In article .com,
wrote:

In order to lift an object by moving air, you need to create enough
force. Force is equal to a change in momentum with respect to time. That
is, you can think of force as being equal to changing the momentum of a
constant mass at a constant rate of acceleration (F = ma), *or* you can
think of it as applying a constant speed change to a flow of mass (F =
m/s * v). But as long as multiplying the two together gives you the same
total force, it does matter from a momentum perspective.

Alan:
Does this mean that a helicopter in hover is continuously pushing
down a mass of air equal to the weight of the aircraft?

No. It means that it is continuously changing the velocity of a mass
flow of air.

Let's say the helicopter has a mass of 1000 kg. To hover, it requires a
force of 9800 Newtons be exerted upward on it. Therefore, the rotors
must exert a force of 9800 Newtons downward on the air. If we assume to
speed at which it makes the air flow -- say 50 meters per second -- then
we can calculate the mass flow involved.

F = M(ass)/s * v; or M/s = F/v = 9800/50 = 196 kg/s.

So if the helicopter is moving the air downward at 50 meters per second,
then the amount of mass to which it must impart that velocity is 196
kg/s. If it only moves the air downward at 10 meters per second, then it
must be moving 980 kg/s at that speed.

Every one knows "F = ma", but few realize that "F = m/s * v" is equally
valid. The same force that will accelerate a fixed mass at a give rate
can also change a mass flow's velocity by a fixed amount.

The corollary
is that a rotary wing or fixed wing aircraft in unaccelerated flight is
displacing a mass of air equal to its weight, i.e., it is not flying
because of low pressure above the wing but because of the upward force
on the airfoil from the displacement of air downward. True?

It's not an either/or situation. Both are true. It flies because of the
low pressure and the low pressure (among other things) directs air
downward.

Russell Thorstenberg
Houston, Texas

--
Alan Baker
Vancouver, British Columbia
"If you raise the ceiling 4 feet, move the fireplace from that wall
to that wall, you'll still only get the full stereophonic effect
if you sit in the bottom of that cupboard."