Ben Schneider wrote:

Paul...

    I am getting close to the point where I need to buy a prop. I am
thinking  MT electric constant speed (though nearly $10,000 UG!) I am
putting a Renesis in with Tracy's RD1C. My knowledge on props is where to
bolt them on, and not much more than that. Any suggestions. Any suggested
questions I need to ask the prop manufacturers at Oshkosh. Bruce
Turrentine has quoted the engine approx 225HP @7500. It is going on an
RV7. What else am I going to need to know? What brand of fixed would you
recommend if I went that route, and why?
   One final thought I had, put a fixed pitch on at first, then move into
a constant speed later on. Good idea, bad idea??

   Ben Schneider
    RV7 Renesis Rotary

Unless you are concerned about takeoff roll and climb rate, all that money
and weight for a variable pitch prop is a waste.  Top speed will be a few
knots better with a good fixed pitch than with an MT.  Variable pitch may
give you 25-30% shorter takeoff roll, and that can be important on a hot
day, high altitude and a grass strip, but consider that you can get a great
fixed pitch prop for about $1500 (two blade) that weighs about 15 lbs.

Until something lighter, less expensive, and reliable comes along if
variable pitch; I'll go with a Catto prop.  Photos of my new Catto are at
http://members.cox.net/alg3/Propeller.htm

Al

I do agree that it appears most fixed pitch props on experimental
aircraft do indeed have acceptable take off performance due to the extraordinary
power to weight advantage when compared to certified aircraft.
I also suspect the fixed pitch prop makers compromise a bit on top speed
and or cruise for a bit better takeoff performance.

Allan Tolle told me he always had three fixed pitch props to choose
from. Cruise, climb and race.

I think a CS prop can be more than just a take off and climb advantage. 
There is a possibility that the pitch can be increased at cruise to the 
point where take off would really suffer but MPG could be optimized. 
In other words a highly optimized fixed pitch cruise prop would give 
much worse takeoff performance.

The optimum situation for MPG is wide open throttle and minimum RPM
for what ever speed you want to cruise. This holds true for piston
engines as well as rotaries as engine friction and pumping losses are
major factors when it comes to BSFC and hence MPG. A CS prop allows 
you to adjust the pitch to optimize these factors. Much the same as 
overdrive does for cars.

Perhaps you need to tell the prop maker what RPM you want to cruise
and assume HP is a straight line function from 0 to 225 at 7500 RPM.
In other words .03 HP per engine rev WOT. 5000 RPM would then be about 150
HP and 1750 prop RPM. If my numbers are correct fuel burn when leaned
to 200 F lean of peak and .47 BSFC or about 11 gallons an hour.
He should then be able to tell you how fast you should be going
knowing the drag on an RV7. My ball park guess would be 175 MPH
or so.
 
Paul Lamar
 
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