Paul wrote:

Hi Don.

Big argument on our news letter.

Is a 72 inch dia 2500 RPM 225 HP prop feasible.
Cruise is 2100 RPM 180 HP.

Are we taking a big efficiency hit trying to do this?
If so how big?

Thanks. 

Paul Lamar

Opps forgot to say the airplane. RV6A optimized for cruise at 2100 RPM and
180 HP. Takeoff hit?

BTW have you seen this paper?

SAE paper on props. 1999-01-1581
Math Modeling of Propeller Geometry and Aerodynamics
J. Phillp Barnes Northrop.

Paul Lamar

"Donald R. Bates" wrote:

    Hi Paul:
72 inch sounds like a typical diameter for this power. If the blade is
properly optimized you should be able to get within 2-3% of peak
efficiency.If you just take something off-the-shelf, you could be close or
as much as 15-20% off peak efficiency if the match is poor. Efficiency is
slightly better as diameter increases until tip speed gets too high into the
transonic region.

Since this is a fairly low rpm cruise the pitch may be high and stay stalled
longer on takeoff and extend the run. You would have to just try it out to
know exactly how long.

Thanks for mentioning my programs. I'm not familiar with the paper you
mentioned. Is it any good? What method or theory is used?

Don Bates
Bates Engineering   http://www.innercite.com/~bateseng/

Thanks again Don.

My guess is a bit more cord out near the tips since they are going slower.

To quote the abstract; "A new implementation of the vortex step method".
Some pretty fancy three D graphics. $25! I'll bring my copy to S&F
so you can look at it.

Well folks there you have it. Find a prop builder and ask them if they 
have Don Bates's program and you will have 25 more HP NA to work with
if you have a 2.85:1 PSRU. 
 
Paul Lamar 

 
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