No One Can Explain Why Planes Stay in the Air - Scientific American


53 bookmarks. First posted by schmacme 21 days ago.


This did the rounds recently.
I’m firmly in the Newtonian camp— the air is def…
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5 days ago by carlfish
In December 2003, to commemorate the 100th anniversary of the first flight of the Wright brothers, the New York Times ran a story entitled “Staying Aloft; What…
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7 days ago by Aetles
In December 2003, to commemorate the 100th anniversary of the first flight of the Wright brothers, the New York Times ran a story entitled “Staying Aloft; What…
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7 days ago by granth
whoa. Didn’t know that.
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7 days ago by jgordon
On a strictly mathematical level, engineers know how to design planes that will stay aloft. But equations don't explain why aerodynamic lift occurs.
article  science  aviation 
11 days ago by kaarlows
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11 days ago by sandykoe
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?

Adding to the confusion is the fact that accounts of lift exist on two separate levels of abstraction: the technical and the nontechnical. They are complementary rather than contradictory, but they differ in their aims. One exists as a strictly mathematical theory, a realm in which the analysis medium consists of equations, symbols, computer simulations and numbers. There is little, if any, serious disagreement as to what the appropriate equations or their solutions are. The objective of technical mathematical theory is to make accurate predictions and to project results that are useful to aeronautical engineers engaged in the complex business of designing aircraft.

But by themselves, equations are not explanations, and neither are their solutions. There is a second, nontechnical level of analysis that is intended to provide us with a physical, commonsense explanation of lift. The objective of the nontechnical approach is to give us an intuitive understanding of the actual forces and factors that are at work in holding an airplane aloft. This approach exists not on the level of numbers and equations but rather on the level of concepts and principles that are familiar and intelligible to nonspecialists.
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12 days ago by dwalbert
No One Can Explain Why Planes Stay In The Air
Do recent explanations solve the mysteries of aerodynamic lift?

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February 1, 2020
AUTHOR
Ed Regis has written 10 science books, including Monsters: The Hindenburg Disaster and the Birth of Pathological Technology (Basic Books, 2015). He has also logged 1,000 hours flying time as a private pilot. Credit: Nick Higgins

IN BRIEF
On a strictly mathematical level, engineers know how to design planes that will stay aloft. But equations don't explain why aerodynamic lift occurs.
There are two competing theories that illuminate the forces and factors of lift. Both are incomplete explanations.
Aerodynamicists have recently tried to close the gaps in understanding. Still, no consensus exists.
In December 2003, to commemorate the 100th anniversary of the first flight of the Wright brothers, the New York Times ran a story entitled “Staying Aloft; What Does Keep Them Up There?” The point of the piece was a simple question: What keeps planes in the air? To answer it, the Times turned to John D. Anderson, Jr., curator of aerodynamics at the National Air and Space Museum and author of several textbooks in the field.

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?

Adding to the confusion is the fact that accounts of lift exist on two separate levels of abstraction: the technical and the nontechnical. They are complementary rather than contradictory, but they differ in their aims. One exists as a strictly mathematical theory, a realm in which the analysis medium consists of equations, symbols, computer simulations and numbers. There is little, if any, serious disagreement as to what the appropriate equations or their solutions are. The objective of technical mathematical theory is to make accurate predictions and to project results that are useful to aeronautical engineers engaged in the complex business of designing aircraft.

But by themselves, equations are not explanations, and neither are their solutions. There is a second, nontechnical level of analysis that is intended to provide us with a physical, commonsense explanation of lift. The objective of the nontechnical approach is to give us an intuitive understanding of the actual forces and factors that are at work in holding an airplane aloft. This approach exists not on the level of numbers and equations but rather on the level of concepts and principles that are familiar and intelligible to nonspecialists.

It is on this second, nontechnical level where the controversies lie. Two different theories are commonly proposed to explain lift, and advocates on both sides argue their viewpoints in articles, in books and online. The problem is that each of these two nontechnical theories is correct in itself. But neither produces a complete explanation of lift, one that provides a full accounting of all the basic forces, factors and physical conditions governing aerodynamic lift, with no issues left dangling, unexplained or unknown. Does such a theory even exist?

