Two questions. 1. I have 2 os 91 four strokes. Marvelous engines. I am putting one in a SIG King Cobra, and ground clearnce is a problem. I presently run a APC 14-7 at about 9400 on 15%. Would a APC 12-12 size give me about the same performance? Or would a 3 bladed prop be the answer? Twelve inches is about the max clearance I have. What is the difference between a OS 32SX, and a OS32SX-H? I know the last one is a helo engine. Is there any problems using the helo engine in a sport plane? I can get a good deal from my LHS. Thanks for your help in this matter.

For the .91, an 12 x 9 or 10 may work, maybe even the 12 x 12, you'd have to try and see. It may load the engine too much. For a 3-blader, we'd look at a 12 x 8 or so, if you can find one. Otherwise a 13 x 6 or 7. Not easy to find. You'd be better off taking a 14-inch 2-blade prop with an 8-inch pitch and cutting it down. The blades will be wider and will offer more load to the engine.

The O.S. engines with the "H" designation mean that it's a heli engine. This means it's usually ported for higher RPM and has a very large heat-sink head. It may over-cool in an airplane installation. It may not accept a propeller, because the prop shaft may not be long enough. It was intended to receive a flywheel/clutch assembly for a heli.