Re: Physics of Flight (and Anderson/Eberhardt)

[William Beaty <billb@ESKIMO.COM>]

I apologize for the lateness of this reply.  I postponed the message and
then didn't check my postponed outbox.


On Wed, 18 Aug 1999, Stefan Jeglinski wrote:

> 2. When the ball bounces off the wall, it imparts an additional small
> impulse to the earth. My guy actually sees this as an increase in his
> speed,

Not necessarily.  If the balls cause the Earth to accelerate, that does
not give acceleration to the trolley.

> If my guy was
> throwing the balls hard enough, he could be hitting a wall quite far
> away without changing the result. In fact, he might already have
> thrown another ball off before the 1st even hits the wall, so that he
> doesn't even have direct knowledge that the ball ever hit the wall.
> If he instead threw the ball at the escape velocity and missed the
> wall, this would be a distinguishably different result w.r.t. his
> final speed though, so perhaps he -would- have direct knowledge,
> albeit *delayed*.
>
> 2. "The air thrown down from a wing can't possibly hit the ground and
> cause the same effect." I would argue why not in principle?

I agree that it can cause the same effect in actual practice, and not just
in theory. For example, if a helicopter takes off from a small ship, the
propwash from the rotor blades shoul push the ship deeper into the water
even when the helicopter is hovering at a significant height above the
ship.

However, I think this misses an essential question:  is there a force-pair
between the helicopter and the ship?  No.  In order for a force-pair to
arise, the air from the helicopter must bounce from the ship and then rise
up and produce a significant force upon the helicopter.  Helicopters don't
work like that.  Such things do occur with a hovercraft.  A hovercraft
flys by pushing against the earth and creating a force-pair between itself
and the earth, and the earth pushes upwards on the hovercraft.  Yes, the
air from a helicopter or an airplane will push against the earth, but the
earth does not return the favor.


John Denker offers an analogy for particle-exchange force interactions in
his critique of the Anderson/Eberhardt paper:  baseballs bouncing between
the earth and a horizontal board.  If we throw a baseball downwards so it
bounces from the earth, and then we place a horizontal board above it, the
baseball will rise up and bounce from the lower surface of the board and
fly downwards again.  For perfectly elastic collisions, the baseball could
keep bouncing forever.  As a result, there would be a net force-pair
between the board and the earth. The force keeps the board aloft.  In my
opinion this is not an example of a reaction engine because a reaction
engine can still hover even when the surface of the earth is not there.

Suppose I were to hover by throwing baseballs downwards.  There would be a
force-pair between my hand and the accelerating baseballs.  This
force-pair keeps me aloft.  If the baseballs strike the earth, then
there's another force-pair here as well, but the second force only serves
to stop the baseballs.  If I never catch any bouncing baseballs, then I am
not using the earth as part of my hovering process.

If the earth was suddenly replaced by a tiny dense blob of neutronium of
similar mass, so that its surface suddenly moved to a point thousands of
miles below me, I would never know it.  I would just keep throwing my
baseballs downwards, and remain hovering.  A hovercraft would instantly
know that the surface of the earth had vanished.


> The air
> undergoes so many other collisions that it's effects seems diminished
> to an infinite degree, but in fact this is the argument of why a jar
> with a fly in it weighs the same on average whether the fly is flying
> or not.

...but does our jar push upwards on the fly?  It does not, it only pushes
upwards on the fly's downdraft.  It obviously does halt the fly's
downdraft.  However, if there is no "back pressure" running upwards along
the columnar downdraft, then the fly doesn't experience an added upwards
force.  Rather than a jar, imagine using a large aquarium tank.  If the
fly is hovering in the center of the sealed tank, and if we suddenly make
the tank vanish, the fly won't suddenly experience an enormous change in
lifting force.  The fly's downdraft might be pushing on the bottom of the
tank, but this does not mean that the bottom of the tank is pushing
upwards on the fly.  In other words, a reaction motor is a very different
animal when compared to a ground-effect machine or to a balloon.
Airplanes aren't held aloft by the surface of the earth.  Airplane wings
are reaction engines.  (This is only true when they fly higher than a
wingspan or so.  Below that altitude they turn into GEMs, ground-effect
machines.)


2-dimensional simulations are indispensible for our understanding of
airfoils, but they have serious limits when used to explain
three-dimensional airplanes.  A 2-dimensional simulation depicts the
flight of a ground-effect machine, not the high-altitude flight of a
3-dimensional airplane.  Is this an important issue?  I think it is.  It
is important because we seem to be we taking sides and debating these two
concepts? :

Bernoulli-ist position:
     Airplanes fly because of pressure differences, and they
     need not deflect any air downwards.

Newton-ist postion:
     Airplanes fly entirely because they deflect air downwards, and the
     pressure-differences are a consequence of the changing
     momentum of the air.


If a wing flys by reaction, then the "Bernoulli-ist"  position is wrong.
This is not a matter of opinion, and the situation cannot be changed by
trying to redefine the meaning of "reaction engine."


Here's another very important issue.  If a wing remains aloft by reaction,
then John Denker's critique of the Anderson/Eberhardt paper contains some
serious errors and needs to be modified:

   Anderson/Eberhardt:
   HOW AIRPLANES FLY: A PHYSICAL DESCRIPTION OF LIFT
   http://www.aa.washington.edu/faculty/eberhardt/lift.htm

   John Denker's critique of the above:
   http://www.monmouth.com/~jsd/fly/lift.htm

This is not a small problem easily ignored.

If Drs. Anderson and Eberhardt made errors, then they need to face this
unpleasant fact and to correct their paper.  However if John Denker is the
one with the errors, then it is he who needs to face this fact and take
appropriate action.

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