Excellent answer A, in cruise flight, there is little difference between a
propeller's slipstream and freestream velocity.

Today's props are very efficient (near 90% max) and create little
turbulence, with the exception of the tip's vortex . I have opened the
window on a c-150 and put my arm out to feel the propwash, and other than a
slight pulsing the only substantial disturbance is where my hand penetrates
the prop's tip vortex ring. Try it some time. This is not to say the flow
across a fuselage behind a prop is laminar, I'm nearly certain it isn't as
laminar boundary layers needs very little disturbance to become turbulent,
but total chaos does not exist within the slipstream behind a prop.

The stagnation point can also be defined as the point on the leading edge of
a surface/component that the velocity normal (perpendicular) to the skin is
0.

The Meredith effect, and ramjets in general, create stream flow by
maintaining a negative pressure gradient (high to low pressure) streamwise
within the duct/engine. This gradient is created by area differentials,
fixed or variable, between the inlet and exhaust. The inlet area is smaller
than the exhaust area, and thrust is a product of the duct's massflow times
the inlet's and exhaust's differential velocities. In the case of a cooling
system, if the internal aero losses are to high, due to friction across
radiators, etc., then the net thrust may be negative.

RJ



wrote in message ...
You'd be surprised how *little* a prop accellerates the air. The thrust
is a
product of how much air is accellerated and the change in velocity. A
prop
going 150 mph is working on a lot of cubic feet of air per second - a cyl
of air
the diameter of the prop and the distance the airplane goes in a second.
Changing the velocity of that amount of air just 10 mph gives oodles of
thrust.
The back prop on a 337 is seeing air that's moving backward at about 15
mph.

The slower the airplane is going, the more it has to accellerate the air
to get
the same thrust, because it's accellerating less air.

The stagnation point is the point on the leading edge of the wing where
the air
is splitting - some goes over the wing, some goes under the wing. Above
the
stagnation point the air goes over, below, under. AT the stagnation point
(which is very, very small) it piles up then falls off one way or the
other.


In article ,
says...

I'm not clear on what the stagnation point is. Maybe you can expand
you explanation more?

My line of thinking was basically this:
Airplane flies in clean air, propeller bites into clean air at high
sub sonic speeds, creates column of high velocity air, but at the same
time, wastes bunches of energy also churning it up. As air moves back
past fuselage and wing roots, churning energy disipates (decaying
exponentially with distance from prop). Now I'm sure that the
airframe itself introduces some turbulence of its own, but this is
distributed all over and is less concentrated at its source(s)

I think a lot of people are thinking about airplanes minus the
propeller or thinking of pusher configurations. This may be one of
the reasons why pusher power plants seem often have over-heating
issues. Taking the 337 for example: On the surface you'd say, the
back of the airplane must be traveling at the same speed as the front
of the aiplane so what's the deal? But going to the second order
effects, the front engine lives in an air flow with speeds much higher
than IAS and the turbulence of the cooling stream for the front engine
is MUCH higher than it is the intake scoop for the rear engine because
of the aformentioned exponential decay of the turbulence in the prop
wash.

On the Meredith Effect. Intuitively I don't see how its supposed to
work. You take air in the front of the scoop, you heat it up, and it
expands (ideal gas law). How does the air know thats only supposed to
go out the rear?

Regards

"RJ Cook" wrote in message
...
Jay,

The turbulence within a boundary layer over an aerodynamic surface
INCREASES
with distance DOWNSTREAM of the stagnation point. The cowl of an
aircraft
would have less turbulence within the boundary layer than nearly
anywhere
else on the fuselage.

RJ