the nice thing about the "lift comes from the Bernoulli effect" model is that (with more than just high school physics) one can design airfoils; and the nice thing about the "lift comes from circulation and circulation makes vortexes" model is that it helps one to design winglets, if one is a whole bunch better at aerodynamics than I am.
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Yep, typical response from a model airplane guy who knows nothing at all about much of anything. Terminal stupid is really hard to cure even with the evidence right in front of their eyes. Exactly how can turning a gas flow pull on an airfoil? Air is not a rope moron. Subsonic lift is the result of what happens to air flow IN FRONT OF THE AIRFOIL pure and simple.
Want to explain supersonic flight by your downwash nonsense? Behind a supersonic airfoil the direction of flow is ALWAYS up without exception.
You might also think a bit about an autogyro where air flow behind the rotor blade is ALWAYS UP without exception.
Yes, a subsonic airfoil can be operated under conditions such that air flow is downwards behind the airfoil. This down wash has everything to do with what the air flow was in front of the wing and nothing at all to do with directly inducing lift. In the particular demo at NASA you could pick conditions such that there was no downwash at all. In fact you could easy pick conditions with their demo where there was upwash. I had no problem at all finding such conditions. But you could not pick conditions where there was downwash due to limits of the experiment they designed.
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On Mon, 18 Jan 2010 09:15:19 -0800, flyrcalot wrote:

Also a typical response from anyone who understands fluid dynamics.

Uh -- sure there, buddy.

How can it not? Turning the gas flow requires that the gas be accelerated, which requires a force -- this force comes from the airfoil, and is called "lift".

Actually, I didn't know there was such a thing as a "rope moron". "Punctuation" is also in Wikipedia, by the way.
Air _is_ a fluid, with mass, and acts like a fluid with mass -- up to and including the fact that to accelerate it requires a force, and to exert a force on it beyond aerostatic forces will make it accelerate. Just, in fact, like any other mass.
Newton was one smart dude.

No. Not even by your model. But if you have _trustworthy_ sources that contradict the NASA sources that Glenn cited, please put 'em down!

Says you. Citations? Reasonably trustworthy ones?

Again according to you. Again -- citations?

Indeed, "due to the limits of the experiment they designed".
You have two choices: either an aircraft is supported by magic, or Newton's laws apply. Since the air is acting on the wings to support the aircraft, the wings must act on the air. Since the wings are acting on the air, and the air has mass, and the air has no support (in the absence of confounding factors like wind tunnel walls), the air must go down. Hence downwash.
That's not a modeler misunderstanding the situation, that's basic physics. There's no need for voodoo -- just a need to understand Newton's laws.
_Every_ serious text on aerodynamics that I have read (and I've read a few) go over the Bernoulli model, then they point out how that downward force on the air causes downwash. No one who's trying to educate an airplane designer tries to say that downwash doesn't exist.
And those folks _aren't_ all modelers.
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Did I ever say a thing about Newton being wrong? Hardly, as Bernoulli is a direct consequence of Newton. Dirt easy to derive the Bernoulli equations directly from Newton if you know a tiny amount of math.
The NASA demo did not use a airfoil that went from wall to wall. In fact the span of the airfoil was only about 1/2 the width of the air tunnel. Not that it really makes a bit of difference in terms of air flow towards the center of the air foil. It does make a difference at the ends of the airfoil as tip vortex needs to have an open end.
In terms of providing citations I am not going to waste my time. You could not understand them if I did and as a result would just say they are wrong. They are scattered all over the net just like airfoils providing subsonic lift without any downwash are all over the net.
One major problem is people read about things like the circulation model and do not understand what a change in point of reference does. In a circulation model you are assuming the air foil is not moving relative to the surrounding air a span away from the airfoil. The circulation model is a perfectly good way to calculate the size of the Bernoulli effect that causes lift. That is exactly what it was developed to do. But that circulation is caused by the change in reference point and does not represent, in the slightest, the actual flow of air past an airfoil moving through the air. You obviously do not understand this aspect at all or you would not have raised the issue. Or maybe you actually think air flows forwards underneath your models wings? It is possible you are that incredibly ignorant. You would not be the first.
I have several college level texts on lift that do not say a thing about downwash causing lift. I have one that says "Anyone who thinks downwash causes lift does not understand lift." You might want to read Von Mises "Theory of Lift" book. It was good enough to be used as a text at Harvard Univ. for many, many years. Of course if you have only had calculus you are not going to understand it. Your failure to recognize that Bernoulli is a direct consequence of, and derived directly from, Newton's Laws tells me you never went to college and took physics so I doubt if you know enough math to understand any book on lift.
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On Mon, 18 Jan 2010 13:10:08 -0800, flyrcalot wrote:
Yup, it's a troll.
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I knew from the topic. I might have to give him credit for research, researching this group............. mk
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On Wed, 20 Jan 2010 14:09:06 -0600, MJKolodziej wrote:

The best way to drive away trolls is to ignore them.
But that leaves their drivel unchallenged.
So I usually refute it a bit, to provide food for thought to the Google Groupers.
Then I (try to) stay out of it.
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flyrcalot wrote: ...

