The Kneeslider has covered a lot of motorcycle powered cars, but we've only briefly mentioned airplanes. After the recent flurry of interest in motorcycles with radial airplane engines, why not look at the flip side and examine airplanes with motorcycle engines? A note the other day from Bob Horn got me thinking about this when he sent me a link to SkyRay Industries, (formerly HogAir) a company building Harley Davidson V-twin conversion kits for homebuilt aircraft. Although at first glance, the Harley engine doesn't strike me as the best choice for an airplane, it may actually work. But motorcycle engines come in such a huge variety, some of them would appear to have a lot of things going for them in this application.
In the same way that motorcycle powered cars are nothing new, motorcycle engines have been used in airplanes for a long time. Ed Heath, founder of the Heath Airplane Company, sold a high wing monoplane back in the 1920s called the Heath Parasol powered by a Henderson motorcycle engine, among others. As a way to expand the market for the airplane, he began selling the Parasol complete or as multiple partial kits and you could even buy just the blueprints and build it as time and money permitted. (His company was later purchased and after WWII the kit idea was refocused on electronics and became the famous Heathkit Company, which sold electronic kits for many years.)
B.D. Maule, founder of the aircraft company today known as Maule Air, built his first plane back in 1931. The Maule M1, as it was called, was also powered by the Henderson motorcycle engine.
Those are just two of many early examples from a time when any semi reliable engine was fair game, buy why don't we see all sorts of motorcycle engines used in airplanes today?
If you have ever had an interest in homebuilt airplanes, the Experimental Aircraft Association (EAA) is a great organization you should look into. Their annual Oshkosh fly-in is the most amazing place to walk around and it's not surprising to see homebuilders experimenting with all sorts of different engines. Just about every automotive engine has been converted for aero use and a few motorcycle engines have made the jump as well, ... a few but not a lot. Given the creativity and ingenuity of homebuilders, you would think motorcycle engines, with their compact dimensions and high power, would be popping up everywhere, maybe there's a reason they're not.
Of all of the companies that could come up with an airplane engine based on their motorcycle engines, Honda would seem to be one. Not only do they have the engineering capabilities but some of their engines seem ready made. Take something out of a Gold Wing, make a tweak here and there and presto, airplane engine. Well, a few years ago, Honda was working on an engine with Teledyne, builders of the widely used Continental engines, and it had been installed in some airplanes and tested. Since that time, Honda has developed their HondaJet but the piston engine is nowhere to be found. Maybe the crossover isn't all that simple.
Motorcycle engines, though very high tech these days and capable of putting out huge horsepower, are not designed to operate like airplane engines. Airplanes can fly for hours on end at a steady 70 percent power, motorcycles usually operate far below that level with occasional bursts of high revs. Motorcycle engines have power but they're not designed to deliver it the way airplanes need it, not that they can't do it and with some gearing to get the revs right, they might do it. But problems with a motorcycle means you coast to the berm and check it out, problems with a plane means you look for a place to set her down, preferably close by and smooth. It's one area where engine choice might be a bit more critical and using an engine designed for that purpose might make a lot of sense. Again, homebuilders do a lot of great work and they may have just the right combination of skills to use something from a motorcycle and make it work reliably.
This is an interesting topic that deserves more attention.