3 ms·

It is great atrticle - but one thing it has backwards - Bernoulli principle is harder to understand than the second Newton law. Also I don't buy that viscosity

by zby 11y ago

It is great atrticle - but one thing it has backwards - Bernoulli principle is harder to understand than the second Newton law. Also I don't buy that viscosity argument - viscosity diverts air not down - but vertically. The move down is simple dynamics: .\ the dot is th air particle - the \ is the wing - when the wing moves the dot is forced down under the wing. The particle moves - and so it gets momentum down. The particle touching the wing is moved by the wing (the wing pushes it down - and the particle pushes it up according to the third law - generating the lift), the other particles are pushed by the particles toughing the lift.

plafl 11y ago
Viscosity is fundamental. A two dimensional airfoil inside inviscid fluid has zero drag (maybe you expected that) and indeterminate lift (that for me was a surprise), unless you force the stagnation point to be on the trailing edge. This mathematical condition is known as Kutta condition, and the physical process that allows it is viscosity. I know the explanation is somewhat cryptic but it's a starting point.
slavak 11y ago
NASA calls this "The Skipping Stone" theory of lift[1], and it is incorrect. What you're neglecting is that not only the bottom part of the wing diverts air downwards, but indeed the top part does as well; this contributes a large part of the lift generated by the wing. [1] https://www.grc.nasa.gov/www/k-12/airplane/wrong2.html https://www.grc.nasa.gov/www/k-12/airplane/wrong2.html
- Retric 11y ago
  Not quite. If you had a zero width wing you get a void behind the wing. Aka you pushed air down so it's not there. This creates a low pressure area behind the wing. But, that low pressure is still pushing down. It just pushes less than the normal air pressure under a wing. You can see this by dropping a single sheet of paper vs a stack of paper. In your model the low pressure above the wing pulls the paper up slowing it's decent. But, if you drop a stack of loose printer paper the top piece falls as fast as the bottom and is not pulled from the stack.
- jessaustin 11y ago
  It's interesting that that page links directly to a "right" page that says basically the same thing. Apparently it's very bad to simplify the explanation so much that one neglects the momentum of the laminar flow over the top of the foil. When I'm explaining it to an 8yo, however, I do neglect that.

by zby 11y ago

plafl 11y ago

Viscosity is fundamental. A two dimensional airfoil inside inviscid fluid has zero drag (maybe you expected that) and indeterminate lift (that for me was a surprise), unless you force the stagnation point to be on the trailing edge. This mathematical condition is known as Kutta condition, and the physical process that allows it is viscosity. I know the explanation is somewhat cryptic but it's a starting point.

slavak 11y ago

NASA calls this "The Skipping Stone" theory of lift[1], and it is incorrect. What you're neglecting is that not only the bottom part of the wing diverts air downwards, but indeed the top part does as well; this contributes a large part of the lift generated by the wing. [1] https://www.grc.nasa.gov/www/k-12/airplane/wrong2.html https://www.grc.nasa.gov/www/k-12/airplane/wrong2.html

Retric 11y ago
Not quite. If you had a zero width wing you get a void behind the wing. Aka you pushed air down so it's not there. This creates a low pressure area behind the wing. But, that low pressure is still pushing down. It just pushes less than the normal air pressure under a wing. You can see this by dropping a single sheet of paper vs a stack of paper. In your model the low pressure above the wing pulls the paper up slowing it's decent. But, if you drop a stack of loose printer paper the top piece falls as fast as the bottom and is not pulled from the stack.
jessaustin 11y ago
It's interesting that that page links directly to a "right" page that says basically the same thing. Apparently it's very bad to simplify the explanation so much that one neglects the momentum of the laminar flow over the top of the foil. When I'm explaining it to an 8yo, however, I do neglect that.