...
Hey flyrcalot
Maybe you are mistaken the "missing" airfoil lift, with the thrust (vectoring) of a propeller, jet engine or a rocket engine.
The jet engine does 2 thing during normal use: * accelerate air (e.g. turbo fans...) * use some air to burn fuel with the consequence - accelerate burned fuel gasses out at the exhaust end.
These engines expels/sends gas at high speed preferable in one direction only.
If (enough high speed) exhaust/air is directed downward the rocket/airoplane will ascend/climb (e.g. vertically) - then airfoils might not be needed/used if the engine in itself generate enough lift. No airfoil downwash needed - engine downwash is enough:
Pure vertical lift:
Harrier Jet Vertical Takeoff:
http://www.youtube.com/watch?v=m_4oD-BltCY

F35 JSF take off:
http://www.youtube.com/watch?v=Xm7_PPE-8nk

Mixed airfoil and thrust lift:
F22 going vertical:
http://www.youtube.com/watch?v=wg1B6vmDDHw

757 Going Vertical:
http://www.youtube.com/watch?v=-m5mCRAc3Fk

-
Lift is a force generated by turning a flow - and/or accelerating gas downwards. Actually impulse: http://en.wikipedia.org/wiki/Impulse
-
How are solar sails possible? If photons have no mass, how can they push on something?: http://www.physlink.com/Education/AskExperts/ae270.cfm?CFID $788988 Citat: "...This momentum is what gets transferred when wind hits a normal sail[airfoil!]. But photons have momentum without having mass!..."
Glenn
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flyrcalot wrote: ... > > airfoils > providing subsonic lift without any downwash are all over the net. ...
Hey flyrcalot
At high speed no traditional wings are needed, the lift the fuselage as an airfoil generates as downwash, is enough:
http://en.wikipedia.org/wiki/Lifting_body
NASA Dryden Fact Sheet - Lifting Bodies: http://www.nasa.gov/centers/dryden/news/FactSheets/FS-011-DFRC.html Quote: "... Aerodynamic lift - essential to flight in the atmosphere - was obtained from the shape of the vehicles rather than from wings as on a normal aircraft. ... To reduce the costs of constructing a research vehicle, the Air Force returned the X-24A to Martin for modifications that converted its bulbous shape into one resembling a "flying flatiron" ..."
Glenn
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On 1/18/2010 10:10 AM, flyrcalot wrote:

So you are over 100?
Anyway, moving air is deflected by the airfoil. As to what the forces are, a simple demo with a hand sticking outside a car moving at speed will show what is going on. You can change pitch and shape by moving and curving the top of the hand. It's even possible to use a flexible thin rubber tube over the hand to simulate the flat bottom of a wing.
Symmetrical wing airfoils are often used in aerobatic models. In this case, pitch has everything to do with lift.
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Amusing to see this age old discussion (downwash vs Bernouilli) still is around.
- is a propellor to be considered a rotating wing?
- What does one notice while standing behind a plane with a prop at full rpm?
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There sure have been a bunch of irrelevant comments. Anyone on here got the guts to do an experiment and report the results? I doubt it. But I will tell you how to do the experiment anyhow.
Take a playing card. Poke a straight pin through the center of the playing card. A push pin like lots of people use to pin balsa together when building will work fine also. Put a drop of CA on the head side of the pin to fix it rigidly at right angles to the plane of the card. Now take a spool of thread. One of those wooden spools that is about 1.5 inches in diameter is best. But I suppose a plastic spool would work also as long as you do not peal off the paper on the end. It is important that the end of the spool is flat. Open the center hole of the spool if it is covered with paper.
Now lay on your back, put the card on top of the spool with the pin down the center hole of the spool and blow up through the hole on the bottom of the spool. See how hard you have to blow to blow that playing card off the top of the spool. If you have any trouble blowing the card off perhaps you should try your air compressor starting at the lowest pressure setting you have on your compressor and slowly increasing the pressure until you blow the card off. Some people have remarkably low lung capacity and can not blow very hard.
Obviously it should be no problem at all to blow the card off the spool. But what happens when you blow with different forces? What is the maximum force at which the card will stay on the spool? Explain the results in terms of how the air turns when it hits the bottom of the card resulting in a Newtonian force.
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I like the one about landing a real airplane, say a Cessna 172 or Piper 140. Pick a day when there is little wind and preferably with clouds. You set a glide up with a more or less fixed descend rate. As the plane gets closer and closer to the ground, the descend rate is usually changed lower by increasing the pitch of the wing. This obviously creates more drag, and slows the plane a bit more, until you reach a magic combination of speed and descend rate called for by the plane's flight chacteristics. Eventually the wing goes into "ground effect". At this point it seems that air trapped under the wing increases lift, and the plane "floats". The pilot may decrease pitch to cause the plane to "land", or allow the float to continue until the plane eventually "lands" anyway. You can increase the pitch to the point that the wing stalls just as the wheels kiss the ground Older models had a manual Flap lever. This was useful in that you could use the flaps to modify or modulate wing shape, airflow, and drag, thus having another tool to deal with the "float". There is an old arguement concerning gettint a plane "on step". Most elevators don't "fly" under "normal" condtions. It seems that "on step" occurs when you can get them to fly, and evidently reduce drag.
wrote:

