Bernoulli's Principle and Airflow
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tgrayson
New Member
Thanks for all of the insight on this topic, It seems there is a lot of speculation. I will take all your info and try to put it all together. Hopefully I don't get asked this on my FAA checkride lol. Thanks again.
Ah, I'm not sure discussing circulation would be the best thing to bring up to your examiner. Most likely, he's never heard of it and might think you didn't know what you were talking about.
As for the speculation, no. This science of this has been well understood for almost 100 years.
USMCmech
Well-Known Member
Thanks for all of the insight on this topic, It seems there is a lot of speculation. I will take all your info and try to put it all together. Hopefully I don't get asked this on my FAA checkride lol. Thanks again.
Lift is the result of the difference of pressure between the top and bottom of the wing. I useually stick with the simple Bernoulli/Newton 70/30 explination for primary students. It's basic, easy to understand, and really all a primary student needs to know.
However I mention that the reall explaintion is much more complex as 767 and tgrayson have explained. This will help with the enginers who want to delve more deply into the topic.
B767Driver
New Member
If you are referring to Newton's 3rd law of motion...leave that out. It really has no effect on lift until very high speed, high altitude flight.
Matt777
Well-Known Member
I love this subject! "How lift works" is extremely interesting.
First, no one should take this personal, because it is not...
Lift is NOT the result of the difference of pressure between the top and bottom of the wing. For that pressure difference ALONE to keep a Cessna 172 airborne- it would have to be travelling at 400 kts.
Lift is an equal and opposite reaction to the huge mass of air being accelerated downward off the trailing edge in the form of downwash.
There IS extremely low pressure above the wing and the pressure below the wing, in comparison, could be considered high, but this pressure differential is not keeping 870,000lbs of 747 airborne. The accelerated airflow above the wing results in very low pressure which creates a vaccum like a big scoop above the wing sucking in tons and tons of air from above and diverting it downward as downwash.
An aerobatic airplane can fly upside down so a wing can lift even with "backwards" camber- all you need is an angle of attack to create a pressure differential that sucks in air and shoots downward as downwash.
I HIGHLY recommend the book "Understanding Flight" by David Anderson and Scott Eberhardt (an aerodynamicist at the UW) that is where I learned all of this.
Here's a 14 page teaser from them http://home.comcast.net/~clipper-108/lift.htm
Very interesting! Let me know what you think of it.
There are tons of pictures on airliners.net that confirm the existence of downwash. Just search for "condensation" or "contrails". I have all the good ones on my hardrive, but I don't have links to all of them on the website to post here.
To be clear, I am never posting to try to be "right" but to have an interesting discussion.
I just changed my avatar- check out the downwash and the low pressure "scoop" above the wing scooping in and diverting tons of air downward for the equal and opposite reaction!
Regarding circulation- I don't want to say that I disagree with it (mostly because I'm not very familiar with it- certainly not enough to give an OPINION on whether I think it is right or wrong), but my suspicion is that circulation is simply describing the low pressure above the wing propagating forward up to and over the leading edge of the wing and sucking some of the high pressure from below the leading edge. And also describing downwash of course.
Does that make any sense or do I have absolutely no clue what circulation is?
I would be VERY interested to see a photo illustrating circulation in action.
That is why I am so convinced of downwash- because of proof that is clearly seen in pictures like my avatar picture.
Thoughts, comments, rude remarks?
I just made this photo album on airliners.net full of the best condensation/downwash pics on the site...
http://www.airliners.net/search/photo.search?album=16683
First, no one should take this personal, because it is not...
Lift is NOT the result of the difference of pressure between the top and bottom of the wing. For that pressure difference ALONE to keep a Cessna 172 airborne- it would have to be travelling at 400 kts.
Lift is an equal and opposite reaction to the huge mass of air being accelerated downward off the trailing edge in the form of downwash.
