Why is it that one could or could not use the cam spindle to drive the prop. That would give a 2:1 reduction without a seperate reduction drive assembly?

I've seen several small engines the size of lawn tractor engines that work that way. Not sure how much you would save after you increased the size of the cam gears bearings to handle the load.

Cory

I was thinking of a toothed belt. That would eliminate the associated pieces of the reduction unit and the duplication of the internal cam belt or gears.

The Continental Tierra http://www.flightglobal.com/airspace...away-5674.aspx used the timing gears as the reduction gearbox. The problem with the design was that the play in the gear train changed the timing relationship; it was enough to make the engine run erratic.

Interesting, how would the play be any different than another small 4-stroke, like the Ryobi I think, that uses the belt to run the cam. I was thinking of a stout belt like would normally be used in the reduction drive not a little flimsey one.

I made a quick sketch of my idea. See attached:

I think the 26cc Ryobi 4S uses a gear reduction for the cam drive vice a belt. I think you would have to strengthen the cam gear elements to make them strong enough for driving the prop. My Ariens sno blower takes the drive wheel power off of the cam gear; the impeller is driven off of the main shaft at full engine rpms.

Richard/Club Saito #635

Been done before, Daymo ?? Damo?? built flat twins with the cam driving the prop.

Damo, made in Sweden, they said it would run without oil in the gas...Good luck on that one...

I recently read the RCM report on the Damo, and it said the initial directions said no oil was needed, but i recall they revised that sometime later to recommend oil for lube.

Sincerely, Richard/Club Saito #635

RCV model engine from UK employs the idea, albeit it is a not a 100% conventional engine. If you look at the picture, you will see that the propeller is fastened on the rotating cylinder valve, which of course rotates at half the speed of the crankshaft, because we are having a 4-stroke engine.
Indeed, the RCV60-SP, which is .60 cu. in. can swing 14x14, 15x12, 16x12 2-bladed props, which are sizes of .80 and up for conventional engines!