Bill Garrett wrote:

Paul wrote:

Vince Welch wrote:

Paul,

There has been quite a bit of discussion on the RV-list lately about proper
prop installation.  It seems that the general consensus is that the blades
on a two bladed prop should be at 2 and 8 O'clock when the number one
cylinder is at TDC.  If this is true, then how should the prop be mounted
when using a rotary engine?

Vince Welch
RV-8A

The proplem with that Vince is our PSRU's are not exact integer numbers
like 2:1 or 3:1. The PSRU is 2.17:1 so however you index the prop
it will be there some of the time but I think it will have to
rotate 31 times before it gets lined up exactly again.
I could be wrong on that number. It is something like that.

On a direct drive piston engine this is not the case of course.

How is the airplane coming? Long time no hear from. Got any construction
pictures?

Paul Lamar

Paul-

I calculate that with a 2.17:1 PSRU it actually takes 300 prop rotations for a
rotor to be in exactly the same place again as it previously was!

My calculations are as follows:
Since the rotors turn only 1/3 the speed of the driveshaft, multiply the
driveshaft speed by 1/3.
Then the rotor:prop ratio becomes (2.17/3):1 or 2.17:3 or 217:300 which is the
ratio in lowest terms.  So every time the prop turns 300 times each rotor makes
exactly 217 revolutions and these are the only times both prop and rotor will be
in the same relative positions.

The important thing in all this IMO is that those rotors are always humming away
inside the engine at only about 72% the rotational speed of the prop!

Your newsletter is great!  I've been a lurker for a long time (plan to put a 13B
into an RV-6A) but as a mathematician couldn't resist doing these calculations.

 Bill

Did you consider each rotor fires each time a rotor face is in alignment with
the spark plugs? :-) Also the engine fires twice per rev of the e-shaft.
BTW I am not exactly sure about that 2.17:1 ratio. Could be 2.173:1 
to mess things up :-)

Interesting stuff. Thanks Bill 

Paul Lamar
 
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