There IS extremely low pressure above the wing and the pressure below the wing, in comparison, could be considered high, but this pressure differential is not keeping 870,000lbs of 747 airborne. The accelerated airflow above the wing results in very low pressure which creates a vaccum like a big scoop above the wing sucking in tons and tons of air from above and diverting it downward as downwash.
An aerobatic airplane can fly upside down so a wing can lift even with "backwards" camber- all you need is an angle of attack to create a pressure differential that sucks in air and shoots downward as downwash.
I HIGHLY recommend the book "Understanding Flight" by David Anderson and Scott Eberhardt (an aerodynamicist at the UW) that is where I learned all of this.
Here's a 14 page teaser from them http://home.comcast.net/~clipper-108/lift.htm
Very interesting! Let me know what you think of it.
There are tons of pictures on airliners.net that confirm the existence of downwash. Just search for "condensation" or "contrails". I have all the good ones on my hardrive, but I don't have links to all of them on the website to post here.
To be clear, I am never posting to try to be "right" but to have an interesting discussion.
I just changed my avatar- check out the downwash and the low pressure "scoop" above the wing scooping in and diverting tons of air downward for the equal and opposite reaction!
Regarding circulation- I don't want to say that I disagree with it (mostly because I'm not very familiar with it- certainly not enough to give an OPINION on whether I think it is right or wrong), but my suspicion is that circulation is simply describing the low pressure above the wing propagating forward up to and over the leading edge of the wing and sucking some of the high pressure from below the leading edge. And also describing downwash of course.
Does that make any sense or do I have absolutely no clue what circulation is?
I would be VERY interested to see a photo illustrating circulation in action.
That is why I am so convinced of downwash- because of proof that is clearly seen in pictures like my avatar picture.
Thoughts, comments, rude remarks?
I just made this photo album on airliners.net full of the best condensation/downwash pics on the site...
http://www.airliners.net/search/photo.search?album=16683
Realms09
Well-Known Member
Newton's third law of motion, as its law moniker suggests, is an integral part of Newtonian physics and has everything to do with lift at all speeds (well, when much less than the speed of light
). The third law states that particles experience equal, opposite, and simultaneous reactions. This is nothing but a statement of conservation of momentum. Conservation of momentum generates three equations for each component of space, and these are what are solved fluid mechanics problems in addition to the conservation of mass-energy.Newton's laws are correct for most aeronautical engineering purposes. In his time Newton applied his laws to fluid dynamics through a simple fluid model (sine squared law), and it is that model which is flawed except at high speed, high altitude (hypersonic) flight.
For more information see "A History of Aerodynamics" by John D. Anderson, Jr.
tgrayson
New Member
Agreed, since pressure is nothing more than the transfer of momentum from the air to the wing, a Newtonian reaction. So saying part of the lift is Bernoullian and part Newtonian is making a false distinction.
What B767Driver was criticizing is Newton's lift formula, which described lift as a bunch of air bullets ricocheting off the bottom of the airfoil, without any understanding of the reduced pressure on top of the airfoil. Newton knew his description didn't match reality.
tgrayson
New Member
This is simply wrong. The wrongness of it can be made obvious in the fact that any force applied to a wing flying through the air only has one mechanism for its transmission.....pressure. Even if you want to model lift as deflecting an air mass downwards, the wing has to apply this force to the air somehow. How does a wing grab a hold of the air and throw it down? Pressure. There is no other way.
One of the most fundamental ideas of aerodynamics is that you can take the sum up the pressure difference around an airfoil and calculate lift directly. The main problem has always been calculating the pressure any any particular point. You will find formulas in all advanced aerodynamics books describing this.
This is a terrible book. Easy to understand, yes, but just because something is easy to understand doesn't make it right. Their fundamental theories are at odds with the aerodynamics of the past 100 years. There is no evidence that the Coanda Effect has any role in lift production. You won't find it mentioned in any other aerodynamics book, except in exotic applications like vectored thrust.
