Air Force Museum error?
11-04-2013, 11:06 AM
#76
A wing produces "lift" because of a differential pressure between the upper and lower surfaces, I think we all agree on that. There appears to be two schools of thought on how that is generated, Bernoulli or Nutonion reaction, it's probably a bit of both. However the pressure difference required is actually quite small. Take a 60 size plane say 700 sqin wing area, if the pressure on the underside is increased from 14.7 psi to 14.71 psi and the pressure on the upper side is reduced to 14.69 psi we have a differential of 0.02 psi multiply this by 700 sqin and we have a lift of 0.02 X 700 = 14 lb. More than enough to lift a 60 size plane which probably weighs about 6 -8 lb. The way the museum worded it is probably not the way I would have worded it but I don't have a serious problem with it either.
11-04-2013, 11:10 AM
#77
Mr Abbot has the answers... even in the first chapter of his book: "Basic on aerodynamics..." or so.
Just ask for "the" Abbot... is quite famous.
In my opinion there is no need to change the contents in order to make them available for average persons; you just explain them differently but a fact is a fact.
The abbot, even going really deep on wing aerodynamics, starts explaining very clearly how works... you have high pressure here, low pressure threre and they want the balance themselves; then you get the lift.
COnclusion: the museum was RIGHT, even if it's not available to the common person understanding.
Cheers
Just ask for "the" Abbot... is quite famous.
In my opinion there is no need to change the contents in order to make them available for average persons; you just explain them differently but a fact is a fact.
The abbot, even going really deep on wing aerodynamics, starts explaining very clearly how works... you have high pressure here, low pressure threre and they want the balance themselves; then you get the lift.
COnclusion: the museum was RIGHT, even if it's not available to the common person understanding.
Cheers
Last edited by bokuda; 11-04-2013 at 12:44 PM.
11-04-2013, 11:17 AM
#78
The error in bokuda's logic is that he is giving credit to the wrong force at play. A force you have not created or modified.
By sucking on a straw to take a drink, you are actively creating low pressure to draw the drink up but you are giving credit to the drink being pushed up via higher pressure. By blowing over a flat piece of paper laying on a table you are actively creating a lower pressure over the top of the paper but you are saying it's the air underneath pushing the paper up. The top of the wing which has a camber is modified to actively create a low pressure area above and therefore lift but you are giving credit to higher pressure underneath the wing which is not modified.
See what I'm saying?
By sucking on a straw to take a drink, you are actively creating low pressure to draw the drink up but you are giving credit to the drink being pushed up via higher pressure. By blowing over a flat piece of paper laying on a table you are actively creating a lower pressure over the top of the paper but you are saying it's the air underneath pushing the paper up. The top of the wing which has a camber is modified to actively create a low pressure area above and therefore lift but you are giving credit to higher pressure underneath the wing which is not modified.
See what I'm saying?
11-04-2013, 11:18 AM
#79
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Can you site a credible source that says lower pressure creates suction? Can you site a source that the vast majority of scientists and engineers accept the notion of " suction?"
I think your proposed rewording isn't bad. But what is the point of getting into an argument about it with a docent, who didn't write it and doesn't have the power to change it? And why call it an error, when it's simply one ordinary way of describing what is happening?
This whole exchange reminds me of a law in New York that says merchants aren't allowed to charge people extra for using credit cards, but they can give a discount to people who don't use credit cards. A judge recently described this as crazy, which seems mild.
11-04-2013, 11:23 AM
#80
Originally Posted by [email protected]
i think our air force knows more about planes then the modler will ever know
BTW: Three former members of my club went on to become ( or will) aeronautical engineers and they are still modeling. One went to the Air Force Academy and is now flying F16's, one went to Emery Riddle in Florida and is now a test pilot for an airplane manufacturer and one is now a junior at Emery Riddle. So your statement is not necessarily accurate.
11-04-2013, 11:29 AM
#81
So when you refer to suction ("sucking on a straw "pulls" the liquid") it's OK, because that's how the "average person" thinks about it. But when the museum does it, they're making a mistake, because engineers don't think of it that way. Hmm.
