Just to follow on with what shupack said, there can be a surprising change in prop speed due to the torque variations over each cycle. But a picture is worth a thousand words. I have a graph at http://members.coolcats.net.au/~scep...stant_rpm.html (courtesy of Brett Buck) which shows the calculated RPM at all points of rotation. The graph of course is specific to that particular engine and it's method of operation and the prop used was a 12x5 wood which would give a greater variation in revs than a heavier prop. The link at the top of the graph goes to another similar graph showing the calculated torque (both positive and negative).