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Old 02-08-2020, 08:51 PM
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how airplanes fly.


I read your recent article about how airplanes fly. I want to add my thought. Should we start as follows: A plane is on the runway. Air pressure at 35 pounds per square inch is pushing both up and down on the wings. As the plane accelerates down the runway the configuration and position of the wings (and perhaps the horizontal tail as well) causes the air pressure on top of the wings to gradually diminish until the difference between the air pressure on top of the wings falls ( in pounds per square inch) to a level where the pressure on the bottom of the wings minus the pressure above the wing X the total area of the wings in square inches surpasses the weight of the plane, then the plane lifts off.
If this is correct, then the question is what causes the air pressure above the wing to diminish. Maybe one factor is the inertia of the air above the wing which would slow down the air filling the partial vacuum developing above the wing.

John Price
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Old 02-08-2020, 09:43 PM
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Referring to this column, I'm guessing:

How do airplanes fly, really? by SDSAB member aerodave, July 12, 2005.

ETA:
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Originally Posted by john price View Post
I read your recent article . . .
John Price
Not exactly recent, though. These Straight Dope articles are all reprints.

Anyhow, welcome to the Straight Dope Message Board, John Price.

Be a good boy, don't piss off the mods and enjoy the ride!

Some more ETA: Hang tight for a bit ... I'm going to hunt up some cites that I've seen before ... There are all kinds of theories about how airplanes fly, and it seems that it's even less certain than we though back 45 years ago when I was first learning this stuff.....

Last edited by Senegoid; 02-08-2020 at 09:48 PM.
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Old 02-08-2020, 10:09 PM
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Oops -- Looks like the cite I was trying to find is, in fact, the same one that aerodave gives at the end of his article. If you click through the pages, it has several pages showing wrong theories and some pages showing some of the better current theories.

Quote:
Originally Posted by aerodave
NASA’s Glenn Research Center has an educational site that covers these topics (and lots of others, like propulsion) at a basic level, with interactive Java applets. The incorrect theories of lift start at: http://www.grc.nasa.gov/WWW/K-12/airplane/lift1.html
Back in the day, we all learned the Bernoulli theory. But I don't recall ever hearing about the "equal transit time" idea. As far as I can figure, that was somebody's idea of what the Bernoulli theory was all about. But I can still believe the Bernoulli theory, at least partly, and I can't see any logical reason why one needs to imagine an "equal transit time" theory to go along with it.
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Old 02-08-2020, 10:57 PM
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We know in detail what an airfoil does but not so much about how it works. Modelers have found that a wing works about the same if you turn it around and fly with the trailing edge first. You do not need an airfoil envelope. Flat plates and curved plates work well as do wings with a curved airfoil for the first third of the wing and a flat plate from there to the trailing edge.

And, check out Magnus effect or Flettner aircraft. They have virtual airfoils. Some great stuff on You Tube.

Last edited by Crane; 02-08-2020 at 11:00 PM.
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Old 02-09-2020, 01:12 AM
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Originally Posted by john price View Post
I read your recent article about how airplanes fly. I want to add my thought. Should we start as follows: A plane is on the runway. Air pressure at 35 pounds per square inch is pushing both up and down on the wings. As the plane accelerates down the runway the configuration and position of the wings (and perhaps the horizontal tail as well) causes the air pressure on top of the wings to gradually diminish until the difference between the air pressure on top of the wings falls ( in pounds per square inch) to a level where the pressure on the bottom of the wings minus the pressure above the wing X the total area of the wings in square inches surpasses the weight of the plane, then the plane lifts off.
If this is correct, then the question is what causes the air pressure above the wing to diminish. Maybe one factor is the inertia of the air above the wing which would slow down the air filling the partial vacuum developing above the wing.

John Price
Slight nitpick, the air pressure would be 15 pounds per square inch, not 35
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Old 02-09-2020, 02:24 AM
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Airplanes fly by pushing down air. It's an F=ma type of thing.
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Old 02-09-2020, 05:59 AM
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I believe the Bernoulli explanation is at last (mercifully) falling out of favor, and the Newtonian one is (deservedly) becoming more common.

To summarize: planes fly because their wings deflect air downward, which (in accordance with Newton's laws of motion) produces an upward force (equal to the weight of the plane when it is in unaccelerated flight).

The best way to grasp this is to observe a helicopter hovering over water: The rotor blades (which clearly are wings) keep the machine airborne by pushing a huge amount of air downward, made obvious by the way the water surface is disturbed.
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Old 02-09-2020, 07:55 AM
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The Bernoulli explanation is the Newtonian one. Bernoulli's equation is just F = ma as applied to fluids (as opposed to discrete objects). What's thankfully falling out of favor is the explanation which is frequently taught as "the Bernoulli effect" but which has nothing to do with it, nor with any other aspect of reality.
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Old 02-09-2020, 08:33 AM
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The best way to grasp this is to observe a helicopter hovering over water: The rotor blades (which clearly are wings) keep the machine airborne by pushing a huge amount of air downward, made obvious by the way the water surface is disturbed.
Yep.