I think your proposed rewording isn't bad. But what is the point of getting into an argument about it with a docent, who didn't write it and doesn't have the power to change it? And why call it an error, when it's simply one ordinary way of describing what is happening?
This whole exchange reminds me of a law in New York that says merchants aren't allowed to charge people extra for using credit cards, but they can give a discount to people who don't use credit cards. A judge recently described this as crazy, which seems mild.
I think your proposed rewording isn't bad. But what is the point of getting into an argument about it with a docent, who didn't write it and doesn't have the power to change it? And why call it an error, when it's simply one ordinary way of describing what is happening?
This whole exchange reminds me of a law in New York that says merchants aren't allowed to charge people extra for using credit cards, but they can give a discount to people who don't use credit cards. A judge recently described this as crazy, which seems mild.
Last edited by bokuda; 11-07-2013 at 07:01 PM.
11-04-2013, 11:39 AM
#82
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I believe you are refering to the heated boots used on the leading edge
The Beech Premier bizjet uses an electromechanical system called EMEDS, an electrically-heated leading edge in conjunction with electric coils within the airfoil that pulse, mechanically blowing any 'runback' ice off the surface. Very innovative new technology.
One reason full scale focuses it's de-icing more on upper surfaces and often "ignore" the undersurfaces is economics of time and money and how very little weight (as well as surface degredation) there can be underneath.
Ice can build very rapidly and form in odd shapes. It destroys the airfoil shape, in extreme events causing a 'shock wave' similar to one produced at the speed of sound. I've experienced tailplane stalls twice due to rapid ice formation and tail boot system failure, and it is terrifying. Lucky to be here.
I figured if we're getting into electrical theory, I'd elaborate on ice.
11-04-2013, 11:56 AM
#83
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If no one points out errors at museums, how would they ever be corrected?
11-04-2013, 12:53 PM
#85
To stem accusations of contradiction: the air going over the upper surface of the wing is creating a lower pressure relative to the lower surface at which is being created a lower pressure than the ambient air pressure, but still higher than that on the upper surface of the wing.
11-04-2013, 01:04 PM
#86
I don't! Wings don't get pushed up. You are not actively creating high than standard pressure under a wing. Wings are designed to actively create lower pressure above the wing. If a wing has lower than standard pressure above (by design) and standard pressure below which force is active? It's lifting not pushing.
11-04-2013, 02:25 PM
#87
This is my last post on this thread. I thank those of you who have been civil in your arguments and statements. At this point I will just agree to disagree. As an educator, I was always careful to make accurate statements and assertions, even if my students would not have known the difference if I hadn't. I would expect museums and other exhibitors to do the same. Not all who visit these places are "average," and average people should be given accurate information anyway. Misinformation can carry over into adulthood and into many other areas. A web search revealed that many college texts and even aerodynamic engineering and military texts still erroneously teach the "different distances" theory where molecules of air that were separated at the leading edge must meet at the trailing edge. Also, apparently modern aerodynamicists do not agree on the Newtonian vs Bernoulli principles as they relate to lift. All the sources I checked, however, agree that angle of attack provides much more of a contribution to lift than either of these.
I still maintain the Air Force placard is not accurate and should be corrected; I have seen nothing to convince me otherwise.
Here is what Wikipedia has to say about suction:
"Suction is the flow of a fluid into a partial vacuum, or region of low pressure. The pressure gradient between this region and the ambient pressure will propel matter toward the low pressure area. Suction is popularly thought of as an attractive effect, which is incorrect (emphasis mine) since vacuums do not innately attract matter. Dust being "sucked" into a vacuum cleaner is actually being pushed in by the higher pressure air on the outside of the cleaner. The higher pressure of the surrounding fluid can push matter into a vacuum but a vacuum cannot attract matter."
