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Old May 08, 2012, 02:49 PM
aeronaut999 is offline
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The Willamette Valley, Oregon
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downwash revisited

Originally Posted by Tom Harper View Post
(10) Are you proposing that the rudder does not work because it has no earth to push on?

No, you are confused about my claims of cause and effect, or I have not stated them clearly. I never said that that the wing works only because the earth pushes back. A wing would still generate lift as a spacecraft passed through a cloud of molecules with no earth nearby. The downwash in this case would continue forever-- the vortices would detach from the gas cloud and keep going forever. My original point all along with this thread, is that if the earth does not somehow feel the downward "push" of the plane in some way or the other-- and I am now realizing that that way of feeling the plane's downward push need not involve momentum-- then the plane's gravity will attract the earth upward and momentum will not be conserved. See posts #22 and #25. I still stand by that claim 100%.

As far as point #3 goes, surely we all agree that if air is exerting an upforce on the wing, then the wing is exerting a downforce on the air? I'm sure we all do.

The whole point of yesterday's post is simply to say that the downforce exerted by the wing onto the air is resisted by the air's resistance to shearing, and so opposing forces arise that greatly reduce the NET downward force on the air, so the air's NET downward acceleration is much less than would be the case without the opposing shearing forces. If we were simply creating propulsive force by, say, shooting bb's out of cannon into the vacuum of space, then the rearward momentum imparted to the bb's is the same as the forward momentum imparted to the spacecraft. But when we complicate the picture with a surrounding fluid, opposing forces arise and the picture becomes more complex. For example consider propwash. The air is exerting a forward force on the aircaft, but in steady cruise this force does not accelerate the aircraft forward, because it is opposed by other forces (drag) and the net forward force on the aircraft is zero. Likewise the prop exerts a backwards force on the air, but other opposing forces are exerted by the fluid on the air molecules near the prop, so the net acceleration of the air near the prop is less than would be predicted in the absence of these opposing forces. To take an extreme case, if we were flying in honey or molasses or jello, then the prop would "screw" its way through the fluid with very little slippage and very little rearward momentum would be imparted by the prop to the fluid. Likewise, if we are flying through honey, the honey's resistance to shearing greatly reduces the downwash velocity (and the downwash momentum) imparted by the wing to the fluid. Yet the wing is still exerting a downforce on the honey, which is still "felt" by a scale sitting underneath the vat of honey.

Just as when we stand on solid ground, our feet exert a downforce on the ground, just as the ground exerts an upforce on our feet, but there is no momentum transfer from our feet to the ground, because the ground is very viscous, you might say.

Seems straightforward to me.

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Last edited by aeronaut999; May 08, 2012 at 04:12 PM.
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