Jeff Spitzer wrote:

At 01:03 PM 5/26/99 -0700, you wrote:

Perhaps it can!

Perhaps the fan drags the air around with it.
Perhaps this happens more or less up to rather high
forward speeds.

Yes, the fan tips are going at what would be supersonic but
the mach number is still below 1 because the air is going in
the same direction.

Air has some viscosity. It is like water going down the
drain in your bathtub to a certain extent. There is a vortex
in the water before it gets to the pipe.

This is because the tip velocity is so much higher than
the forward velocity.

Everybody assumes the air molecules travel in a stright
line until they get to the fan or prop blade but I suspect
that is wrong.

BTW this is perhaps why bumble bees don't fly.
Perhaps they swim. :-)

Paul Lamar

I have looked at several prop and fan design programs, including the one by
Drela, and there is no consideration of pre-rotation.  I don't believe it
exists.  Your viscosity reasoning ignores the significant influence of the
"disk" which essentially insulates the upstream from the downstream.  There
is some local upwash just as in a wing but that would actually make the
local velocity higher.

BTW, bumble bees (and many other insects) fly by the use of unsteady
aerodynamics and a process called something like "pull and squirt".  The
ornithopter/biomechanics guys have a name for it.  The wings start the
stroke at the top with the tops of the wings together.  By pulling them
apart rapidly, a vacuum is formed between them and over the body.  The
wings may or may not make thrust on the down stroke depending on the bug.

The majority of thrust is produced as the wings approach each other again
on the upstroke.  The wings twist so that the leading edges meet first.  As
the wings bang together (literally) the air is expelled to the rear as the
trailing edges meet.  There is no corrolation to steady flow aero.  Where
did you think the buzzing came from?  BTW, this works much better at low
reynolds numbers.

Jeff Spitzer

That's really interesting stuff Jeff. Perhaps somebody will build
a 100 X scale rotary powered bumble bee to buzz around in. :-)

Thanks for sharing that with us. I have been wondering about that
for the past 55 years.

Paul Lamar
 
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