Ken Powell wrote:

Paul,
I have been following the discussion of prop tip speeds but none of
the charts presented an answer that I could see to the following
question?  (I am a programmer and I am sometimes mathamatically
challenged!)
-- On a 200 MPH airplane (RVx), at what propeller RPM do the tips
enter the high drag transonic speed that we need to avoid?
1.  for a 68" prop
2.  for a 70" prop
3.  for a 72" prop

I also noticed last week that one of the charts seemed to present
evidence that prop diameter changes of this small magnitude were not
significant, but the results of real world tests seem to indicate
that they do.  What is the 'real' story here?
Please give me some guidance here; there doesn't seem to be an easy
answer.  I know that 'thinning' the tips will allow higher RPM, but
I would like the issue addressed for standard wood props like we
would buy from most of the prop makers.
Thanks,
Ken Powell

The tip speed is the vector sum of the tip tangential speed combined
with the forward speed of the airplane. I am a little tired at the moment
(been wailing on the 182 all day) so I'll do the math a little later unless
some one else would like to run the numbers. At high angles of
attack the mach number where things start to get nasty are as low
.65 mach. At low angles of attack things could still be reasonable
at .75 mach. This suggest that a bit more twist than is theoretically called
for may be beneficial in a prop. This is with a 66-210 airfoil
which has a thickness ten percent of the cord. A typical wood prop
may have an old fashion Clark Y airfoil perhaps 12 to 15% thick
as I recall. 

Forget it with wood props. They are not strong or stiff enough for very 
thin (less than 5%) high mach number state of the art props. If you want the 
last word in props you will need to go with carbon fiber or titanium.

Paul Lamar
 
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