Ben Schneider wrote:

    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??

Paul Lamar responded:

A constant speed prop will cover a lot of sins :) As I recall the RD1C
is 2.85:1 is that correct?. If so 7500 RPM is doable. Good idea asking
the prop people at OSH. Just say max RPM is 2600, 220 HP and type of
airplane. You don't need to go through all the rotary and gear box

stuff.

You might want to mention "no significant torque excursions and no
torque reversals whatsoever."  Some of the props cannot live on 4 banger
Lycoming/Continental engines but would be fine on a Wankel (even without
the PSRU).

Russell Kent

With these steep reductions you are backing into some rather large props,
and steep
pitch numbers. With the RV7 the flight drag should be about the same as the
Cruiser 4 place ie with 220hp you should get about 210 mph sl max. Putting
a 68 inch dia [in] my program suggests about a 90 inch pitch (chord line) blade
widths
and angles about as below
blade width 30% rad  8.5 in   angle  54.5
                    50% rad 8.5 in    angle  48.4
                    70% rad 6.5 in    angle  31.4
                    90% rad  5.5 in   angle  25.2 (backing off angle for
mach is BS)
Top and cruise are virtually the same prop and 70 % power should be about
2300 rpm and just under 190 TAS
Vance

"(backing off angle for mach is BS)" Why? Don't just hit and run :)

Paul Lamar
 
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