Ken responded:

Yes, I understand that.  I was just stating that there is not a lot of
difference in prop efficiency  between prop diamenters of 64" and 76",

so it

doesn't matter a whole lot if you run a 2.17 or a 2.85 redrive.  The

important

variable here is the airfoil section of the prop needs to be efficient

up to

about .85 mach.  IIRC, Bill F. stated that a  Clark-Y is awful above

about .75,

as are the 2000 series.  BTW, I do not think that you can turn a prop

with a

Clark-Y or 2000 series airfoil 900 FPS at 12000" without running into

the drag

problems assosiated with tip speeds approaching mach.  Does anyone know

what the

speed of sound is at 12000 ft and say 30 degrees F.

Ken Powell

Yvon Cournoyer wrote:

Is this any help?
Yvon

I said, and still believe all of the stuff above about the high Mach losses.
That said, when you devalue the L/D at the tips(for high M) and run the
whole blade,
you often wind up with a lot more flexible (i.e. wider useful speed range,
more thrust
under a wider range of conditions)  prop with a 24XX (not a lot different than
a Clark Y, much more data available) than with the 16 series.  I think it would
be a good Phase 2 prop to have the outer couple of stations use the 16 series.
These foils are specifically designed for transonic speeds, such as prop tips.
I think we might pick up a few pounds thrust and might also be a bit quieter
at cruise.   I spent a lot of time writing the equations for my sim program and
believe the results are reasonable, go up when they should, down when they
should, smoothly transition over speed, altitude, horsepower and RPM, and cover
a *great* many effects with actual wind tunnel data,   BUT
it has never been actually tested with a real prop.

I've been writing sims for over 25 years and mostly they are pretty good, but
sometimes things get away from you.    SO - we'll see, but I really do think
the design will be pretty good "out of the box".  Might be wrong, tho.

Bill

 
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