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"We still can't make up our minds as to how wings work, so this isn't overly novel. Sure, we can explain the outcome, but whether it's Navier-Stokes or Bernoull
by tobinfricke 12y ago
"We still can't make up our minds as to how wings work, so this isn't overly novel. Sure, we can explain the outcome, but whether it's Navier-Stokes or Bernoulli or something else, we can't make up our minds."
Time to trot out my favorite quote about how airplanes fly, from Stick and Rudder by Wolfgang Langewiesche, page 9, published 1944:
»The main fact of all heavier-than-air flight is this: the wing keeps the airplane up by pushing the air down.
It shoves the air down with its bottom surface, and it pulls the air down with its top surface; the latter action is the more important. But the really important thing to understand is that the wing, in whatever fashion, makes the air go down. In exerting a downward force upon the air, the wing receives an upward counterforce--by the same principle, known as Newton's law of action and reaction, which makes a gun recoil as it shoves the bullet out forward; and which makes the nozzle of a fire hose press backward heavily against the fireman as it shoots out a stream of water forward. Air is heavy; sea-level air weights about 2 pounds per cubic yard; thus, as your wings give a downward push to a cubic yard after cubic yard of that heavy stuff, they get upward reactions that are equally hefty.
That's what keeps an airplane up. Newton's law says that, if the wing pushes the air down, the air must push the wing up. It also puts the same thing the other way 'round: if the wing is to hold the airplane up in the fluid, ever-yielding air, it can do so only by pushing the air down. All the fancy physics of Bernoulli's Theorem, all the highbrow math of the circulation theory, all the diagrams showing the airflow on a wing--all that is only an elaboration and more detailed description of just how Newton's law fulfills itself--for instance, the rather interesting but (for the pilot) really quite useless observation that the wing does most of its downwashing work by suction, with its top surface. ...
Thus, if you will forget some of this excessive erudition, a wing becomes much easier to understand; it is in the last analysis nothing but an air deflector. It is an inclined plane, cleverly curved, to be sure, and elaborately streamlined, but still essentially an inclined plane. That's, after all, why that whole fascinating contraption of ours is called an air-plane.«
- SamReidHughes 12y agoOkay but there's a difference in the nature of the "pushing down" of a rocket, a plane, and a hovering helicopter, and in how much of an impulse is actually inflicted on an external mass.
- tobinfricke 12y agoIs there?
- SamReidHughes 12y agoYes. For an extreme example, imagine the wing of an airplane (or anything) sitting on the ground. It's putting a force on the ground, the ground's putting a force on it, and its altitude remains constant. The ground isn't getting any extra downward momentum, and the plane isn't getting any extra upward momentum. Now consider a hovercraft. It sits on air. It does not push as much air downward as an airplane of equal weight would push. The amount of air it has to displace is probably proportional to the length of its perimeter (around the sides) multiplied by the height of the cushion of air it sits on multiplied by the air pressure underneath. Double the size of the hovercraft in both horizontal linear dimensions, and you've got double the perimeter, same air pressure (the machine's weight growing proportionally to area) and same cushion height, meaning you're holding four times the weight up with twice the air displacement. Now consider a low-flying airplane. This is like an inefficient hovercraft, with the ground effect in play. Less air gets "pushed downward" than the same plane flying higher off the ground. Consider a high-flying airplane. The ground effect is gone, but air's still got viscosity. Fly around the world forever and you'll notice that the amount of air beneath the plane is not actually increasing. Hovering helicopters have to push more air downward than moving helicopters because they have to fight the stream of downward moving air that they've created for themselves. If air had virtually no viscosity (suppose atoms were really tiny) then you'd have to fly by somehow forcing air to move downwards. If air had an extremely high amount of viscosity (relative to the mass and power of human-scale mechanical devices) then you wouldn't move it downwards at all -- planes would move around in the air like an amoeba or have a cylindrical conveyor belt surface. Our atmosphere is somewhere between these extremes.
- belovedeagle 12y agoI'm truly baffled at how people keep claiming that we don't understand how wings work when this obvious explanation is out there, one that any 8-year-old could give you, and it's absolutely right. "We don't understand how planes could fly upside down"---nonsense; it's the same way they flight right-side-up: by pushing the air down (or pulling, if you like).
- mikeash 12y agoThe problem is that people see the various different explanations for how a wing works (pushing the air down, pressure differential, speed differential, circulation, what have you) and they think they're competing theories, not just different perspectives on the same phenomenon. Edit: and of course it doesn't help at all that the silly and trivially disproven equal transit time theory continues to be the go-to explanation for how wings work for so many attempts to teach it.
- renox 12y agoWell, in my school (in France) the explanation was the 'equal transit time' which of course didn't satisfy me at all (it took me a long time to get a satisfying explanation). So when schools teach this kind of BS, you can expect that the confusion will last a long time..