The second part.
You can't see circulation, because the air doesn't actually flow in the direction that circulation says. That's because the circulation is superimposed over the relative wind. The net result of circulation is that the air over the top of the wing is faster and the air below the wing is slower. This is exactly what you would expect if circulation were real.There are a number of real effects that demonstrate that circulation is real. There is a starting vortex created every time lift changes. And, as you mentioned, the trailing vortices. And slow motion photography can show the circulation getting started. Realm09 had some interesting MIT videos about that. I'll see if I can find the link.
B767Driver
New Member
This is a terrible book, that I have no idea how it got published, especially with the credentials of the author. It also has everything to do with propagation of false theories and explanations of the subject.
B767Driver
New Member
Newton's third law of motion, as its law moniker suggests, is an integral part of Newtonian physics and has everything to do with lift at all speeds (well, when much less than the speed of light
Newton's laws are correct for most aeronautical engineering purposes. In his time Newton applied his laws to fluid dynamics through a simple fluid model (sine squared law), and it is that model which is flawed except at high speed, high altitude (hypersonic) flight.
For more information see "A History of Aerodynamics" by John D. Anderson, Jr.
Agreed. See TGrayson's defense of my statement above..
B767Driver
New Member
I think it's important for anyone understanding how lift if created (or explaining especially at the CFI level) to understand that lift is not created by downwash. Yes, downwash is a byproduct of lift...and the magnitude of the downwash vector is indicative of that of lift creation...but it is not the reason lift is produced. It's also important to understand that lift is not produced by air deflecting off the bottom of the wing (commonly called Newton's 3rd law).
To be concise, simple and correct see how TG summed up one of his posts....1) Law of Conservation of Matter, 2) Bernoulli's Principle and 3) Circulation. If you have questions on any of those ingredients, I'm sure he'd be happy to expand.
Matt777
Well-Known Member
I wasn't disputing the low pressure at all- it creates the "scoop" that the authors of that book described.
The authors of this book take the position that while this method of calculation works wonderfully for engineering calculations, it does not describe lift production. That is their opinion- I'll get to mine at the end of this post.
Well I would have never EVER imagined that a book written by two aerodynamicists with access to wind tunnels would be wrong!!! I'm very surprised! I'd like to take to position that they have THEIR foot in THEIR mouth- not me (God forbid!)!
The second part.
There are a number of real effects that demonstrate that circulation is real. There is a starting vortex created every time lift changes. And, as you mentioned, the trailing vortices. And slow motion photography can show the circulation getting started. Realm09 had some interesting MIT videos about that. I'll see if I can find the link.
Thanks for that info- yeah, I don't dispute that circulation is present.
Just after I posted I opened up that terrible book
for the first time in about a year and read the section on circulation- they say exactly what you've described. So here is why I was convinced by this book:
#1. The authors' credentials. and #2. Photos seem to confirm the existence of downwash.
Like these: http://www.airliners.net/photo/British-Airways/Boeing-777-236-ER/1091105/L/
http://www.airliners.net/photo/Thomas-Cook-Airlines/Boeing-757-2Y0/1093402/L/
http://www.airliners.net/photo/Thai-Airways-International/Airbus-A340-642/1222607/L/
http://www.airliners.net/photo/Hawaiian-Air/Boeing-767-3G5-ER/0766803/L/
http://www.airliners.net/photo/Martinair-Cargo/McDonnell-Douglas-MD-11CF/1414267/L/
And *especially* the photos of various bizjets flying over the fog layer over the Salt Lake that show the presence of downwash and vortices well below the level at which the aircraft flew over.
Since reading this book that I thought was written by a couple of smart qualified guys (until now) I thought that downwash was the cause of lift.
I reasoned out (hypothesised or invented/imagined) that the way the pressure is distributed around the wing determines the center of pressure, while the downwash is what the wing is reacting equally and opposite to.
