Making Motorcycle Engines Fly... Litterally.

Hello All,

Recently, I have been taking a very serious look at building a
kit-plane. Among the many aspects of this has been the search for an
engine. All of the various kits in my price range call for an 80-140
horse powerplant, obviously- lightweight and reliablility are key.

For many reasons (cost actually being among the least), certified
aircraft engines are not particularly appealing to me, and I'm
currently researching my alternatives.

There is quite the cottage industry currently in Auto->Aviation
adaptations, centering primarily around Subaru and Mazda rotary
engines... This works great in the 400lb, 200hp+ class of engines, its
not particularly useful to my selection of lighter airframes.

Hense my question-

I am looking for an engine with the following properties-
- Total Gross mount Weight < 240lbs (Engine + intake/induction +
exhaust + cooling)
- power ~ 150hp max... able to run at 115hp output for extended periods
of time (I'll use reduced gearing to go from engine speed to prop
speed, so I don't care what speed the engine needs to run at)
- Decent fuel burn - 5-8 gph at 75% power (the aformentioned 115hp
setting)
- RELIABLE

Are there any particular motorcycle engines that I should look at?

After a few days of looking, There has been one which has really caught
my eye- The BMW 'flying brick' motors, K1100 and K1200... Everything
I read online lauds them for outstanding reliability... They produce a
decent 100-130hp that could easily be geared down to my target RPM, and
they seem to have a long history of aftermarket forced induction, which
I would hope let me reduce some of the strain on the engine by giving
it a modest 4-8psi boost and then running at a lower rpm for my target
115hp cruise power...

The one thing I cannot find for the life of me, about any motorcycle
engine, is the engine weight and dimensions itself... How much does the
K1200 engine weight? (all accessories attached, no transmission). What
are its external dimensions... would a modestly turbocharged variant
fall under my target weight?

What about thoughts on reliability? Ease of conversion?

Any and all input would be greatly appreciated:)

-ESC

scott.cragg.TakeThisOut@gmail.com wrote:

> Any and all input would be greatly appreciated:)

I've done design studies on homebuilt aircraft and thought about
4-stroke motorcycle engines and 2-stroke engines and converted
automobile engines, too. I found some flat-four 2-stroke engines (I
think Hirth made them in Germany) that put out 130 horsepower, but all
economy flew out the window at that power. Even the Rotax 582 2-stroke
twins would use about 7 gallons per hour.

I think that you are really more interested in cost than anything else.
That Rotax 4-stroke (914?) boxer engine can come equipped with a
turbocharger and sells for over $10K the last time I looked. The
normally-aspirated Rotaxes consumed a frugal 4 gph.

Have you ever checked the price on a motorcycle engine? They are NOT
cheap if you buy a new engine. The price of an engine would be around
75% of the cost of a whole new motorcycle. That makes the Rotax engine
look more attractive.

And, 115 horsepower continuous output is a lot to ask from a
conservatively-tuned motorcycle engine.

But I would think that you could pick up a decent condition Yamaha
FJ-1200 air cooled engine for only about $1000 or so. They weigh about
200 pounds, even with the transmission. The drive from the crankshaft
to the clutch driven gear is about 1.7 to 1.

I think Yamaha still sells brand new FJ-1200 motors to owners of
Legends racing cars.

High performance short stroke motorcycle engines that put 150
horsepower to the rear wheel really aren't designed for that continuous
duty cycle but you see motorcycle racers running around the high
banking at Daytona doing 180 MPH at full throttle.

With the short stroke, and low flywheel weight, the motorcycle can
accelerate rapidly, and the reason that it doesn't *stall* when the
rider rolls off the throttle is because the motorcycle's *inertia* is
transmitted from the rear tire contact patch to the crankshaft.

You might discover that it was difficult to make small changes to your
power setting without having your engine stall if you used a 150
horsepower sportbike engine in your homebuilt aircraft. That's
something to think about before selecting such an engine.

Nobody mentions that the high performance sportbike engine has to be
torn completely down and rebuilt after an hour or two at full output.

..On the highway, large displacement street motorcycles that get about
45 miles per gallon are running at maybe 3500 to 4000 RPM at only 1/4
throttle opening for economy.

So they are using about 1.3 to 1.5 gallons per hour at cruising speed.
At 100+ mph, mileage drops off to about 6 gallons per hour.