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

[brian whatcott <inet@INTELLISYS.NET>]

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.    Noting a difference between aerostats
and aerodynes is well-taken.

...
> I was responding to somebody's earlier message that stated that the
> exhaust is *required* to apply force to the planet, and that if it did
> not, Newton's laws would be violated.....
> (And maybe I misunderstood it in the first place.)

I too read John's message in that way for a moment.

> ... Now suppose we fly an aircraft over the earth at a height which
> is small, but which is well outside of the "ground effect" distance.  In
> that case the wings will launch a pair of wake-vortices downwards.  The
> earth will not experience a significant force until that vortex-pair
> collides with the ground.
>
> I agree that at great altitudes the downwards motion of the aircraft
> wake-vortices must gradually be slowed by interacting with the atmosphere,
> and this will cause the atmosphere to push downwards upon the earth.
> However, if the aircraft is fairly close to the ground, the "pressure" it
> applies to the earth is in the form of a "footprint" where the vortex-pair
> is crashing into the ground and being disrupted.

> William J. Beaty

I may have mentioned at some point that I was indeed caught in a wing tip
vortex some years ago, before separation rules were introduced into air
traffic control practise.
 I was cleared to take off almost immediately behind an airliner of
moderate size (that I can suppose was heavily laden).
 At one moment rising off the ground at 50 feet, the next, turned 90
degrees and heading for the tower.
   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.

brian whatcott <inet@intellisys.net>
Altus OK