So, based on all the books you've read and the expertise shown here could you steer me to some references that disprove the importance of downwash.
I ask this because, in my mind, when I look at these photos it confirms the downwash theory. Of course there is no way for a picture to convince me of the reduced pressure being responsible for 100% of the lift since that low pressure IS visible in photos, but the strength of that pressure cannot be known (or "seen") in these photos... yet I DO see the downwash- so I know it exists. So how do we actually know that downwash is not as strong as these aerodynamicists with their wind tunnels and computers claim?
Could you take a few minutes to steer me to the facts- whether it be a link or recommending a book or something?
thanks!
Matt777
Well-Known Member
Re: *****Newton's 3rd law having application to air impacting the underside of the wing: I definitely never ever thought that was the case!!! I'm not quite that gullible!
It looks like I've got some research to do...
As far as doing some homework, you could probably do a search on this very website for tgrayson & B767Driver's posts on this very subject. Lots and lots of good information from them here that you can then take out and compare with other sources. I think you might just be suprised on how sharp these two guys are on this subject.
Realms09
Well-Known Member
Matt,
Your pictures show downwash because downwash does in fact exist - no doubt about it. Authors go wrong when they mistakenly tout this as the cause of lift, which is a correlation vs. causation sort of fallacy. Pressure acting on the wing surface (air molecules colliding with wing molecules) applies an equal and opposite force to the wing and to the air. Air and wing molecules collide in this manner on the front, back, top, and bottom of the wing. It is this molecular interaction, pressure, that when summed over the entire surface is the cause of the lift force and the cause of downwash.
1. Classical mechanics governs fluid flow of aeronautical interest. This includes Newton's three laws of motion, conservation of mass, and conservation of energy.
2. When the principles of (1) are used to described a fluid flow a relationship between pressure and flow speed is evident. As speed increases pressure decreases. This is known as Bernoulli's principle.
3. In order for there to be lift on a body in a flow, there MUST be a net pressure difference when the pressure is summed over its surface. Based on (2), any net pressure difference around the body must have an associated flow speed difference. This is the idea of circulation.
4. The net pressure difference causes a net force, lift, on the wing. It also causes the air mass to experience a downward force which results in downwash.
Any deep understanding of fluid dynamics must proceed from an understanding of classical mechanics. Towards that end I recommend a physics I textbook or selected chapters from the Feynman Lectures on Physics, vol I.
Your pictures show downwash because downwash does in fact exist - no doubt about it. Authors go wrong when they mistakenly tout this as the cause of lift, which is a correlation vs. causation sort of fallacy. Pressure acting on the wing surface (air molecules colliding with wing molecules) applies an equal and opposite force to the wing and to the air. Air and wing molecules collide in this manner on the front, back, top, and bottom of the wing. It is this molecular interaction, pressure, that when summed over the entire surface is the cause of the lift force and the cause of downwash.
1. Classical mechanics governs fluid flow of aeronautical interest. This includes Newton's three laws of motion, conservation of mass, and conservation of energy.
2. When the principles of (1) are used to described a fluid flow a relationship between pressure and flow speed is evident. As speed increases pressure decreases. This is known as Bernoulli's principle.
3. In order for there to be lift on a body in a flow, there MUST be a net pressure difference when the pressure is summed over its surface. Based on (2), any net pressure difference around the body must have an associated flow speed difference. This is the idea of circulation.
4. The net pressure difference causes a net force, lift, on the wing. It also causes the air mass to experience a downward force which results in downwash.
Any deep understanding of fluid dynamics must proceed from an understanding of classical mechanics. Towards that end I recommend a physics I textbook or selected chapters from the Feynman Lectures on Physics, vol I.
tgrayson
New Member
One thing to keep in mind is that no matter how well established a scientific topic is, you will find one or two people with good credentials who dispute it. You will find one or two PhD's who dispute Einstein's theory of relativity or Quantum Mechanics or Evolution or the age of the Earth or that germs cause disease, etc.