This is from the U. of Minnesota Physics and Astronomy website:
"When we use a vacuum cleaner we often talk about the dirt being "sucked up" off the floor. It may not be comfortable at first, but the idea of suction is a false concept right from the start. There is no such thing as suction, (emphasis mine) just areas of high pressure next to areas of lower pressure and the Bernoulli effect. Students that write on the back of a desk that "...this class sucks..." should not be taken very seriously because they have learned little about the world. The vacuum cleaner creates high speed air above the dirt, the pressure above the dirt drops due to the Bernoulli effect and the air under the dirt pushes the dirt particles up into the air stream. Most modern day vacuums have some method of vibrating the carpet to make sure that the dirt has some air under it to do the lifting."
I still maintain the Air Force placard is not accurate and should be corrected; I have seen nothing to convince me otherwise.
Here is what Wikipedia has to say about suction:
"Suction is the flow of a fluid into a partial vacuum, or region of low pressure. The pressure gradient between this region and the ambient pressure will propel matter toward the low pressure area. Suction is popularly thought of as an attractive effect, which is incorrect (emphasis mine) since vacuums do not innately attract matter. Dust being "sucked" into a vacuum cleaner is actually being pushed in by the higher pressure air on the outside of the cleaner. The higher pressure of the surrounding fluid can push matter into a vacuum but a vacuum cannot attract matter."
This is from the U. of Minnesota Physics and Astronomy website:
"When we use a vacuum cleaner we often talk about the dirt being "sucked up" off the floor. It may not be comfortable at first, but the idea of suction is a false concept right from the start. There is no such thing as suction, (emphasis mine) just areas of high pressure next to areas of lower pressure and the Bernoulli effect. Students that write on the back of a desk that "...this class sucks..." should not be taken very seriously because they have learned little about the world. The vacuum cleaner creates high speed air above the dirt, the pressure above the dirt drops due to the Bernoulli effect and the air under the dirt pushes the dirt particles up into the air stream. Most modern day vacuums have some method of vibrating the carpet to make sure that the dirt has some air under it to do the lifting."
11-04-2013, 02:29 PM
#88
i don't! Wings don't get pushed up. You are not actively creating high than standard pressure under a wing. Wings are designed to actively create lower pressure above the wing. If a wing has lower than standard pressure above (by design) and standard pressure below which force is active? It's lifting not pushing.
11-04-2013, 02:59 PM
#89
I think I will just continue to build really light and you know the rest...I do have a question though, do I vacuum or use blow gun to remove dust particles from my wings?
How about a nice game of chess...
Bob
How about a nice game of chess...
Bob
11-04-2013, 03:57 PM
#90
well guys - no matter what words you like to use the bottom line is :
the pressure difference - top to bottom does the heavy work ( lifting)
which side contributes what percentage is a loosing argument because the NET difference is all that counts and you can't change top or bottom without changing the required response of the other side to provide an equivalent differential pressure.. (same lift)
It is YIn and Yang
it ain't magic it's just physics at work
all the airfoils developed over the last 100 years are nothing more than reshaping of a plate, to improve the lift to drag ratio for particular speeds and loads
Lift is still - just pressure difference
the pressure difference - top to bottom does the heavy work ( lifting)
which side contributes what percentage is a loosing argument because the NET difference is all that counts and you can't change top or bottom without changing the required response of the other side to provide an equivalent differential pressure.. (same lift)
It is YIn and Yang
it ain't magic it's just physics at work
all the airfoils developed over the last 100 years are nothing more than reshaping of a plate, to improve the lift to drag ratio for particular speeds and loads
Lift is still - just pressure difference
Last edited by rmh; 11-04-2013 at 06:18 PM.
11-04-2013, 05:12 PM
#92
On a visit to the Air Force Museum last summer I saw a placard in the aerodynamics section that said the curvature of a wing's airfoil causes lower pressure on the upper surface of a wing than the lower and the wing is "literally sucked upwards." I had quite a discussion with one of the docents who could not understand why I saw this as an error.
What do you think of this?
I was concerned because the Air Force should obviously be knowledgeable about aerodynamics and this sign is read every day by the public.
What do you think of this?
I was concerned because the Air Force should obviously be knowledgeable about aerodynamics and this sign is read every day by the public.
