Re: angular momentum,


The Thing wrote:
On Mon, 04 Dec 2006 19:48:57 -0800, jmorriss@xxxxxxxxxxx wrote:


The Thing wrote:
On Mon, 04 Dec 2006 06:35:49 -0800, jmorriss@xxxxxxxxxxx wrote:


The Thing wrote:
On Sun, 03 Dec 2006 21:15:26 -0800, jmorriss@xxxxxxxxxxx wrote:


The Thing wrote:
On Fri, 01 Dec 2006 14:29:50 -0800, Fallingeagle wrote:

a rotor on a helicopter is horisontal, and a little one on the back is
verical... how do they effect the motion of the helicopter and why?

If you take a close look at a cross section of the rotor blades
you see that they are shaped like airplane wings, bulging upward
on the top (for the big rotor) and flat on the bottom. Helicopters
go up because the rapidly moving air across the rotor surface
drops the air pressure on top of the rotor relative to the air
pressure on the bottom. This is because the air on the top surface
of the rotor moves faster relative to the rotor surface than the
air on the bottom and pressure is related to these speeds. This
is the very same reasons airplanes fly.

Or, as an alternate interpretation...

Helicopters and airplanes fly because they both cause air to be
directed downward, and by Newton's Third law, if the wing/rotor exerts
a downward force on the air, the air exerts an upward force on the
wing/rotor. The cross-section of the wing/rotor has more to do with
accomplishing this redirection of air as economically as possible...

Same could be said of airplane wings. One could make them flat for
example and use angle of attack to achieve the desired effect. In
the end the upward force is due to the pressure difference on the
rotor blade. I'm not certain there is much of a downward wind
developed by an airplane wing in level flight. Applying Newton's
law to the downward moving air would crash most aircraft

Huh? Newton's Third Law doesn't apply to the forces acting on a wing
and the air moving over it? Look up "downwash"...

Huh? Did I say Newton's Third Law was somehow invalid? Are you
saying that the pressure differential isn't what's keeping the
plains in the air? Look up "forces"....

Sorry... I took your quote:


"Applying Newton's law to the downward moving air would crash most
aircraft..."

to mean that Newton's Third Law didn't apply.

No, the way you suggest using it might not be the full
story though.


An airplane is kept in the air as long as the product of the mass of
air deflected downward by the wing and/or engines each second, times
the air's downward velocity is equal to the weight of the plane.
Action - reaction.

Add up the work being done moving all that air and ask yourself
how a glider manages to stay up so long? No way is the initial
impulse given to the glider going to last more than a few seconds
at that rate. I believe there are more (or different) mechanisms
at play here.

Gliders <DON'T> stay up. They are constantly dropping. Think "glide
ratio". They stay up <for a while> because of very efficient large
wings, and because of updrafts sought out by the pilot They can
briefly climb by swapping kinetic energy for potentila energy.


Why does the air moving across the top of the wing move faster than the
sir moving across the bottom of the wing?\

The answer is to get to the other side in the same time it takes
the air traveling beneath the wing. Shape for a wing isn't everything


Why would the air need to get to the other side at the sane time as its
previous neighbour? Evidence is that over the top air gets to the
trailing edge <before> the neignbouring air that went under the wing.
See the Wickpedia article on Lift(force) or the references therein:

http://user.uni-frankfurt.de/~weltner/Flight/PHYSIC4.htm

and

http://www.av8n.com/how/htm/airfoils.html



One thing I've learned about engineers is they
don't do *** if it doesn't improve performance. Why make all those
funny wing shapes if flat would do?


Some funny shaped wings <are> flat and do quite well; some are
symetrical in cross section. Some <change> cross-section in different
flight regimes, with flaps and slats. Some planes can fly for extended
periods of time inverted.

Despite the dirty details of how the air and the wing interact, and
they can get very dirty indeed, Newton's Third Law always holds:
Multiply the mass of air diverted downwards each second by its downward
velocity and you get the lift This isn't in addition to the Bernoulli
Lift; this <is> the Bernoulli Lift!

.

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