On Sat, Aug 21, 2010 at 7:34 AM, Jeff Whaley <jwhaley@datacast.com> wrote:
> What is the Rotaries best prop orientation at the prop flange relative to TDC?
> The Lycomings are typically entering the compression stroke at 11-10 o'clock and 5-4 o'clock.
> The 13B goes through 3 compression strokes per e-shaft revolution - does this predicate a 3-bladed prop is the ideal match?
> Jeff

The orientation idea comes from the older 4 cylinder airplane engines that had to be hand started. So the position of the blade made a difference in that the person starting the engine needed a good body position to start out. It was also a good idea, because in the event of an engine stoppage, that very same position had the prop near horizontal, and that kept a blade from digging in on landing.

 

In the rotary powered craft, the engine will be at TDC twice per crank shaft rotation (for a two rotor) so, 2 times the reduction ratio tells you how many times it gets to TDC per propeller revolution. 3 times for a 3 rotor, and 4 times for a 4 rotor.

 

A 2 rotor with a 2.78:1 reduction unit will see TDC 2 X 2.78 = 5.56 or about every 65 degrees of prop rotation. 

 

This suggests that hand propping might be more difficult because of the mechanical disadvantage of the reduction unit, and should the engine fire, the length of time available to remove your hand might be very short, should the very next rotor face fire as well.

 

Lynn E. Hanover