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

cjcampbell

wrote:
Dylan Smith wrote:
On 2006-02-24, Greg Esres wrote:
There is a *net* downward momentum of air.

I have several aerodynamics books that say differently.

I guess that depends on what you mean by "net" downward movement. Air
does move downward from an airfoil. There is no difference between a
fan blade and wing.

Otherwise there is no lift.

If there is a pressure difference between the top and bottom, you will
have lift. Your airfoil is blisssfully unaware of the air with which
it has no contact.

Define 'contact' and 'aware.'

But air acts as a fluid. The airfoil certainly DOES have an effect on
air that it has no contact.
If you think there is no downward movement of air from an airfoil, stand
underneath a hovering helicopter some day. Or behind the propellor of a
plane - the prop is also an airfoil.

You might be able to get lift out of an airfoil in an enclosed tube with
no downward movement of the air, but that won't happen in the real
world.

In the real world airplanes have flown with pressure sensors
on the wings, confirming lift from the Bernojuli effect in actual
flight.

In the real world there are many photographs of huge canyons carved in
layers of cloud and smoke as airplanes fly over them, as well as
photographs of ripples and spray in water below them. The downward
deflection of air is caused by the low pressure area above the wing, so
of course the Bernoulli effect is confirmed. The downward flow of air
is predicted by Bernoulli.

This does NOT disprove the notion that there is localized downward
flow from some parts of the aircraft. However, there is no NET flow
of air down or up from airplane wings or helicopter blades. Otherwise,

ambient pressure at ground level would steadily increase as more
and more aircraft pushed the air down...

No it would not, once the aircraft was out of ground effect. The
downward flow dissipates rapidly after the aircraft has passed.
Otherwise you could say that all the air is being sucked out of the
space above airplanes and nothing is moving in to replace it, so that
eventually everything above heavily travelled altitudes will become a
vacuum. Are you saying that a fan will eventually increase the ambient
pressure on one side of the room and leave a vacuum on the other side?
It would make half of my living room kind of uncomfortable, wouldn't
it? Air moves in from the sides and quickly equalizes the air pressure.