Re: Battleship downforce and JumboJet downwash.

[William Beaty <billb@ESKIMO.COM>]

On Fri, 10 Dec 1999, brian whatcott wrote:

> And I expect you respond that all the seabed feels the increased
> pressure - but only the increase in water pressure provided in the
> just the ordinary way, by the increased head from the rise in water level
>  when the ship was launched.

Ah, but how long does it take for the increased head to propagate across
the ocean?  It might be fairly fast, since tsunamis move fast in deep
water.  Does the launching of a battleship initiate a worldwide
femto-tsunami?

> It reminded me though, of the storm in a tea cup on this list when
> someone was arguing about the extent of the downflow from a passing
> jumbo jet. Did it/didn't it reach the ground?
> Of course it does/doesn't  - sort of thing....
>
> So let me share a modest insight: though the down thrust from the
> wings of a big airplane never directly reaches the ground
> (from reasonable heights of several thousand feet) the answer to
> where its weight is reacted is of the same kind as the one for the
>  destroyer sailing over the sea bed.
>
> Yes! the ground carries the airplane's weight - but the mechanism is
> just the ordinary one - the pressure of air in this case.


In the case of a hot air balloon, I would agree with the above.  In the
case of an airplane, the situation more resembles the "continuous
launching" of a ship, and the "footprint" of increased pressure cannot
propagate everywhere instantaneously.

We're back to the "hovering bird in a cage" problem.

Here's another angle: when a large bumblebee hovers about 10cm above the
ground, how large is the footprint of increased pressure?  If the ground
is covered with dry dust, you can SEE this footprint.  It's a circular
patch which is several cm in diameter, where the dust is being blown
radially outwards as the jet of air below the bee delivers its momentum to
the ground.  Fascinating to watch!  As the bee moves around, this
footprint follows it.  I imagine that if the bee moved horizontally at
20kph, the footprint would still follow along, but would strike the ground
far behind the bee.  (Wouldn't it take the form of wingtip-vortices, where
the "footprint" is the spot where the vortices collide with the ground?)

If the bee instead was to fly slowly upwards, I imagine that the footprint
would become wider and wider because the jet of air beneath the bee would
undergo turbulent mixing, and deliver its momentum to a very large region
on the ground.  In that case the situation would begin to resemble that of
a hot air balloon, because the jet of air from the hovering bee never
reaches the ground, but instead collides with the air which rests upon the
ground.


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William J. Beaty                                  SCIENCE HOBBYIST website
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