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

On Wed, 18 Aug 1999, brian whatcott wrote:

Bill voices the conceptual difficulty of connecting a rocket exhaust
(or a bird's wing flutter) with a ground reaction.
I find this helpful:
If I eject one molecule downwards at high speed, I can expect it to
share with two molecules at lower speed, and so on in increasing
quantities until at appreciable distances beneath, it will be
instrumentally difficult to detect the small pressure increment over
the large area in question.

Not a conceptual difficulty exactly. My thought-experiments force me to
conclude that such a force is not part of the force that causes flight.
Instead, it is simply the force which (eventually) stops the downwards
motion of the parcels of air which had been "launched" by the hovering
bird, aircraft, etc. Is a bird like a balloon because both of them push
downwards upon the earth? No. 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.)


... Suppose a
rocket hovers several lunar-diameters above the moon. We can aim the
exhaust at the moon, or we can divide the exhaust into two diagonal
streams which miss the moon. This has little effect on the force-pair
between the rocket and its exhaust, although if the exhaust-streams miss
the moon, then the attraction exterted by the rocket upon the moon will
cause the moon to accelerate towards the rocket (imperceptably, because
the moon's mass is so enormous.)

William J. Beaty


I can see that Bill wanted to argue for the case where there
is no air to dilute the pressure due to the efflux.
We all can accept I'm sure that the rocket is not gaining thrust
by pushing on the air. Rather, we are merely noticing a consequence
of firing a rocket exhaust towards the Earth (or Moon as the case
may be.) If we remove the exhaust several planetoid diameters
but still aim it so that it all impacts the planetary surface,
then its effect though diluted by the large area of application
is the same as if it were close.
If the exhaust does not impinge upon the planetoid, then it
does not contribute a push.
(Or am I missing something here?) :-)


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. I'll have to search through old
messages to find that quote. (And maybe I misunderstood it in the first
place.)


Here's an interesting aspect to flight... Suppose we throw a baseball
towards the earth. The earth experiences very little force until the
baseball collides. Now suppose that we use a smoke-ring generator to
launch a ring-vortex downwards towards the earth. Again, the earth
experiences very little force until the ring-vortex collides with the
ground. 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. What if we fly a large,
heavily loaded aircraft right over a large building with a weak roof?
Conceivably it could punch the roof downwards! This is the "macroscopic"
version of the demonstration where a smoke-ring launcher is used to knock
over little cardboard targets. Airplane wings are smoke-ring launchers,
and as they fling a vortex downwards, they experience a lifting force.
To calculate lift, we can look at the surface velocities on the wings, or
we can look at the downwards movmement of the air in the wake behind the
plane.


((((((((((((((((((((( ( ( ( ( (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