The prop program by Dokus and Dubner shows a 2.2% loss of efficeincy
between a 76.4" prop (at 2700 RPM) and a 64" prop (at 3225 RPM).
This would indicate that there is not much difference between the 2.17
ratio and the 2.85 ratio.  These numbers are for a tip speed of 900 ft
per second.  900 ft per second is 80% of Mach at sea level on a 'normal'
day (1114.4 fps).  This also explains why the canard guys go so fast
with small diameter props.

Ken Powell

Ken,

  What you describe above is not what Paul is advocating (as I understand
it) and so is not a relevant comparison. The relevant comparison is the
76.4 inch prop at 2700RPM for a tip speed of 900 ft/sec(using your baseline
for sake of argument) versus a 76.4" prop at 2100RPM for a tip speed of
700fpm (example from one of Paul's previous emails). In order to do this it
would be necessary to increase pitch, chord, number of blades, etc...
though I'm not sure which of these variables would be best to change.

Mike Wills

Correct Mike. Pitch and perhaps cord.
Paul Lamar

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

 
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