Air Force Museum error?
11-02-2013, 12:54 PM
#51
Senior Member
Actually, the weight of the ice is not irrelevant. The stall speed is affected by the weight the wings are trying to carry. The way the wings deal with additional weight is to demand greater AOA. There really aren't too many things in aerodynamics that are responsible by themselves for anything. The suction from the upper, the pressure from below, the surface drag, the wing loading, etc etc are all part of the discussion.
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.
Nothing in aerodynamics is sound byte simple. One liners seldom come close to answering any question.
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.
Nothing in aerodynamics is sound byte simple. One liners seldom come close to answering any question.
11-02-2013, 06:38 PM
#52
While I cannot disagree with what you're saying in the first paragraph, the weight of ice is negligible for the conversation. A clean wing can carry the extra weight. A contaminated wing may not.
But we are straying from the main topic. The AF sign is correct in its theory albeit in layman's terms.
But we are straying from the main topic. The AF sign is correct in its theory albeit in layman's terms.
11-02-2013, 07:03 PM
#53
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I'm going to walk a middle path and suggest that it depends on the airfoil. If the shape is somewhat critical then even a small build up of ice may produce some early separation and thus limit the maximum achievable lift coefficient or perhaps simply push up the drag coefficient such that it's tough to get up to speed needed.
11-02-2013, 09:42 PM
#54
The devices against ice formation are located mainly close to the leading edge of the wings and tails.
The reason may be that those are the areas more prone to ice formation during flight, or that the shape of those areas are critical for lift, or both.
This thread discussed some ideas about that:
http://www.rcuniverse.com/forum/airp...ale-wings.html
The reason may be that those are the areas more prone to ice formation during flight, or that the shape of those areas are critical for lift, or both.
This thread discussed some ideas about that:
http://www.rcuniverse.com/forum/airp...ale-wings.html
11-03-2013, 08:13 AM
#55
e I believe you are refering to the heated boots used on the leading edge The answer is mostly in how they work, Air is pumped into them which causes them to expand and crack the ice. They are heated which melts the remaining ice and the areas on each side. Often the upper area just behind the boot is also heated. The heat is not just contained at the heating element but travels along the skin for a signicant differance. When the ice is cracked the air flow gets into the cracks and then blows the ice away. The heat melts the remaining ice at the surface and now there is a sharp edge to the ice where the air flow can get under the ice which is now held only by the viscosity of water from the melted ice, so a much larger area behind the boot eventually slakes off. Because of the curveture of the upper surface more the ice edge is to the airflow so more ice slakes off of the upper edge even if there is no additional heat behind the boot.
11-03-2013, 04:25 PM
#56
"air molecules at the leading edge must reunite at the trailing edge."
I never believed this even back in the early sixties when it was taught to me. I argued with the teacher that there was no reason why air molecules had to rejoin at the trailing edge. The accelerated air molecules could be replaced or join up with random molecules from the surrounding atmosphere. The teacher would not accept my argument. Not that I'm a know-it-all, but it seemed common sense to me. It still does.
I never believed this even back in the early sixties when it was taught to me. I argued with the teacher that there was no reason why air molecules had to rejoin at the trailing edge. The accelerated air molecules could be replaced or join up with random molecules from the surrounding atmosphere. The teacher would not accept my argument. Not that I'm a know-it-all, but it seemed common sense to me. It still does.
Last edited by bokuda; 11-03-2013 at 04:58 PM.
11-03-2013, 04:30 PM
#57
So vacuum cleaners don't suck, huh. Tell that to Dyson. Unless you are an Aerodynamics Engineer with at least a Master's degree you have only an inkling of knowledge of what you are talking about. Aerodynamics is extreemly complex with many, many factors involved. Sometimes simplicity is the best answer.
BTW: there is no need to be demeaning. I'm happy to discuss this, not to take insults.
11-03-2013, 04:39 PM
#58
The suggestion that the thread might be locked was based on the first few posts that did nothing but lament over the use of the term "suction". The forum is about aerodynamics and how it pertains to our model flying. Not about semantics of word usage. The issues of what word was used is no more pertinent to this forum than discussions about retractable landing gear or how to mix the correct shades of colour for WWII scale models. It was simply off topic for THIS forum. And that was why I suggested a new direction for the thread or closure. Not being nasty, just trying to keep the discussions for this forum area on topic.
Last edited by bokuda; 11-04-2013 at 04:59 AM.
11-03-2013, 04:53 PM
#59
The principles of flight are the same for RC as it is for full scale aircraft. If you want to test the importance of the upper surface of a wing spray some wall texture (to simulate ice) on top of one of your models, let it dry and go fly it. My guess is it won't fly well due to airflow separation over the wing.
