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? 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?
Russell Thorstenberg
Houston, Texas