Motorcycle Engine Powered Airplanes

Heath Parasol

Skyray Industries Harley Davidson airplane engineThe 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?

BMW 1150 airplane engine installationIf 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.


  1. says

    I’d like to see someone take a run-out Lycoming or Continental flat-four, cut it in half, put new jugs on it, and put it on a motorcycle. It’d be like a BMW boxer twin, but infinitely cooler (and far more practical than putting a radial engine in a bike).

    Unfortunately, it would probably be godawful expensive. The engines alone are upwards of $15,000 new, and it’d require a lot of extra work to make a bike engine.

    Back on the subject — the really difficult part about making an automotive engine work in an airplane is durability. Most aircraft engines are designed to be run at 75 percent power all day, every day, for 2000 hours or more between rebuilds. Most automotive-style engines are built to run at about 30-40 percent power for continuous duty, or for much shorter duty cycles at higher power output. Racing engines can (usually) handle essentially 100 percent power output for the course of 24 hours or so, but then they need a complete rebuild.

    Running an auto engine at a low power setting is certainly feasible — there’s a very popular conversion for the Republic Seabee that replaces the stock engine with a GM LS6 (IIRC; I know it’s a ‘Vette engine) and the people who’ve done it find reliability to be fine — but you do lose a LOT of power by doing that. A typical aero engine rated for 150 BHP will happily run all day at 120 BHP (80% power) for 2000 hours between overhauls. If you did the same thing with your average 150-HP motorcycle engine, you’d have to rebuild it after 200 hours. You could run it at 30% power, but then you’d need an engine rated for 400 BHP to get the same 120 BHP continuous power output, and that’s simply not practical in most cases.

    Then there’s the whole weight thing. Aero engines tend to be made out of aluminium, and motorcycle engines — especially the twins — tend to use lots of iron and steel. This makes their power-to-weight ratio fairly poor.


  2. aaron says

    at least the harley engined plane doesn’t have to worry about the engine spraying oil on it’s tires…. truly an odd choice when the requirements are power, weight, and reliability.

  3. Bob Horn says

    My 1988 Evo engine unit weighed 120 lbs, but I wouldn’t have used that for flying. I don’t know what the newer counterbalanced twins weigh. Either way, the heavy flywheel does wonders for the prop drive reliability – the biggest headache in doing aviation engine conversions.

    The latest counterbalanced one sounds like a much better aircraft engine than a VW, Ford Model A (as used in the Pietenpol plans), etc….

    The only steel parts are pretty much the same parts as any other motorcycle engine – except there’s no primary or transmission to worry about. It’s a dry sump already – add EFI and go aerobatic. Cooling should be a, uh, breeze. I think it would go a lot longer than 200 hours between overhauls. The average hours/year for the sport pilot are rather low.

    If power & weight were the primary characteristics of an aviation powerplant, the 2 stroke engine or pulse jet would be perfect. They aren’t.

  4. sfan says

    FYI… some here may already know that BMW was originally founded as an aircraft engine manufacturer and its circular blue & white logo is said to symbolize a white propeller rotating with blue sky in the background.

  5. says

    A pulsejet is an impractical powerplant for most purposes because of its fuel consumption; in order to get a reasonable run time, you have to carry so much fuel that the power-to-weight benefits are completely negated. Furthermore, modern home-built pulsejets are typically run off propane, and carrying a large bottle of highly flammable gas through the air at 100 MPH isn’t the safest thing going.

    Two-strokes have been used, although they’re not particularly popular any more. I suspect this is largely because two-stroke engines of appropriate power ratings are becoming increasingly difficult to find. Bear in mind that the homebuilt aviation crowd is, by and large, not interested in developing an engine from scratch, but rather re-using available parts. A purpose-built two-stroke would have some key benefits, but the sad reality is that it doesn’t exist.


  6. says

    Greg, that’s exactly what I was thinking with my first comment. I think an airplane engine in a bike would be awesome. This was mentioned the last time The Kneeslider talked about airplane engines in bikes, but it’s worth pointing out again: several years back, a guy built a bike around two cylinders from a Rolls-Royce Merlin V12.


  7. john says

    Propane isn’t that dangerous, it dissipates readily which can’t be said of gasoline or aviation fuel. Lots of 4wd rock crawlers run propane, and often end up on their sides or upside down. The safety of propane is one reason it’s used, being able to keep the engine running at extreme angles is another. Tanks made for motor vehicles on the other hand are quite heavy and way overbuilt to survive a crash.

    People under estimate the safety of gasoline due to familiarity.

    Pulsejets are supposed to be very finicky to keep them running, which if true I imagine is a big drawback on takeoff. Anyone actually built a pulsejet? I’ve seen the cheesy plans for sale, but don’t know of anyone whose built one.

