AviationBanter - View Single Post - The Impossibility of Flying Heavy Aircraft Without Training

David CL Francis

On Fri, 3 Mar 2006 at 05:30:06 in message
.com,
wrote:

Newton had three laws of motion, you're ignoring the first.
Is there a net change inmomentum of the fan? If not,
how can there be a net change of momentum of the air?

I am ignoring nothing. The above statement is wrong. You agree below
that energy is put into the air. In the case of a fan that energy goes
into increasing the velocity of the air. The rate of change of momentum
(mass flow times velocity increase) produces forces that increase the
momentum of the air. Energy changes momentum. Momentum destroyed turns
back into energy.

This argument is hung up on the idea that the air returns to a steady
state eventually - which it does! But not quite back to where it was
because of losses Nevertheless energy is lost and replaced by the
engines of the aircraft.

There no question that energy is put into the air. There is
no net change in momentum, of the air. otherwise all the
air would pile up on one side of the fan and there would
be a vacuum on the inlet side. Air moving through the
fan in one direction is offset by air moving around the fan
in the other direction.

The air slows down and looses energy and momentum far away from the
aircraft - so what? Any small drop in pressure at the fan also reaches
back and develops flow some way in front of the fan. For lift purposes
it does not matter much. The air may or may not make its way back to the
inlet again, some of it will.

In open air the volume of air moving around the fan is larger,
but moving at a lower speed than the air moving through the
fan so that the momenta of the flow in either direction is equal
magnitude and opposite in direction to the flow in the other
direction.

Except for losses that occur due to friction and eddies that float away
to dissipate themselves elsewhere. But the answer must be still be so
what? Momentum is not conserved because energy has been added. Are you
saying that a land vehicle with a horizontal fan to drive it along rails
will not accelerate and move? Will the vehicle not build up momentum
because of this?
--
David CL Francis