cjcampbell wrote:
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.
For a fan in open air the momentum of the air moving through the
fan is equal and opposite to the momentum of the air moving around
the fan to replace the air removed from the front of the fan. There is
no net momentum change in the air. For ducted flow that returns the
air to the front fo the fan, the net momentum is also zero. Net flow
and
net momentum through any closed loop is zero--else the 'loop'
is not 'closed'.
Followjng a wing in level flight, the downward momentum of the
air in the downwash is equal and opposite to the upward momentum
of the air to either side that moves up to replace the air that washes
down. There is no net momentum change in the air.
...
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,
Cool! Got any links to some? How about pictures of airplanes
flying just below the ceiling?
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.
'Dissipation' is flow. If you include that dissipation into your
sum, there is no net flow. Otherwise, as stated above, the
ambient pressure at ground level would steadily increase and,
as you note below, the pressure higher up woudl steadily drop.
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.
Precisely my point. The downwash hypothesis sucks. It _is_
quite intuitive, it makes a lot of sense, but nature is not bound
by intuition or common sense.
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?
Are you saying that if there is net flow from one side of the room
to the other the pressure of both sides will stay the same?
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.
Precisely. There is no net flow.
--
FF