Re: Battleship downforce and JumboJet downwash.

[William Beaty <billb@ESKIMO.COM>]

On Sat, 11 Dec 1999, brian whatcott wrote:

> 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.

I wonder if air-pressure changes, as well as solar & lunar tides, can be
subtracted out? Maybe the remaining signal would be "mystery noise", but
maybe it might also contain obvious patterns.  For example, how large is
the tide caused by Jupiter?  Do lakes "slosh" at 1f, 2f, etc., like the
overtones of an organ pipe?


> 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.

My analogy was poor.  I was imagining that an aircraft, since it must
continuously alter the momentum of the oncoming air, must also
continuously broadcast a footprint which continuously expands as the plane
moves forward.  On the other hand, if we launch a ship, the footprint
expands a single time and would eventually cover the ocean (as if a
ship-sized hunk of water had been suddenly deposited onto the ocean.)
Instead of a "continously-launched boat", I should have said "a
hydroplane."  When a hydroplane lifts up, bouyancy isn't the cause,
therefor the underwater wing must fling water downwards within the lake.


> 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.

I don't understand.  Do you mean that we traditionally explain only the
lift during ground-effect mode, but not the lifting force created during
flight at high altitudes?   I'm discussing a bumblebee which hovers at
many wingspans height.



> > 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.
>
>
> 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...

I suppose.  I was imagining that the jet of air decending from the
bumblebee was colliding with the stationary air below the jet, air which
rests upon the ground.  This would force that air downwards and press upon
the ground.  Maybe the region of increased pressure would be cone-shaped.
If so, then whenever the jet reaches the ground unimpeded, then it creates
a tiny disk-shaped zone of high pressure, but when the same bee hovers at
higher and higher altitude, then turbulence communicates the momentum of
the jet to a larger and larger "cone" of air, and the base of this
downward-moving "cone" of turbulent air presses upon the earth.  The air
launched by the bee might never reach the ground, but its momentum would
still be communicated to the ground.

Ah, I know where temperature comes in.  In the long run, the jet of air
must collide with the ground inelastically.  This might take the form of a
turbulent motion which eventually is halted by viscosity, resulting in a
slight temperature rise in the air and a slight expansion.  This warm air
would rise, and the pressure over a wide region would rise slightly (as it
does for a hot air balloon.)  But this isn't part of the force
communicated (indirectly) by the bee's air-jet, and it has a different
footprint.  Instead, this force is connected with the inefficiency of
winged flight, and with bouyancy of the heated air.  If the bee's air-jet
was smaller in diameter but moving faster, then it might deliver the same
bee-lifting mv, but the mv^2 energy loss would be smaller, and the amount
of air-heating would be smaller.


A parting shot in the "Bernoulli war":  is the physics of a hovering
helicopter radically different than the physics of a hovering Harrier jet?
(Some say that the helicopter flys by pressure-difference, and it need not
fling any air downwards.)


((((((((((((((((((((( ( (  (   (    (O)    )   )  ) ) )))))))))))))))))))))
William J. Beaty                                  SCIENCE HOBBYIST website
billb@eskimo.com                                  http://www.amasci.com
EE/programmer/sci-exhibits          science projects, tesla, weird science
Seattle, WA   206-781-3320          freenrg-L taoshum-L vortex-L webhead-L