A helicopter stays aloft because it pushes air down. An airplane stays aloft using the same principle: it pushes air down.
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Old 02-09-2020, 09:38 AM
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Re: Shape of wing.

People have been studying wing shape for over a century now. And your standard airliner wing cross section is still basically the same as it was in the 1920s.

This shape provides more lift at cruising speeds than a flat wing and therefore saves fuel.

Sure you can fly a plane with flat or inverted wings, but you're going to burn more fuel.

It's absurd to deny this reality of wing shape. The standard shape increases lift!
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Old 02-09-2020, 10:14 AM
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Re: Shape of wing.

People have been studying wing shape for over a century now. And your standard airliner wing cross section is still basically the same as it was in the 1920s.

This shape provides more lift at cruising speeds than a flat wing and therefore saves fuel.

Sure you can fly a plane with flat or inverted wings, but you're going to burn more fuel.

It's absurd to deny this reality of wing shape. The standard shape increases lift!
IANA aerospace engineer. But it's my understanding that a "barn door" shaped wing can (and will) provide lift by deflecting air down, but there are problems when the wing has a simply flat geometry (i.e. no airfoil shape):

- The bottom surface of a flat wing will deflect air down, thus providing lift. But the air won't follow the profile of the top surface, and thus the top surface won't deflect air down and provide lift.

- The air above a flat wing is very turbulent, thus producing drag (and inefficiency).

An airfoil is shaped so that, in addition to the bottom surface, the top surface of the wing also provides lift. The top surface is shaped such that the air wants to follow the profile. (I am not sure how this occurs, exactly, but it does.) And if the air follows the top surface of the wing, it will be deflected down. In addition, the air is trying to "pull away" from the top of the hump on the top surface of the airfoil, but is unsuccessful in doing so. So it creates a slight vacuum between the air and the top of the hump, thereby providing additional lift.

One more thing: in addition to being inefficient, a barn door shaped wing would have a terrible stall angle compared to an airfoil.
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Old 02-09-2020, 03:29 PM
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It's absurd to deny this reality of wing shape. The standard shape increases lift!
I think the right way to say this is that airfoil shapes are solutions to the problem of making wings efficient. If you don't care about efficiency, you can produce large amounts of lift with a crude approach, up to and including "barn door" wings.

And note that there is no standard shape - airfoils vary a lot in their details, according to the job(s) the wing is expected to do. Airfoil research is ongoing - future wings will not be the same as today's, which in many cases are quite different from those of the past.
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Old 02-09-2020, 07:02 PM
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how airplanes fly.


Most of the time, pretty well.
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Old 02-10-2020, 07:42 AM
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It is the speed of air flow that produces the lift. Bernoulli's principle, which says that if air speeds up the pressure is lowered. Thus a wing generates lift because the air goes faster over the top creating a region of low pressure, and thus lift.
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Old 02-10-2020, 08:20 AM
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Lift is a function of area and angle of attack, not airfoil shape. The airfoil shape determines drag.

The air that is deflected down at the trailing edge of the wing does not produce lift. The angle of deflection is inversely proportional to the aspect ratio of the wing. A long, thin wing produces the same lift as a short wing of the same area, but has little or no downwash.
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Old 02-10-2020, 05:07 PM
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... a wing generates lift because the air goes faster over the top creating a region of low pressure, and thus lift.
This is pretty much the standard "Bernoulli-based" explanation. It's not wrong, but it's a bad explanation for two reasons:
It encourages the listener to overlook the important fact that to create that region of low pressure, an enormous amount of air must be continually deflected downward.

It amounts to explaining the unknown (how a plane flies) in terms of the unfamiliar (Bernoulli theory of fluid flow). By contrast, Newtonian action-reaction is familiar - something every toddler encounters every day: "When I push on the chair, it moves away from me, and I move away from it."
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Old 02-10-2020, 05:17 PM
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A long, thin wing produces the same lift as a short wing of the same area, but has little or no downwash.
Regardless of aspect ratio, no downwash = no lift.