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flyrcalot wrote:

...

What if the pin gets in your eye?
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flyrcalot wrote:

Could you please tell me (us) the purpose of this demonstration?
-
The purpose of this video is not to show that the "Coanda effect", but to show that the upper surface of a wing defect air downwards and in return gets lift:
Coanda Effect demonstration of working principle:
http://www.youtube.com/watch?v=S-SAQtODAQw

NASA, Glenn Research Center: http://www.grc.nasa.gov/WWW/K-12/airplane/wrong2.html Quote: "...the upper surface contributes more flow turning than the lower surface..."
Glenn
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The purpose of you doing the experiment is you might learn something. But I seriously doubt if many on here have the intellectual curiosity to do a simple experiment and fewer still will have the guts to report the results if they do the experiment. Particularly if they have poor lung capacity and have to resort to an air compressor.
By the way, the Coanda effect is a direct consequence of Bernoulli and nothing else. If it were not for Bernoulli there would be no such thing as a Coanda effect. That is why Coanda is called an effect and Bernoulli is called a law. Effects are the result of laws. So you might as well erase anything you thought you knew about lift caused by Coanda and just think in terms of Bernoulli.
You do need to turn some air to get lift. In subsonic flight the air you need to turn is in front of the leading edge of the airfoil. You turn that air up. I guess an upwash should cause a downforce on the airfoil according to local thinking. If downwash behind causes lift it follows that upwash in front would give negative lift. There are a gazzilion pics of smoke trails on the net that show that upwash in front of the wing. Strange that there are not pics showing downwash behind the wing. The best try that I have seen shows tip vortex even thou the claim is it shows downwash. In supersonic flight you also turn some air. That air is turned up behind the trailing edge. But not up very far. Just to about the mid point of the form thickness of the wing at the particular angle of attack, and angle of attack is always tiny in supersonic flight.
Want another experiment? Take a prop that you fly on some modestly powered plane like a trainer with an FP40 on the nose and measure the full throttle air flow it generates with the plane teathered. I do not care about what units for air flow you use. Cubic meters per day is fine if you wish. I can convert units just fine. Now turn the prop backwards and repeat the measurement. Now try flying the plane with the prop backwards. You may be surprised at the results.
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| The purpose of you doing the experiment is you might learn something. | But I seriously doubt if many on here have the intellectual curiosity | to do a simple experiment and fewer still will have the guts to report | the results if they do the experiment.
http://www.rudezone.net/images/notThisShitAgain.gif
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wrote:

I love it when trolls give out long detailed instructions for experiments that have nothing to do with the subject at hand while at the same time throwing in ad homonem attacks simply to make more smoke. The one thing that is clear is that there is no sunshine where this one is keeping his head.
Detailed research results from NASA scientists have been cited and you don't have time to provide any to support your assertions of correctness based on a users casual observations at a SHOW? Pull your head out at least long enough to take a deep breath (and tell us where the lift went when you did that).
If you get enough food and fresh oxygen to that brain, it might occur that you have drawn the wrong conclusions from your uneducated observations.
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