What's the difference between heli and airplane engines of similar displacement? Can a heli engine be used successfully in an airplane? (I was looking at a YS heli engine (50) and wondering if it could replace an OS 55AX or 52FS in a small pattern plane.)

This is not an authoritative answer, but I think you'll find the Heli engines have larger heads to promote better cooling. The additional cooling area on the front of the plane may cause the engine tuning to be more difficult. This adds weight to the engine, so when put on a plane, it will be heavier to the nose. Not sure if the output shaft is different between an heli and a Plane.

Brad

It can be done. I have put an OS 50 Hyper on a Bobcat 50. The weight of the head will be a little more, but not that much. The biggest problem is that heli motors run richer than airplane motors. This is mostly a problem on idle to about 1/2 throttle. It will run very rich in that range. Other than that it will work just fine.