The short wing acts energetically on a relatively small volume of air, thus embedding a relatively large amount of energy in the wingtip vortices that are an inevitable consequence of lift. The long thin wing acts more gently on a relatively large volume of air, with consequently lower induced drag. But there ain't no such thing as (unaccelerated) winged flight without downwash.
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Old 02-12-2020, 07:34 AM
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I don’t have a dog on this catwalk, but I have been reading you guys' discussion with interest. So, of course, the internet gods provided this article on my home page this morn. From Scientific American, it discusses that "No One Can Explain Why Planes Stay in the Air".
Quote:
Originally Posted by Sci...American
What Anderson said, however, is that there is actually no agreement on what generates the aerodynamic force known as lift. “There is no simple one-liner answer to this,” he told the Times. People give different answers to the question, some with “religious fervor.” More than 15 years after that pronouncement, there are still different accounts of what generates lift, each with its own substantial rank of zealous defenders. At this point in the history of flight, this situation is slightly puzzling. After all, the natural processes of evolution, working mindlessly, at random and without any understanding of physics, solved the mechanical problem of aerodynamic lift for soaring birds eons ago. Why should it be so hard for scientists to explain what keeps birds, and airliners, up in the air?
https://www.scientificamerican.com/a...ay-in-the-air/
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Old 02-12-2020, 09:49 AM
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I don’t have a dog on this catwalk, but I have been reading you guys' discussion with interest. So, of course, the internet gods provided this article on my home page this morn. From Scientific American, it discusses that "No One Can Explain Why Planes Stay in the Air".


https://www.scientificamerican.com/a...ay-in-the-air/
If the wing was entirely flat and perpendicular to the ground and pushing air like a bulldozer out of the way but generating no lift would it be surprising that there would be a pressure difference in the fluid on the two sides of the wing? On one side you are compressing the fluid on the other side you are pulling away from the fluid.

Put the same flat wing at an angle to the ground other than parallel, while in motion, and you still are pushing fluid forward and down while pulling away from a fluid on the opposite side of the wing.

Last edited by octopus; 02-12-2020 at 09:49 AM.
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Old 02-12-2020, 10:10 AM
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shunpiker has it right


If downwash produced lift then long thin wings would produce less lift than short broad ones. They don't. Lift is a function of area.

If lift was created by pushing down on the air then parking at the end of a runway to watch 747s take off would result in you car being flattened by the supporting column of air. I've done it and my ears did not even pop.
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Old 02-12-2020, 10:11 AM
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I believe the Bernoulli explanation is at last (mercifully) falling out of favor, and the Newtonian one is (deservedly) becoming more common.
The Bernoulli hyothesis sounded like BS to me when I learned it in my teens. Sure, the faster moving air reduces pressure to cause lift; you can prove that by blowing air through a straw held over a piece of paper. The question I had was why the air is moving over the top of a wing faster just because it has a longer path -- were the molecules that got split having conversation and the one on top wanted to continue it?
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Old 02-12-2020, 10:14 AM
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Shunpiker has it ...
While I appreciate the accolades, I really don’t have my finger on the pulse of this discussion. Heck, I barely have a pulse.
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Old 02-12-2020, 10:34 AM
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If downwash produced lift then long thin wings would produce less lift than short broad ones. They don't. Lift is a function of area.

If lift was created by pushing down on the air then parking at the end of a runway to watch 747s take off would result in you car being flattened by the supporting column of air. I've done it and my ears did not even pop.
No. Air isn’t solid and just because the wing is pushing down on the air doesn’t mean that the column of fluid under the projected area of the wing will squish you.

For a very simple layman’s explanation here is NASA’s take. https://www.grc.nasa.gov/WWW/k-12/Su...nearticle.html
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Old 02-12-2020, 11:35 AM
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The Bernoulli hyothesis sounded like BS to me when I learned it in my teens. Sure, the faster moving air reduces pressure to cause lift; you can prove that by blowing air through a straw held over a piece of paper. The question I had was why the air is moving over the top of a wing faster just because it has a longer path -- were the molecules that got split having conversation and the one on top wanted to continue it?
I always took this to be a manifestation of the continuity principle. See here for more detail. If the molecules that take the longer path didn't catch up to those that took the shorter path, you'd end up with a mass imbalance.
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Old 02-12-2020, 11:39 AM
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Yes that is a simplistic description by someone at NASA who creates web pages.

Hmmmm…. so air isn't solid and pushing it down supports a multi-ton machine but to do that the machine would have to fall in order to push down a compressible fluid far enough to raise it's pressure enough to support it's weight and do it without the compressed air squirting out on all sides of the column.
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Old 02-12-2020, 12:25 PM
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Yes that is a simplistic description by someone at NASA who creates web pages.

Hmmmm…. so air isn't solid and pushing it down supports a multi-ton machine but to do that the machine would have to fall in order to push down a compressible fluid far enough to raise it's pressure enough to support it's weight and do it without the compressed air squirting out on all sides of the column.
As the wing moves through the air it pushes down on it. The plane doesn't have to fall at all. If it's flying level it isn't falling. In level flight the lift is in balance with the weight and the lift comes from a net force on the wing that is equal to and opposite the force on the fluid surrounding the plane. The physics is undeniable. For there to be an upward force on the wing there has to be a downward force on the air supporting the wing. There has to be an equilibrium if the acceleration of the wing is 0.

