Can heli engines be converted for airplane use?

In my experience I'd say not.

Heli engines have a bigger head (heat sink) and are designed to cool without propwash so would never get to the correct operating temp. Unless you had in hidden in a cowling perhaps but the short shaft on a heli engine may make this difficult.

Heli engines have a higher rpm too and produce more torque. I think they are more like a nitro car engine than a plane engine.

Finally, why the hell would you want to?

Yes they can. I've seen it done. I never saw the point, but it's been done. Heli engines do have a bigger head, but they also have their own version of propwash in the form of a cooling fan, so there won't be a problem there. Heli engines are usually more aggressively ported that airplane engines, but it's not a huge difference. Which engine you are talking about will determine whether there's a difference in the porting or not. If you stick with smaller props it will work fine. The difficulty is in the prop shaft. If a prop will go on and it's long enough to get a prop washer, nut and spinner on, then you're good.

So, Carl, crash your first airplane and now your an expert????? Ha ha. Yea, I have to agree with what you said. I'm not really familiar with heli engines, but I would think they wouldn't adapt well to planes. That's why they are heli engines, just like engines for cars. Stick with the engines designed for planes.

Thanks to all for the quick responses. I did not realize the differences between heli and airplane engines.

The actual differences are not that much. It's not commonly done to put heli engines on planes because heli engines cost more. The carburetor setup on some of them (not all) has a finer midrange adjustment because heli engines spend so much time at part throttle. That adds some cost, along with the bigger cooling head. The benefit of a heli engine is that they are made to run well on 30% fuel and they are ported to top out around 18,000 rpm so the power to weight ratio is better if you're looking for top performance in a speedy plane. Most plank flyers just move up to the next displacement for that, but using a heli engine is an option to get some more power without adding more weight.

Of course it can be and is done on occasion. I know of two flying in our club now in airplanes. It is a big pain in the behind fitting some sort of prop drive bushing that may involve some machineing and that's the reason you don't see it too often.

The cooling head is no problem to functionality simply because as noted by Jester but it is big and ugly, not likely to fit any cowlings.

Also as noted by Jester the generally aggressive port timing will require shorter but courser propellors than what we normally would use except perhaps pylon racers.

John