Ken Powell wrote:

I can't resist - I just have to throw my $.02 into the debate.
The ideal ratio would be 2.5 to 1, but since this is not
available we are forced into choosing between 2 ratios that are
not optimal.
   I maintain that the Ross box in the 2.85 ratio needs a thrust
bearing for the input shaft and no one has figured out how to do
this yet so this is not an option for many of us.  Now if Tracy
were building the 2.85 ratio I would at least take a better look
at this option.
  That leaves only the 2.17 ratio:  a.)  Tracy's box has proven
to be reliable (the Ross drive in this ratio also needs the
thrust bearing modification)  b.)  there is a lot of prop data
available because of the direct drive VW's that turn up to about
3600 RPM. We need to turn a prop about 3226 RPM (7000 rpm /
2.17).
  Using the prop program by Ed Docus and Joe Dubner gives the
following results: (max tip speed = 900 ft per second)

1.)  3225 RPM / 200 mph =  diameter = 64"
                           efficiency = 88.3 %

2.)  2700 RPM / 200 mph = diameter = 76.4 "
                          efficiency = 90.5 %

I vote for the short prop along with the 2.17 ratio.  You can't
use a 76" prop on a RV-4 anyway!

Question for Bill Freeman or Vance - What airfoil should a prop
of this description use (900 ft per second tip speed)?  A
Clark-Y appears to be to 'draggy' at these higher tip speeds.

For those of you who don't remember, I am building a RV-4 with a
PP / NA motor.  I plan to trun it 7000 rpm on takeoff and cruise
at 6000 rpm.

Ken Powell

 
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