Re: [Phys-l] Bernoulli's Principle

[Brian Whatcott <betwys1@sbcglobal.net>]

At 02:00 PM 12/21/2007, you wrote:
> Rick Swanson wrote:
>
> > 2.  If something interacts with the air and goes up, some air must go
> > down.  It's a conservation of momentum thing.
>
> What do you mean by "some air"?  If "some air" goes down and "some air"
> goes up, such that the air, on the whole, is at the same level, does
> that counter conservation of momentum?

A layer of air ahead of a lifting wing rises in front of the leading edge
and  curves over the wing, departing down past the trailing edge, a
net mass flow downwards. A lower layer of air rises before the leading
edge but does not make the cut, and departs more slowly under the
 wing and downwards contributing about a third as much mass times
acceleration.     The flow is less smooth when the asymmetrical airfoil
is upside down, so that its angle of attack needs to be greater, leading
to a slightly exaggerated tail low impression on inverted fly pasts,
with greater power expended to maintain speed.
The effective center of lift is well forward of the wing's center line
so that the wing tends to rotate in an unstable sense. If the aircraft
 is reasonably loaded, a pitch displacement of this type is countered
by an opposing moment from the tail section, so that dynamic stability
is maintained. At the wing tips, the comparatively fast moving, less
dense layer above is invaded by the compatively denser lower layer
beneath the wing, This provides a rotating impetus to the upper and
 lower layers which spin into vortices.


> One thing that bothers me about the "air must be projected downward" is
> illustrated when an airplane flies horizontally.  What is the difference
> between an airplane flying horizontally and simply taxiing horizontally
> on the runway?  Is mass being projected downward in each case?  And what
> if the plane was "flying" in water?  Is the water projected downward,
> and what would that look like to someone observing the water surface?

The fluid though which the plane flies behaves in much the same way
 whether liquid or gas, until speeds at which the gas can be compressed.
The wing lift which is proportional to the fluid's acceleration in the
downward sense, is proportional to air (fluid) speed squared, so it is
 low at low speed, so that the gear  bears most weight at taxi speeds.



> I think we need to keep in mind that the Earth is there.  And, as John
> Denker so elegantly pointed out, air is a fluid.
>
> Air exerts a force when there is a difference in pressure within the
> fluid.  For certain situations, a difference in pressure is *consistent*
> with differences in speed or a circulation.  However, in my mind, that
> does mean that differences in speed or circulations *cause* the pressure
> difference or can be used as "explanations" for the pressure difference.
> This is like saying that, due to a=F/m, an acceleration *causes* or
> *explains* the force.  All it means is that one is consistent with the
> other, right?


I must say, this para finds a responsive note from me at least.
 I rebel to read what physical effects *MUST* do because of
 some model formulation. (Circulation is a model of the fluid
dynamics in this view)


> Suppose you hold a wood board out the car window while driving down the
> road.  If the board is oriented with its plane oriented vertically, what
> "causes" the pressure on the leading side to be greater than the
> pressure on the trailing side?  Is your explanation of this much
> different when the plane of the board is at an angle?  If so, why?  This
> is where I am confused.

Pressure varies across flat and curved surfaces. Where the leading
layer of approaching fluid stagnates on the perpendicular surface,
 then accelerates to both sides to escape turbulently to the rear,
there is a loss of kinetic energy in the flow as seen from the board's
perspective.     This can be treated as high drag and low lift
 condition in a very high angle of attack surface.

> ----------------------------------------------------------
> Robert A. Cohen,

I hope this arm-waving response may help a little.



Brian Whatcott    Altus OK    Eureka!