  8. tony Fairbridge says

    The Douglas 350cc flat twin of the 30’s,40’s and 50’s was used in a number of lightweight single seat aircraft, and the 1100 BMW is currently available from several aviation-related companies set up for aircraft use, and is quite widely used. The 750cc(?) Agusta V-Twin was used in the prototype “Corby Starlet” single seat aircraft, but was replaced by the 1200cc VW in later aircraft. The Kawasaki snowmobile and waverider motors are close relatives of the Kawasaki motorcycle engines and are widely used in lightweight avaiation.

  9. Mike says

    I think you will find that during WWII Vincent motorcycles made engines for drones. The drones typicaly had a very short life because they were used as targets to train pilots in the art of arial combat.

  10. Jerry Yancey says

    I have over 500 hours in a Thorp T-18 with a 3.8 liter Ford V6 engine in it. I sold the airplane when I lost my medical in 2001 and the new owner threw out the engine and put in a Lycoming and resold the airplane at a profit. People are brainwashed that only an “AIRCRAFT ENGINE” can be relied on.

    A 1/2 Lycoming motorcycle has been built and is not that expensive if you use all unairworthy run out parts as the builder apparently did. He used a crank that had been in a prop stoppage and cracked cases and used parts. The camshaft had to be relobed after being cut down so it was no big deal. He had a lathe, a vertical mill and could gas weld the cases. Apparently it’s pretty awkward to ride as the width is very excessive. The rider used a VW transmisssion like the Amazonas motorcycle.

    Modern car engines like the LS-1 and Northstar can run at 1 bhp/cid for several thousand continuous hours, if the oil and coolant are kept in the temperature range and clean. The biggest problems are designing a reduction drive, and coping with all the electronics in the modern generation of engines. But no one is interested in a cheaper airplane since the homebuilt field, aside from LSA compliant aircraft and ultralights, is dominated by airplanes like the RV series, where a long line of yuppie buyers forms for every one being built, and the buyers want “good old Lycoming power”.

    What we need now aren’t more and better homebuilts, but type certificated aircraft that flight schools can rent out that are idiotproof enough to drive the insurance premiums down.

  11. says

    As a pilot, ex-mc racer, and airplane builder, I’m here to state that it is hard to beat a flat-4 Lycosaur. In small planes, the O-200 (flat 4, 200 cubes,100 peak hp, typ operatinr rpm 2400, redline 2700) weighs about 200 lb. It is hard to make a reliable AC motor weigh less than 2 lb per peak hp and have it last 2000 hours. Note the rpms–due to the characteristics of efficient propellors, a purpose-designed AC engine wont ever see 3000 rpm. If you have a redrive so that you can turn more rpm, you gain weight and this costs efficiency. A fuel-injected Lycosaur has about a 0.4 lb fuel/hp-hr appetite…actually this is likewise hard to beat. All new generation piston AC motors are…wait for it…diesels.

    JP Straley

  12. George says

    I wonder how the Yamaha V-twin the guy used in the Neuport 12 repro – KC Dawn Patrol – has worked out, long term ? I keep hoping for a affordable Wankel for aero use. I don’t trust my ( very ) modest mechanical skills to convert a Mazda Wankel for aero applications. Has anyone done so ? Or a Corvair conversion ?

  13. Charley says

    There are hundreds of Corvair Powered aircraft flying, Pietenpols are the largest user or these converted engines. The Pietenpols were first designed to use Model A Ford engines, this is what I am putting in mine. Also there are many reliable conversions for Rotary engines. And as for the Nieuport 12, well the last pictures I saw of it, were of it destroyed in a corn field, nobody was hurt thankfully, and I believe it was due to engine failure.

  14. Colin says

    Okay need to debunk some mis-information. I fly and teach in certified airplanes everyday. Lycoming never made an O-200; Continental did. It was tested by William Wynne and found to have less torque at the prop than a 2700cc/164cid Corvair engine. Biggest problem they are having is the stress of the prop on the crank is breaking them. Lyc made an O-235 with only the latest versions achieving the above/at 100hp mark. They are no where fuel efficient as they burn between 6 and 8 gallons per hour at cruise and 9 to 10 gallons per hour in climb. Check their POH’s if you doubt. Performance charts are located in Chapter 5. Modern interations of auto/motorcycle conversions are gaining in both popularity and safety. Geo/Suzuki engine shave been used for years now behind several different redrive units. The engine was designed to run at or near its peak torque rpm, because like a motorcycle engine, it was designed with short stroke, and large pistons for its size. It produces a flatter torque curve which when geared correctly gives it great use on the road, and by using a redrive multiplies that torque to give excellent prop torque, with 3 to 4 gallons per hour economy. MANY air trikes and ultra lights are flying behind these engines instead of the Rotax 2 strokes due to increased reliability. The current Rotax certified 912S engines first came out of the snowmobile world and were adapted by homebuilders, and then later Rotax saw a market and ramped up for producing them factory. The power to weight ratio of many of the late model auto engines and motorcycle engines being used is very good since they are also using lots of aluminum. The biggest problem with conversions is that there is no standard being followed in many cases since it is in the experimental category, and although Tony Bingelis and several authors have written great guide books on construction methods and practices such as wiring, many builders still go to their local hardware store to buy parts. Most if not all of these engine failures could be traced to using inferior parts, improper fuel line rotuing or wiring, or bad mismatch of components not thoroughly tested on the ground prior to flying. Those that have the correct testing usually do not have the unexpected failures (several engines have had suspected carb icing since their version had no carb heat dna the pilot was not properly schooled on carb icing; investigation does not yield anything since the ice melts prior to findings).

