Richard White wrote:

Paul is right about my geometric calculation, as I assumed
local airflow is zero knots.  I guess I should have been more
clear that the numbers I gave above are for an engine on a
test stand at sea level.  I don't believe that the airflow can be
decelerated any slower than standing still.
[he says, trying to inject a little humor here]

I do want to say one little thing here. 

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
 
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