11-04-2013, 06:02 PM
#94
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On a visit to the Air Force Museum last summer I saw a placard in the aerodynamics section that said the curvature of a wing's airfoil causes lower pressure on the upper surface of a wing than the lower and the wing is "literally sucked upwards." I had quite a discussion with one of the docents who could not understand why I saw this as an error.
What do you think of this?
I was concerned because the Air Force should obviously be knowledgeable about aerodynamics and this sign is read every day by the public.
What do you think of this?
I was concerned because the Air Force should obviously be knowledgeable about aerodynamics and this sign is read every day by the public.
I've got at chance to watch and fly at an Air Force airshow with my F-18 Hornet model and and I'm so impressed how great their skills and knowledge are... https://www.youtube.com/watch?v=dy08DytcYGI That is the simplest way for the public to understand the dynamics of flight.
Last edited by rctech2k7; 11-04-2013 at 06:06 PM.
11-04-2013, 07:12 PM
#97
Again, pressure is a concept, a scalar value associated to the things that are really tangible to our senses: forces and surfaces.
Copied from http://en.wikipedia.org/wiki/Pressure
"Pressure is a scalar quantity (a one-dimensional physical quantity, i.e. one that can be described by a single real number). It relates the vector surface element (a vector normal to the surface) with the normal force acting on it. The pressure is the scalar proportionality constant that relates the two normal vectors."
In order to give a magnitude to pressure, we always need a surface and a force acting perpendicularly against that force.
That force comes from the movement of the molecules of the fluid in contact with that surface.
Those can only push as they collide with the surface.
The acceleration of the airstream above the wing liberates the top surface from a good amount of those collisions, making the collisions that happen against the lower surface more effective.
That applies to the AOA between zero and critical only.
After the layer of air detaches from the top surface, the magic created by the acceleration of the airstream disappears and the slower molecules come back to their habitual hitting on the top surface, counteracting the action of the molecules on the bottom skin of the wing.
Lower P over the top surface means lower force pushing down that surface.
Higher P over the bottom surface means higher force pushing up that surface.
The internal structure of the wing (even if it is a flat plate) connects both forces solidly.
The lifting force = higher force pushing up - higher force pushing up
I disagree with the part of your improved placard's statement regarding the curvature of the wing as the cause of the low P: are the thrust, the chamber, the shape of the leading edge and the AOA what determine the magnitude of the acceleration of the top airstream (higher speed than wing's - decreased P) and the deceleration of the bottom airstream (lower speed than wing's - increased P).
You still did better than me: regarding a good, understandable and accurate explanation for that placard, ....... I cannot find a decent one.
My apologies again, kind Sir; you made me think in more accurate terms.
I misinterpreted your first post as the increased P on the bottom of the wing the only one to be considered.
11-04-2013, 08:02 PM
#99
I consider lift and drag to be the same thing - pressure differences- just doing different things
terrible explanation -
but as LIFT - lowered pressure is useful - as long as a higher pressure is opposed to it- the plate will move from higher to lower pressures as mother nature tries to achieve equilibrium..
However pressure differences opposite to intended direction of flight - is a different kettle of fish
(high pressure in front - low pressure behind)
Oddly enough all of these differences are necessary to some degree.to have lift and stability.
N/Y?
.
terrible explanation -
but as LIFT - lowered pressure is useful - as long as a higher pressure is opposed to it- the plate will move from higher to lower pressures as mother nature tries to achieve equilibrium..
However pressure differences opposite to intended direction of flight - is a different kettle of fish
(high pressure in front - low pressure behind)
Oddly enough all of these differences are necessary to some degree.to have lift and stability.
N/Y?
.
11-04-2013, 08:44 PM
#100
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I believe that the museum sign is an over simplification designed to combat the idea (less common now) that the airplane is held up by a cushion of
compressed air, like that used by ground affect vehicles, and that the shape of the top surface of the wing does not matter. Look up Ekranoplans for
some really large and strange aircraft that use air compressed between the under side of the wing and the ground to stay airborne.
compressed air, like that used by ground affect vehicles, and that the shape of the top surface of the wing does not matter. Look up Ekranoplans for
some really large and strange aircraft that use air compressed between the under side of the wing and the ground to stay airborne.