    The new generation of snowmobile engines is taking the conversion market by storm, as they are only just over 100 pounds at 115#, add redrive and other components, and installed ready to run weight is 145 to 165#. Much lighter than the O-200 at 200 or especially the O-235 which weighs 250 pounds. These engines output is 120 hp, but the best part is that through the reduction drive they get upwards of 450 foot pounds of torque (3 to 1 multiplication). NO small certified engine makes that kind of torque at anywhere near that weight. At 4 to 5 gallons per hour consumption at cruise, and due to its flat torque curve the snowmobile engine is still making an incredible 200 foot pounds of torque at 75% power. This torque value is more than the O-200 or the O-235 make at their redline.

    Certfication processes make certifying the older engines with new stuff too expensive. New engines are being certified with FADEC, fuel injection and converted diesels (from what industries: you guessed it auto and industry; same place alot of the aircraft engines used in WWII came from). The only reason auto manufacturers have not embraced the airplane market is that several times through history when they have they have lost a great deal of money due to such a thin market, and the greed of civil attornies that would litegate for virtually any cause. Since the limitation laws have been passed to protect against these suits more of the support industry is returning, and has lead to the birth of LSA.

  15. Colin says

    Oh one more thing; according to the Part 23 certifcation process, the Lycoming or Continental engines when certified DO NOT have to last their estimated TBO of 2000, but rather must only last a durability test of 50 hours. And that is all they test for. All other information relates to inspection and checking of compression at each 50 or 100 or annual depending on the inspection program it is under. If auto conversion engines were maintenanced as well as their airplane counterparts, there would be very few if any that would fail.

  16. mike wolf says

    WOW, thats really cool. I have a ?. Can you use a ski-do engine on a Trik ultralight 2 seater?

  17. says

    a wankel engine can be modified to be used as a 200 horsepower or split and used as a 100 horsepower engine for sport planes or experimental aircraft. The rotary plates where the rotor rotates and becomes the piston must be taken to a place where the plate can be heated to high temps and fired then cooled slowly, then the plates can be used in a very reliable wankel rotary engine.without upposing cylinders the plane is smooth and very crisp responses. the engine i,m refering to was put in a sauna ray ll and with a g-rating of 4.4 wing load we had to be careful as the engine would perform more than the plane could handle.. carl.

  18. Jim says

    If using a Mazda wankel in an airplane is anything like a car, the fuel consumption will be awful. Also, I don’t know whether they ever solved the tip seal problem. It’s admittedly anecdotal, but the couple of engines I had didn’t make 50,000 miles before having to be rebuilt due to tip seal leakage, which resulted in a significant compression drop. Figuring an average of 50 miles per hour, that’s 1000 hours of automobile use.

  19. Cooper says

    Anybody recall that Porsche produced an aircraft engine in the 80’s? They were available in Mooney’s. And they were a failure. Here’s a critical look at them:
    The Lycoming/Continental designs truly are ancient [dating back to the 1930’s], but they’re a known quantity.

  20. William Duncan says

    Well designed car engines will work fine in the GA duty cycle because that is exactly how the Germans drive their cars on the Autobahn. It also corresponds with, of all things, a school bus. Road and transit buses use heavy duty diesels, but school buses (in the US) are built on truck chassis, and before the medium duty diesels became popular, they all had big block V8 gas engines just like muscle cars, pickups, station wagons and boats.

    School buses have terrible aerodynamics and on perfectly level ground use 50% or more negine power at high speeds (hence their dismal economy.) Up any hill at all it goes drastically up. I drove a schooly for our church’s music ministry for years with a rebuilt 377 (350 crank, 400 block) Chevy on propane and we would go for 30 to 60 minute intervals at 26-27″ Hg at 2600-2900 rpm in the flat lands of Nebraska and Kansas.

    Dave Blanton of Wichita was the most successful engine converter, although his projects did have issues, and much of what he wrote is still true. He believed that torsional vibration was not an issue if one could start and run the engine throughout its RPM range with no propeller attached, and I suspect he was right.