Re: To hover, a reaction-motor pushes on the earth?

[William Beaty <billb@ESKIMO.COM>]

On Thu, 19 Aug 1999, brian whatcott wrote:

> At 17:11 8/18/99 -0700, Bill wrote:
>
> > ... A bird (or an aircraft) is fundamentally
> > different than a balloon because the earth pushes upwards upon the balloon
> > and causes the balloon to stay aloft, while this is not true of birds or
> > aircraft (or rockets.)
>
> Interesting to see the effect of gravity described as
> (comparitively) repulsive.


Hmmm.  When an object is resting upon the earth, aren't there two separate
force-pairs involved, gravity as well as a contact force? When a balloon
(or submarine) is suspended without ascending or descending, the "contact
force" between the earth and the object must obviously be mediated by the
surrounding fluid.  If I understand it correctly, this "contact force"
directed downwards upon the earth is simply the water or air pressure, as
increased by inserting an extra volume into the fluid (and therefore
increasing the mass of the ocean or atmosphere as a whole).  Heyyy...  If
we inflate a helium balloon, the surrounding earth should immediately feel
the increased downwards pressure, but the footprint of the pressure should
expand outwards at a particular speed.  What speed?  Well, it might be
like pouring a bucket of water into one end of a very long swimming pool.
It takes awhile for the bottom of the far end of the pool to feel the
increase in pressure.  But does the change in pressure propagate at the
speed of sound, or at the speed of a surface wave on the fluid?


> At one moment rising off the ground at 50 feet, the next, turned 90
> degrees and heading for the tower.

Nasty!

Here's an aspect to "lifting force" that I've always wondered about.  Does
the wrong concept cause deaths?  If we believe that an aircraft is lifted
by pressure differences, then we might expect that the wake turbulence is
similar to that of any large, fast-moving streamlined body.  On the other
hand, would pilots see nothing wrong with flying beneath a hovering
rocket?  The downwards-moving exhaust stream might do bad things to an
aircraft.  Now if *all* airplanes are actually like rockets, then
obviously we would want to avoid coming anywhere near the stream of
"exhaust" which keeps them aloft.  It might do bad things if you tried to
fly through it.  (I guess the problem is even worse than a "rocket-based"
viewpoint would predict, if the vortices spin violently as well as moving
along.)


>    But there is a point to this reminiscing: wing tip vortices are said to
> demonstrate surprizing integrity, at first drifting down at a few degrees
> angle to the flight path of the airplane which generates them: but on
> reaching the ground, they can hold their angular momentum, and drift
>  bodily downwind.   (This sort of data is carried by FAA notes)
>
>  You can see that this picture does not quite correspond with the rather
> violent effect you noticed from a vortex ring generator: it is the torque
> rather than down-force that I found worrying.


I've heard several stories about the vortices spawned at takeoff.  I
wonder if their vertical speed is different than that of the vorticies
created by high-level flight?  If the craft is mostly being lifted by
ground-effect, then no net-downwash is required.  Therefore I'd predict
that the vortices would just hang above the field and drift along with the
breeze.


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