Another way to think of it is to take a simple flat bottom trainer wing. The bottom is flat and the top has a camber to it. The air passing below the wing is relatively unmodified whereas the air over the top is modified because it travels a greater distance causing higher velocity and lower pressure as a result. So clearly the wing is a lifting device, not a pushing up device.
Yet another example is is to blow over a sheet of paper while it lays flat on a table. As you blow over the sheet of paper you are creating a low pressure area and the paper will lift off the table. Nothing underneath it to push it up.
Another way to think of it is to take a simple flat bottom trainer wing. The bottom is flat and the top has a camber to it. The air passing below the wing is relatively unmodified whereas the air over the top is modified because it travels a greater distance causing higher velocity and lower pressure as a result. So clearly the wing is a lifting device, not a pushing up device.
Yet another example is is to blow over a sheet of paper while it lays flat on a table. As you blow over the sheet of paper you are creating a low pressure area and the paper will lift off the table. Nothing underneath it to push it up.
11-03-2013, 06:03 PM
#60
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A case in point is highly flat and polished gauge blocks used in metal machining setups. These blocks of steel are scrupulously cleaned of oil then slide together to avoid any air between the blocks. The technique is called "wringing". When done right there is almost no air between the blocks and outside air pressure holds them together to the point where a stack of a couple of pounds worth of blocks can be lifted by the top block without the others falling free.
11-04-2013, 05:16 AM
#61
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The Air Force is attempting to educate the public about how a wing works. To the average person, "suck" means pull; as sucking on a straw "pulls" liquid up the straw. In my opinion this erroneous information and should be corrected. Why not provide a proper explanation which is correct and just as simple? Semantics is not the issue, correct information is.
11-04-2013, 05:26 AM
#62
When you suck on a straw, the liquid is pushed up the straw by the air over the glass, which has a higher pressure than the lower pressure inside the straw that you have just created. It's not as if there's some invisible rope created by "sucking," which then "pulls" the liquid upward. You have just committed the very "error" that you are accusing the Air Force of having made. (Not really an error, just two different ways of describing the same thing.) The issue is in fact semantics: You and the Air Force have said the same thing in different ways.
11-04-2013, 05:51 AM
#63
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So now you're conceding that the Air Force's explanation was accurate, and that the only problem was that "the average person" wouldn't have understood it? A few posts back you said this wasn't about semantics, it was about erroneous information. Glad to see you've changed your mind.
11-04-2013, 05:59 AM
#64
Your arugument is just as absurd - the vast majority of the scientific and engineering community accept the premise that BY DEFINITION a lower pressure area creates a suction.
11-04-2013, 06:13 AM
#65
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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 guille2006; 11-04-2013 at 06:26 AM. Reason: contents
11-04-2013, 07:36 AM
#68
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A great place, the USAF Air Museum. I used to jog around it until the security fanatics stopped us. Perhaps a poor choice of words using "sucked" but it is easily understood by the average lay person. I too agree with the basic principle given. But then I am just an old Air Force person while non-rated, flew a whole lot of miles on wings both commercial and military. They worked every time. So I kinda think they know what they are doing.
11-04-2013, 07:52 AM
#69
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The definition of electrical current flowing from "+" to "-" is called Conventional Current. The definition of current flowing from "-" to "+" (which is what physically happens) is called Electron Current. We tend to think of + flowing to -, but in actuality it is the electrons doing the work physically moving toward a more positive (less negative) source. Either way will work in trying to understand components and circuits, but not aerodynamics. 
Last edited by Antares100; 11-04-2013 at 07:53 AM. Reason: error typing
11-04-2013, 08:02 AM
#70
In my humble opinion, any wing must be forced to go through the air at certain angle respect to the direction of movement in order to induce in the surrounding air a reactive force which opposes that movement (drag-useless) and impulses the wing up (lift-useful), counteracting the down impulse of weight.
It seems that the statement in discussion tries to explain the way in which part (lift only) of that reactive force is caused.
As discussed here earlier, some pushing up reaction happens against the bottom surface of the wing, while some pull up reaction happens against the top surface, being the magnitude of the pull up reaction substantially bigger than the magnitude of the pushing up one.
The trust that forces the wing to move and overcome drag is very important, because without that force the Air Force would show in that museum only gliders that descend gracefully.
That thrust is responsible for the speed of the airplane, abundance of which is as important for winning battles as for providing lift.
That certain angle respect to the direction of movement is very important because:
From 0 to around 10 degrees lift grows and drag stays relatively low.
Above around 10 degrees lift gets abruptly reduced and drag increases significantly (more as angle increases).
At 90 degrees there is no lift and drag is maximum.
The tail of the airplane is responsible for keeping that certain angle with precision and according to the speed of the plane and weight.
How would you improve the statement on the placard (as posted by the OP), in order to make it more accurate but not too complicated to the average visitor of the museum?