If these people actually had data that supported their point of view and was not explainable by the conventional theories, they might revolutionize the science. This occurs sometimes, but this hasn't occurred by Andersen and Eberhardt. The world yawned.
And note the book was not published by a reputable scientific publisher.One interesting thing is that this book of theirs originated with an article they wrote in the EAA magazine. They presented their theory in a much abbreviated fashion, but what was interesting is that they really didn't claim their theory was correct, but was easier to understand. They shot down the Bernoulli theory of lift, but they did so by constructing a straw man. In other words, they criticized the "hump theory" of lift, which is a misrepresentation of the conventional theory anyway. And claimed that conventional theory could not explain invented flight, which is false.
Their article was very popular, which is probably why they wrote the book. Perhaps they wanted to see how far they could take their own reinterpretation or perhaps they just wanted to make money?
I'm going to give you a link to a newsgroup thread. There are two significant participants: John Denker and John Lowry, both physicists. Lowry has written a book on aerodynamics which is very conventional. Denker has an online book which is also conventional and he addresses the Coanda stuff. He also has links to sections of his book which might be useful. Here's the link:
http://groups.google.com/group/sci....lowry+"understanding+flight"#49a63f42514668b6
Matt777
Well-Known Member
Thanks for the link- this is very interesting!
First, I made a mistake in my post about a 172 having to travel at 400mph, as stated in "Understanding Flight"! As I mentioned, I hadn't read the book in over a year...
My mistake is that I forgot that the authors were making that calculation based on the equal transit times theory (which everyone knows is wrong). So I wish I had not mentioned the 400mph thing at all- it is equivalent to posting a theory that the sky is green!
One observation about that very interesting thread: The original poster is totally confused because he says that Anderson claims that Bernoulli is not present in lift production- that it is all Coanda and viscosity. The poster is totally wrong because he never claimed that.
As I've made clear, I'm no longer 100% convinced by these guys (duped?),
BUT they never made that claim.
They say that Bernoulli IS present, being the obvious cause of the acceleration of air over the upper surface. The authors never reject Bernoulli, but are claiming that the viscosity of the air (faster air "falling over" slower air at the wing surface) results in the Coanda effect, which causes the air to follow the upper surface of the wing and then depart the trailing edge continuing downward at that angle as downwash.
They claim that Bernoulli is not the direct cause of lift (the low pressure ALONE being the cause of lift), but is the cause of the accelerated air and the low pressure area above the wing that sucks in other air from above to be diverted down as downwash. They never dismiss Bernoulli- their position is that his theory is being abused.
They also don't dismiss ground effect, as claimed by the confused poster. They claim that the ground prevents the wing from getting/using as much air as it usually does- meaning less upwash and therefore less induced drag associated with the volume of upwash air being diverted- and likewise less induced drag from the downwash (and vortices) being blocked at the ground.
Remember I'm not sticking up for them! I have no allegiance or bias for them, BUT the poster has no idea what he's talking about and is inventing positions (and therefore arguments) that the authors never took.
Now I see that Jon Denker is the guy with av8n.com- I love that site! I have been using his material on his site ALONG WITH the downwash theory to understand lift, which shows that his theory has a lot in common with the downwash theory, though he is not aware of that fact because of the confused poster.
It seems that denker mainly just doesn't believe the downwash is significant- it seems to me that, based on what I've read so far (well over half of that thread) they actually agree on everything else.... EXCEPT this one other thing:
---------
> he [Anderson] thinks lift is the product on air being viscously
> "pulled" (accelerated) down across the top of the wing. That is
> totally impossible
We agree what he said is impossible. There are several reasons
why it's impossible.
> because air does not support tension forces (just
> as a length of yarn does not support compression, or "push").
