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

[William Beaty <billb@ESKIMO.COM>]

On Sat, 14 Aug 1999, John Denker wrote:
> billb wrote:
> > How can any aircraft remain suspended above the ground (or
> > remain in level flight)?
> >
> > It can either push upon the earth, or it can employ action/reaction in the
> > way a rocket does.  A helium balloon pushes indirectly upon the earth.  So
> > does an aircraft in "ground effect" flight.
>
>
> So does *everything*.  Gravity is a force between the earth and the
> aircraft;  the only way to counteract it is a force (indirect or otherwise)
> between earth and aircraft.  The only question is how indirect it is going
> to be.

Everything flys by pushing against the earth?  I strongly disagree.
Perhaps my terminology is a bit confusing.  When I say that a balloon
pushes "indirectly" on the earth, I mean that the balloon itself does not
touch the earth, therefor the forces are indirect.  However, there
certainly is a direct force-pair between the balloon and the earth, so
maybe I should say that the balloon pushes "directly" on the earth?  The
force-pair is 'instantaneous' if we neglect the small delay which comes
about from the velocity of sound.

Here's a simpler way to think about it: if we place a large tank of water
on scales, and then we place a neutrally-bouyant submarine into the tank
of water, do those scales register the weight of the submarine?  Obviously
yes.  The submarine pushes down on the bottom of the tank, and the tank
pushes equally upwards on the submarine, therefor the submarine does not
free-fall to the bottom of the tank.  In my mind I extend the physics of
the submarine to the physics of a neutrally-bouyant helium balloon, and I
see that the balloon can remain aloft only because there is a force-pair
between the balloon and the surface of the earth.  The earth supports the
balloon's weight.

A reaction-engine is entirely different.  If a rocket hovers high above
the ground, and if its exhaust eventually contacts the ground, then the
rocket is *not* using the surface of the earth in order to remain aloft.
Why would I say such a thing? Well, first of all I am extremely aware that
rockets fly by pushing against their own exhaust.   :)

Second: in order for the rocket to push upon the earth, a force-pair must
exist between the rocket and the earth.  But there is none.  Instead,
there is a force-pair between the rocket and the exhaust which it emits.
There is a *different* force-pair between the exhaust and the earth which
it strikes.  The two force-pairs are widely separated both in space and in
time, and there is no significant force being communicated backwards from
the earth to the rocket.  Yes, if the exhaust were to bounce off the earth
and be caught by the rocket, then this would act like a long range
force-pair between earth and rocket.  If a rocket hovered 10cm above the
earth, then this situation would apply, since the "reflected" exhaust
would then interact with the rocket.  However, since a high-flying rocket
is not struck by the exhaust that bounces from the earth, there is no
force-pair between the rocket and the earth.

Another point.  Whenever a rocket hovers near a massive object, there is
no requirement that the rocket's exhaust strikes the object.  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.)

I say again: a helium balloon employs a fundamentally different process
than a rocket in remaining aloft.  There *is* a force-pair between the
balloon and the ground, but there is none between the rocket and the
ground.  Neutral density craft push against the earth in order to remain
aloft, while reaction motors do not.



I believe that the controversy between the "Bernoulli-ist" and the
"Newtonist" explanations of flight definitely revolve around the above
concepts.  The "Bernouli-ists" insist that airplanes need not deflect any
air on average.  In your own discussions you state that "upwash" creates
lift, and that "downwash" creates even more lift, and therefor an aircraft
can fly even when upwash equals downwash.  This is not correct, since
whenever an airplane is in level flight very high above the earth, there
is no significant force-pair between itself and the earth, and equal
amounts of upwash and downwash won't change this fact.  The aircraft would
fall downwards unless either it lowers its density and behaves as a
balloon, or if it creates some net downwash and therefor applies some
downwards-directed "delta-vee" to some mass.  Pressure-difference alone
cannot lift a craft.  We can only fly by using the pressure-difference
which arises on a balloon's surfaces, or the pressure-difference across an
object which is constantly accelerating mass-parcels downwards.

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