"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".
It seems that the statement in discussion tries to explain the way in which part (lift only) of that reactive force is caused.
As discussed here earlier, some pushing up reaction happens against the bottom surface of the wing, while some pull up reaction happens against the top surface, being the magnitude of the pull up reaction substantially bigger than the magnitude of the pushing up one.
The trust that forces the wing to move and overcome drag is very important, because without that force the Air Force would show in that museum only gliders that descend gracefully.
That thrust is responsible for the speed of the airplane, abundance of which is as important for winning battles as for providing lift.
That certain angle respect to the direction of movement is very important because:
From 0 to around 10 degrees lift grows and drag stays relatively low.
Above around 10 degrees lift gets abruptly reduced and drag increases significantly (more as angle increases).
At 90 degrees there is no lift and drag is maximum.
The tail of the airplane is responsible for keeping that certain angle with precision and according to the speed of the plane and weight.
How would you improve the statement on the placard (as posted by the OP), in order to make it more accurate but not too complicated to the average visitor of the museum?
"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".
11-04-2013, 09:12 AM
#71
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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, 10:22 AM
#72
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, 10:45 AM
#73
So now you're conceding that the Air Force's explanation was accurate, and that the only problem was that "the average person" wouldn't have understood it? A few posts back you said this wasn't about semantics, it was about erroneous information. Glad to see you've changed your mind.
11-04-2013, 10:53 AM
#74
A similar argument is which way electricity travels along a wire: + to - or - to +? Or do the electron moves to fill the holes or do the holes move to overrun the electrons? Do you agree that electrons are negative particles that are attracted to positive particles (or holes)? Yet BY DEFINITION current flows from + to -.
Your arugument is just as absurd - the vast majority of the scientific and engineering community accept the premise that BY DEFINITION a lower pressure area creates a suction.
Your arugument is just as absurd - the vast majority of the scientific and engineering community accept the premise that BY DEFINITION a lower pressure area creates a suction.
The reasons for the designations of + and - in electricity are irrelevant to the premise of this thread.
(Notice that I did not refer to your assertion as "absurd.")
11-04-2013, 11:02 AM
#75
In my humble opinion, any wing must be forced to go through the air at certain angle respect to the direction of movement in order to induce in the surrounding air a reactive force which opposes that movement (drag-useless) and impulses the wing up (lift-useful), counteracting the down impulse of weight.
It seems that the statement in discussion tries to explain the way in which part (lift only) of that reactive force is caused.
As discussed here earlier, some pushing up reaction happens against the bottom surface of the wing, while some pull up reaction happens against the top surface, being the magnitude of the pull up reaction substantially bigger than the magnitude of the pushing up one.
The trust that forces the wing to move and overcome drag is very important, because without that force the Air Force would show in that museum only gliders that descend gracefully.
That thrust is responsible for the speed of the airplane, abundance of which is as important for winning battles as for providing lift.
That certain angle respect to the direction of movement is very important because:
From 0 to around 10 degrees lift grows and drag stays relatively low.
Above around 10 degrees lift gets abruptly reduced and drag increases significantly (more as angle increases).
At 90 degrees there is no lift and drag is maximum.
The tail of the airplane is responsible for keeping that certain angle with precision and according to the speed of the plane and weight.
How would you improve the statement on the placard (as posted by the OP), in order to make it more accurate but not too complicated to the average visitor of the museum?
"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".
It seems that the statement in discussion tries to explain the way in which part (lift only) of that reactive force is caused.
As discussed here earlier, some pushing up reaction happens against the bottom surface of the wing, while some pull up reaction happens against the top surface, being the magnitude of the pull up reaction substantially bigger than the magnitude of the pushing up one.
The trust that forces the wing to move and overcome drag is very important, because without that force the Air Force would show in that museum only gliders that descend gracefully.
That thrust is responsible for the speed of the airplane, abundance of which is as important for winning battles as for providing lift.
That certain angle respect to the direction of movement is very important because:
From 0 to around 10 degrees lift grows and drag stays relatively low.
Above around 10 degrees lift gets abruptly reduced and drag increases significantly (more as angle increases).
At 90 degrees there is no lift and drag is maximum.
The tail of the airplane is responsible for keeping that certain angle with precision and according to the speed of the plane and weight.
How would you improve the statement on the placard (as posted by the OP), in order to make it more accurate but not too complicated to the average visitor of the museum?
"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 would improve the placard by saying "The curvature of the wing's airfoil causes lower pressure on the upper surface of a wing than the lower. The higher pressure on the lower wing surface pushes (or forces) the wing upward.
(It is debatable whether the intelligence of the average person can handle this explanation in two sentences. It could be stated in one longer sentence with the same caveat!)