Now people say what about the lower pressure on top of the wing? Great. How does that pressure on the wing get lower to begin with? If the plane is sitting on the ground there is no pressure difference on the wing. Curved wing? Not necessary. You can fly with non curved wing. What matters more is angle and pushing the air in front and under the wing which compresses it and moving away from the air on top and behind the wing which causes a pressure drop. It takes time for air to flow into the space that the wing is moving through. Mass is conserved and if there is compression in front of the wing that means somewhere, and that somewhere is behind and on top of the wing, air has to expand. Just think of a plane that's flying with a flat angled wing instead of being distracted by curved airfoil mumbo jumbo.

Make a paper airplane with angled wings. Throw it level to the ground and watch it climb. Where's it getting the lift?

Now what the air does after being pushed forward and down is where things get messy.

Last edited by octopus; 02-12-2020 at 12:27 PM.
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Old 02-12-2020, 07:44 PM
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DesertDog, what you learned is not in fact the Bernoulli hypothesis. I'm sure that the people who taught you thought that it was the Bernoulli hypothesis, but they were wrong. The Bernoulli effect is just Newton's laws, and like any correct way of phrasing the physics behind airplane lift, involves significant downdraft from the wings.
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Old 02-12-2020, 10:37 PM
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If lift was created by pushing down on the air then parking at the end of a runway to watch 747s take off would result in you car being flattened by the supporting column of air.
There is no supporting column of air.

Instead, there are wingtip vortices, created continuously as the high pressure under each wing rolls around the wingtip and into the low pressure found on top of the wing. This effect is apparent (from about 0:16 to 0:40) in this video.

Though they won't damage your car, such vortices can be felt on the ground, and can persist for a while. Small aircraft must use caution when taking off or landing shortly after a heavy plane. Here's a useful video (with useless music) that concerns an accident caused by this issue.
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Old 02-13-2020, 10:33 AM
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DesertDog, what you learned is not in fact the Bernoulli hypothesis. I'm sure that the people who taught you thought that it was the Bernoulli hypothesis, but they were wrong. The Bernoulli effect is just Newton's laws, and like any correct way of phrasing the physics behind airplane lift, involves significant downdraft from the wings.
Pretty sad when even the wrong answer is given a wrong explanation. I'm beginning to think planes fly on pure faith.
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Old 02-13-2020, 04:20 PM
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Pretty sad when even the wrong answer is given a wrong explanation. I'm beginning to think planes fly on pure faith.
I've been tempted, while reading this thread, to write a lengthy essay (even longer than this post).

No need for that. Everything I might have written is there on-line. DesertDog's link to that Wikipedia page should be required reading -- the whole page, not just that section of the page. Not to mention the NASA link in aerodave's article and the related link that octopus gave a few posts above.

That Wiki page further includes links (among others) to the page on Lift (force) and also to Venturi effect. These are also required reading.

The Wiki page on Lift includes a lengthy portion on "simplified" explanations in plain English for us common schlubs, followed by some more technical stuff. The common "Newtonian" and "Bernoulli" explanations are described, and also criticized as being, not wrong, but incomplete. The silly "equal transit time" theory is debunked, including a cute animated graphic with moving colored dots. (I think the NASA page also includes this somewhere.)

I note that the Venturi effect has gone, heretofore, unmentioned in this thread, and in many on-line discussions of lift. I consider this to be evidence of not knowing what one is talking about. This is a necessary effect to understand toward understand the "Bernoulli" effect. This, along with compression of "stream tubes" above the wing, and not "equal transit time", seeks to explain why airflow above the airfoil is faster than airflow below, and nothing about it implies equal transit time.

The Venturi effect states that an incompressible fluid flowing through a tube with a constriction, will flow faster through the constricted region. Air flowing over an airfoil differs in two blatantly obvious ways: Air is extremely compressible, and the "tube" through which air flows over the wing is constricted only below (by the upper surface of the wing) and seemingly not above. It's like the lower half of a constricted pipe with the upper half cut away. The Venturi effect seems to apply, at least somewhat, because the compressible air complies at least a little bit with the Venturi hypothesis, and the upper layers of air above the wing have an internal pressure that partially constrains the flow of air over the wing a bit like in an enclosed pipe. Nothing in this explanation needs to imply any equal transit time theory. The above-cited Wiki links discuss these points.
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Old 02-22-2020, 11:21 PM
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Thirty posts, and not one mention of a treadmill?

Fine. Carry on then.
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Old Yesterday, 08:16 AM
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I'm sure airplanes carry treadmills all the time. No problem.
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