Re: Battleship downforce and JumboJet downwash.

[brian whatcott <inet@INTELLISYS.NET>]

> 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.
At 18:41 12/10/99 -0800, Bill Beaty wrote:

> 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?

For an order of magnitude, I first think of tides swelling several
times per day.   But they are mediated by details of topography,
and this distributed differential force in a circumferential belt,
rather than a point force.

If one could instead, provide a telemetered water pipe with an orifice
sufficient to low pass out the ripples due to wind zephyrs on the surface
of a smallish lake, there would be a chance of testing the transmission
time for the water table rise due to launching a large displacement vessel,
 I suppose.
   Moeover, the water utilities do something of the kind, and pump the
data into little UHF antennas for assessing the water shed.
     The data collection center  would be an interesting kind of place
for high school students to visit, I imagine.

People have monitored the water levels of wells, which can be puzzling,
 by all accounts. Barometric pressure and temperature changes can
contribute their own effects, apparently.

///
[Brian]
> > 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.

A continuous ship launching necessitates a continual rise in water level
(at least on a semi-local scale). This is not the way it happens, I'd say.
> 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. ///

Though pilots notice distinct floatation effects in low wing planes when
landing, it is usual to model an effect on lift for any plane when height
above the ground gets less than about one wingspan.

> If the bee instead was to fly slowly upwards////
> the jet of air from the hovering bee never
> reaches the ground, but instead collides with the air which rests upon the
> ground.

> William J. Beaty

This vigorous air mixing with adjacent layers may reasonably be called
a temperature rise at some level of remove. And a temperature rise
involves a pressure increase in this circumstance...


brian whatcott <inet@intellisys.net>
Altus OK