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

Backing off the angle does nothing to reduce the Mach number, it just
reduces
the lift, and in worse case takes it to negative lift. If you are not
getting
any lift out there just cut the D*** thing off and reduce the drag. A very
small
reduction out there can be benificial in reducing the induced drag tip
losses
but that would be a lot less than what was done to the LAEV blades. The mass
flow per inch is really getting great out there, and it does not make sense
to
excessively unload it.
Vance

What about the tip vortex? The tip vortex contains a lot of the energy
absorbed by the prop. Cl^2/(pi * A) It is really bad on a prop because of
the moment arm and speed of the tip. Also the tip segment drag is way reduced 
according to this plot from Von Doenoff. Again this increment of drag has a 
long moment arm.

I would like to see a plot of torque absorption as a function of radius.
I suspect the majority of it happens out near the tips.

BTW do you have a plot of angle of attack as a function of speed at some constant
RPM for the prop design you just uploaded?

Paul Lamar
 
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