TWO COMPETING THEORIES
By far the most popular explanation of lift is Bernoulli’s theorem, a principle identified by Swiss mathematician Daniel Bernoulli in his 1738 treatise, Hydrodynamica. Bernoulli came from a family of mathematicians. His father, Johann, made contributions to the calculus, and his Uncle Jakob coined the term “integral.” Many of Daniel Bernoulli’s contributions had to do with fluid flow: Air is a fluid, and the theorem associated with his name is commonly expressed in terms of fluid dynamics. Stated simply, Bernoulli’s law says that the pressure of a fluid decreases as its velocity increases, and vice versa.

Bernoulli’s theorem attempts to explain lift as a consequence of the curved upper surface of an airfoil, the technical name for an airplane wing. Because of this curvature, the idea goes, air traveling across the top of the wing moves faster than the air moving along the wing’s bottom surface, which is flat. Bernoulli’s theorem says that the increased speed atop the wing is associated with a region of lower pressure there, which is lift.

Illustration depicts two classic explanations of lift—Bernoulli’s theorem and the Newtonian principle of action and reaction—along with their flaws
Credit: L-Dopa
Mountains of empirical data from streamlines (lines of smoke particles) in wind-tunnel tests, laboratory experiments on nozzles and Venturi tubes, and so on provide overwhelming evidence that as stated, Bernoulli’s principle is correct and true. Nevertheless, there are several reasons that Bernoulli’s theorem does not by itself constitute a complete explanation of lift. Although it is a fact of experience that air moves faster across a curved surface, Bernoulli’s theorem alone does not explain why this is so. In other words, the theorem does not say how the higher velocity above the wing came about to begin with.

Art depicts two recent attempts at more complete explanations of lift: co-dependency of lift’s four elements, and the cause of low pressure above the wing
Credit: L-Dopa
There are plenty of bad explanations for the higher velocity. According to the most common one—the “equal transit time” theory—parcels of air that separate at the wing’s leading edge must rejoin simultaneously at the trailing edge. Because the top parcel travels farther than the lower parcel in a given amount of time, it must go faster. The fallacy here is that there is no physical reason that the two parcels must reach the trailing edge simultaneously. And indeed, they do not: the empirical fact is that the air atop moves much faster than the equal transit time theory could account for.

There is also a notorious “demonstration” of Bernoulli’s principle, one that is repeated in many popular accounts, YouTube videos and even some textbooks. It involves holding a sheet of paper horizontally at your mouth and blowing across the curved top of it. The page rises, supposedly illustrating the Bernoulli effect. The opposite result ought to occur when you blow across the bottom of the sheet: the velocity of the moving air below it should pull the page downward. Instead, paradoxically, the page rises.

The lifting of the curved paper when flow is applied to one side “is not because air is moving at different speeds on the two sides,” says Holger Babinsky, a professor of aerodynamics at the University of Cambridge, in his article “How Do Wings Work?” To demonstrate this, blow across a straight piece of paper—for example, one held so that it hangs down vertically—and witness that the paper does not move one way or the other, because “the pressure on both sides of the paper is the same, despite the obvious difference in velocity.”

The second shortcoming of Bernoulli’s theorem is that it does not say how or why the higher velocity atop the wing brings lower pressure, rather than higher pressure, along with it. It might be natural to think that when a wing’s curvature displaces air upward, that air is compressed, resulting in increased pressure atop the wing. This kind of “bottleneck” typically slows things down in ordinary life rather than speeding them up. On a highway, when two or more lanes of traffic merge into one, the cars involved do not go faster; there is instead a mass slowdown and possibly even a traffic jam. Air molecules flowing atop a wing do not behave like that, but Bernoulli’s theorem does not say why not.

The third problem provides the most decisive argument against regarding Bernoulli’s theorem as a complete account of lift: An airplane with a curved upper surface is capable of flying inverted. In inverted flight, the curved wing surface becomes the bottom surface, and according to Bernoulli’s theorem, it then generates reduced pressure below the wing. That lower pressure, added to the force of gravity, should have the overall effect of pulling the plane downward rather than holding it up. Moreover, aircraft with symmetrical airfoils, with equal curvature on the top and bottom—or even with flat top and bottom surfaces—are also capable of flying inverted, so long as the airfoil meets the oncoming wind at an appropriate angle of attack. This means that Bernoulli’s theorem alone is insufficient to explain these facts.