That's not the best argument. That depends on points of view,
and depends on taste. If you measure _absolute_ pressure,
there is only push. If you measure _gauge_ pressure, there is
both push and pull. De gustibus non disputandum.
------------------
Can you expand of what they are saying? Why can air not be sucked down from above the wing? If there is low pressure where the air is accelerated, due to bernoulli, wont ambient, higher pressue, air from above the wing want to equalize into that low pressure area and then get caught up in the accelerated bernoulli-caused airflow and then proceed to the trailing edge where it either (according to the two viewpoints):
a. doesn't do anything important as is departs the wing because the low pressure makes 100% of lift
or
b. departs downward as downwash diverting tons of air down, to which there is an equal and opposite reaction upward in the direction of lift.
I want the truth!
thanks for guiding a non-expert!
Matt
First, I made a mistake in my post about a 172 having to travel at 400mph, as stated in "Understanding Flight"! As I mentioned, I hadn't read the book in over a year...
My mistake is that I forgot that the authors were making that calculation based on the equal transit times theory (which everyone knows is wrong). So I wish I had not mentioned the 400mph thing at all- it is equivalent to posting a theory that the sky is green!
One observation about that very interesting thread: The original poster is totally confused because he says that Anderson claims that Bernoulli is not present in lift production- that it is all Coanda and viscosity. The poster is totally wrong because he never claimed that.
As I've made clear, I'm no longer 100% convinced by these guys (duped?),
BUT they never made that claim.
They say that Bernoulli IS present, being the obvious cause of the acceleration of air over the upper surface. The authors never reject Bernoulli, but are claiming that the viscosity of the air (faster air "falling over" slower air at the wing surface) results in the Coanda effect, which causes the air to follow the upper surface of the wing and then depart the trailing edge continuing downward at that angle as downwash.
They claim that Bernoulli is not the direct cause of lift (the low pressure ALONE being the cause of lift), but is the cause of the accelerated air and the low pressure area above the wing that sucks in other air from above to be diverted down as downwash. They never dismiss Bernoulli- their position is that his theory is being abused.
They also don't dismiss ground effect, as claimed by the confused poster. They claim that the ground prevents the wing from getting/using as much air as it usually does- meaning less upwash and therefore less induced drag associated with the volume of upwash air being diverted- and likewise less induced drag from the downwash (and vortices) being blocked at the ground.
Remember I'm not sticking up for them! I have no allegiance or bias for them, BUT the poster has no idea what he's talking about and is inventing positions (and therefore arguments) that the authors never took.
Now I see that Jon Denker is the guy with av8n.com- I love that site! I have been using his material on his site ALONG WITH the downwash theory to understand lift, which shows that his theory has a lot in common with the downwash theory, though he is not aware of that fact because of the confused poster.
It seems that denker mainly just doesn't believe the downwash is significant- it seems to me that, based on what I've read so far (well over half of that thread) they actually agree on everything else.... EXCEPT this one other thing:
---------
> he [Anderson] thinks lift is the product on air being viscously
> "pulled" (accelerated) down across the top of the wing. That is
> totally impossible
We agree what he said is impossible. There are several reasons
why it's impossible.
> because air does not support tension forces (just
> as a length of yarn does not support compression, or "push").
That's not the best argument. That depends on points of view,
and depends on taste. If you measure _absolute_ pressure,
there is only push. If you measure _gauge_ pressure, there is
both push and pull. De gustibus non disputandum.
------------------
Can you expand of what they are saying? Why can air not be sucked down from above the wing? If there is low pressure where the air is accelerated, due to bernoulli, wont ambient, higher pressue, air from above the wing want to equalize into that low pressure area and then get caught up in the accelerated bernoulli-caused airflow and then proceed to the trailing edge where it either (according to the two viewpoints):
a. doesn't do anything important as is departs the wing because the low pressure makes 100% of lift
or
b. departs downward as downwash diverting tons of air down, to which there is an equal and opposite reaction upward in the direction of lift.
I want the truth!
thanks for guiding a non-expert!
Matt
One thing to keep in mind is that no matter how well established a scientific topic is, you will find one or two people with good credentials who dispute it. You will find one or two PhD's who dispute Einstein's theory of relativity or Quantum Mechanics or Evolution or the age of the Earth or that germs cause disease, etc.
If these people actually had data that supported their point of view and was not explainable by the conventional theories, they might revolutionize the science. This occurs sometimes, but this hasn't occurred by Andersen and Eberhardt. The world yawned.
One interesting thing is that this book of theirs originated with an article they wrote in the EAA magazine. They presented their theory in a much abbreviated fashion, but what was interesting is that they really didn't claim their theory was correct, but was easier to understand. They shot down the Bernoulli theory of lift, but they did so by constructing a straw man. In other words, they criticized the "hump theory" of lift, which is a misrepresentation of the conventional theory anyway. And claimed that conventional theory could not explain invented flight, which is false.
Their article was very popular, which is probably why they wrote the book. Perhaps they wanted to see how far they could take their own reinterpretation or perhaps they just wanted to make money?
I'm going to give you a link to a newsgroup thread. There are two significant participants: John Denker and John Lowry, both physicists. Lowry has written a book on aerodynamics which is very conventional. Denker has an online book which is also conventional and he addresses the Coanda stuff. He also has links to sections of his book which might be useful. Here's the link:
http://groups.google.com/group/sci....lowry+"understanding+flight"#49a63f42514668b6
tgrayson
New Member
I posted the link mainly to provide you with Denker input regarding this book. In his post, he expresses disdain for two of Andersen and Eberhardt's ideas:
1) Coanda effect
2) Downwash
He's pretty much the only knowledgeable person who addresses #1, because he's probably the only one that knows there's anyone that is claiming that flight requires this phenomenon.
There are a couple of problems with this. First, scientists have long modeled the air flow around the wing; in the early days, they lacked some of the sophisticated theory we have today and had to make some simplifying assumptions. One that they made is this: air has no viscosity. This is referred to as the "inviscid theory" and the interesting this is, the air still manages to navigate around the airfoil, no viscosity necessary.
Problem is, with no viscosity, no lift occurs.
Still, it removes one of justifications for bring the Coanda effect into the picture, since it isn't necessary to explain why air follows the contour of the wing.Couple of things here. First, what causes an acceleration? According to Newton, F = ma, so a = F/m. In other words, an acceleration requires a force. If the air is being accelerated, there is already a force acting on the air. But how can a force already exist prior to the air moving when we're trying to explain the existence of that force by the moving air?
Second, downwash is created by wingtip vortices; no vortices, no downwash. When a wing is placed in a wind tunnel, often the ends are allowed to touch the sides of the wind tunnel, which prevents the formation of wingtip vortices and thus eliminates downwash. Yet these wings still generate lift. How could that be?
I'm not sure of the validity of that statement. Air does have *some* viscosity and so must support *some* tension. Also, a length of yarn could support *some* compression, just not much.
His point seems moot, anyway, because we know the Coanda Effect is real and can produce "lift" in some situations. I believe the tail rotor on some helicopters use the Coanda effect.
The best criticism of Andersen and Eberhardt is that the Coanda effect is unnecessary to explain why the air hugs the surface of the airfoil of a conventional airfoil. It does so when the air contains no viscosity and thus the Coanda effect impossible.
The irrelevance of downwash is illustrated by the fact that wings in a wind tunnel have no downwash, yet generate lift.
Johnsc81
New Member
I love this subject! "How lift works" is extremely interesting.
First, no one should take this personal, because it is not...