The other theory of lift is based on Newton’s third law of motion, the principle of action and reaction. The theory states that a wing keeps an airplane up by pushing the air down. Air has mass, and from Newton’s third law it follows that the wing’s downward push results in an equal and opposite push back upward, which is lift. The Newtonian account applies to wings of any shape, curved or flat, symmetrical or not. It holds for aircraft flying inverted or right-side up. The forces at work are also familiar from ordinary experience—for example, when you stick your hand out of a moving car and tilt it upward, the air is deflected downward, and your hand rises. For these reasons, Newton’s third law is a more universal and comprehensive explanation of lift than Bernoulli’s theorem.

But taken by itself, the principle of action and reaction also fails to explain the lower pressure atop the wing, which exists in that region irrespective of whether the airfoil is … [more]
science  learning  misjudgement 
13 days ago by enochko
In December 2003, to commemorate the 100th anniversary of the first flight of the Wright brothers, the New York Times ran a story entitled “Staying Aloft; What…
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13 days ago by johnrclark
In December 2003, to commemorate the 100th anniversary of the first flight of the Wright brothers, the New York Times ran a story entitled “Staying Aloft; What…
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14 days ago by mattl
On a strictly mathematical level, engineers know how to design planes that will stay aloft. But equations don't explain why aerodynamic lift occurs. There are two competing theories that illuminate the forces and factors of lift. Both are incomplete explanations. Aerodynamicists have recently tried to close the gaps in understanding. Still, no consensus exists.
Science 
14 days ago by zephyr777
"No One Can Explain Why Planes Stay In The Air." via
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15 days ago by burritojustice
In December 2003, to commemorate the 100th anniversary of the first flight of the Wright brothers, the New York Times ran a story entitled “Staying Aloft; What…
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16 days ago by nzeribe
Scientific American is the essential guide to the most awe-inspiring advances in science and technology, explaining how they change our understanding of the world and shape our lives.
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17 days ago by thx1138

Mark Drela, a professor of fluid dynamics at the Massachusetts Institute of Technology and author of Flight Vehicle Aerodynamics, offers an answer: “If the parcels momentarily flew off tangent to the airfoil top surface, there would literally be a vacuum created below them,” he explains. “This vacuum would then suck down the parcels until they mostly fill in the vacuum, i.e., until they move tangent to the airfoil again. This is the physical mechanism which forces the parcels to move along the airfoil shape. A slight partial vacuum remains to maintain the parcels in a curved path.”

“The reduced pressure over a lifting wing also ‘pulls horizontally’ on air parcels as they approach from upstream, so they have a higher speed by the time they arrive above the wing,” Drela says. “So the increased speed above the lifting wing can be viewed as a side effect of the reduced pressure there.”
review  theory  plane  flight  mecaflu 
18 days ago by aries1988
On a strictly mathematical level, engineers know how to design planes that will stay aloft. But equations don't explain why aerodynamic lift occurs. There are two competing theories that illuminate the forces and factors of lift. Both are incomplete explanations.
Pocket 
19 days ago by driptray
Me: how do planes stay in the air?
you, an engineer: ¯\_(ツ)_/¯
Me: ?A?>ASASMALKSJFLKDJ
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20 days ago by miaeaton
Scientific American is the essential guide to the most awe-inspiring advances in science and technology, explaining how they change our understanding of the world and shape our lives.
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20 days ago by ivar
It is 2020 yet we still don't have a consensus explanation as to why planes are able to fly:
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20 days ago by danbri
No One Can Explain Why Planes Stay in the Air: On a strictly mathematical level, engineers know how to design planes that will stay aloft. But equations don't explain why aerodynamic lift occurs. There are two competing theories that illuminate the forces and factors of lift. Both are incomplete explanations. via Pocket, added at:February 05, 2020 at 12:44AM
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21 days ago by LordEnzo
On a strictly mathematical level, engineers know how to design planes that will stay aloft. But equations don't explain why aerodynamic lift occurs.

There are two competing theories that illuminate the forces and factors of lift. Both are incomplete explanations.

Aerodynamicists have recently tried to close the gaps in understanding. Still, no consensus exists.
aerodynamics  lift  airplanes  explanation 
21 days ago by drmeme
In December 2003, to commemorate the 100th anniversary of the first flight of the Wright brothers, the New York Times ran a story entitled “Staying Aloft; What…
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21 days ago by bferg