Lift is NOT the result of the difference of pressure between the top and bottom of the wing. For that pressure difference ALONE to keep a Cessna 172 airborne- it would have to be travelling at 400 kts.
you are correct. I remember reading someone mentioning the 70/30 rule above. Aircraft lift is not entirely based off of a low pressure envolope on the top of the wing. It is a combination of several factors, mainly the bernoulli equation then, to a much lessor degree, newtons laws dealing with the molecules striking the surface of the wings and creating a "cushion" of air for the wings to skim across. Now for those that say that the Newtonian theory is the predominant reason for flight they have not spent that much effort learning about aerodynamics. Boundary layer aerodynamics limit how much impact Newton can have on the lifting capacity of wings.
Lift is an equal and opposite reaction to the huge mass of air being accelerated downward off the trailing edge in the form of downwash.
Downwash is just the result of the accelerated airflow (decrease the pressure, the velocity increases) off the wing, the fact that is travels downward is associate with the angle of attack that that wing is traveling at. This is where Newton's Laws come into play.
There IS extremely low pressure above the wing and the pressure below the wing, in comparison, could be considered high, but this pressure differential is not keeping 870,000lbs of 747 airborne. The accelerated airflow above the wing results in very low pressure which creates a vaccum like a big scoop above the wing sucking in tons and tons of air from above and diverting it downward as downwash.
The typical wing surface area of a 747-400 is around 5650 sq.ft. that comes out to be 813600 sq inches. Pressure differentials can be measured in psi. so the wing loading comes out to be about 1.069 pounds per square inches. so if the pressure differential is even less than 2 psi, the airplane will fly.
An aerobatic airplane can fly upside down so a wing can lift even with "backwards" camber- all you need is an angle of attack to create a pressure differential that sucks in air and shoots downward as downwash.
Aerobatic aircraft are a whole different set of rules, they have wings that have the same camber on the tops as the bottom and rely heavily on the angle of attack of the wing and Newtons laws to maintain flight. Acrobatic planes typically need either a higher wing surface area ratio, or a faster more pawerful engine to overcome the decreased dependence on bernoulli and increased dependence on Newton.
I HIGHLY recommend the book "Understanding Flight" by David Anderson and Scott Eberhardt (an aerodynamicist at the UW) that is where I learned all of this.
Here's a 14 page teaser from them http://home.comcast.net/~clipper-108/lift.htm
Very interesting! Let me know what you think of it.
There are tons of pictures on airliners.net that confirm the existence of downwash. Just search for "condensation" or "contrails". I have all the good ones on my hardrive, but I don't have links to all of them on the website to post here.
Contrails, are a very know reality of flight. however there are two types of contrails. High Altitude Exhaust Contrails, and the type I think you are actually talking about, Wingtip vortice contrails. Contrail form in the low pressure center of the wingtip vortices off the end of the wings. for avery similar reason that Fog forms in a 2-liter bottle when you open the cap.
To be clear, I am never posting to try to be "right" but to have an interesting discussion.
I just changed my avatar- check out the downwash and the low pressure "scoop" above the wing scooping in and diverting tons of air downward for the equal and opposite reaction!
Regarding circulation- I don't want to say that I disagree with it (mostly because I'm not very familiar with it- certainly not enough to give an OPINION on whether I think it is right or wrong), but my suspicion is that circulation is simply describing the low pressure above the wing propagating forward up to and over the leading edge of the wing and sucking some of the high pressure from below the leading edge. And also describing downwash of course.
Does that make any sense or do I have absolutely no clue what circulation is?
Not sure about ciculation either. Most of the applications for Circulation aerodynamics is reserved for the Automotive industry, particularly Formula 1 cars, and Sports (baseball - Curveballs, changeup, etc.) I haven't studying the applications for aerospace applications yet.
I would be VERY interested to see a photo illustrating circulation in action.
That is why I am so convinced of downwash- because of proof that is clearly seen in pictures like my avatar picture.
Thoughts, comments, rude remarks?
I just made this photo album on airliners.net full of the best condensation/downwash pics on the site...
http://www.airliners.net/search/photo.search?album=16683