Making Motorcycle Engines Fly... Litterally.

External Since: Mar 04, 2006 Posts: 2

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

External Since: Feb 20, 2006 Posts: 4

Forget it,.. unless you want to get particularly skilled at glider
flying.

Motorcycle motors have tremendous power-to-weight ratios, but they are
designed to run at high power settings for only short periods of time.

It is a matter of heat transfer. These engines are very efficient when
run according to the way the experts designed them. They specifically
did NOT design them to run at max power for hours.

You could derate them a lot, but then to get the efficiency you'd have
to change everything -- cam, intake, exhaust, fuel, ignition,
compression, etc. Then, after you have invested $10k in those mods,
you'd have a motor that was heavier than an aircraft motor designed by
folks who know what they are doing,.. you'd have spent more, and you
would have a completely unknown/untested powerplant. YOU are the one
that puts your tender pink ass in the machine that uses that motor.
Most of us would rather have the R&D be a little less threatening.

You are not the first guy that thought of this. You are perhaps the
millionth. If you ever got to the point of actually testing the thing,
you would be one of hundreds, most of whom, after many hours and bucks,
reverted to motors designed by engineers for aircraft. In the long
run, it actually is cheaper to buy an aircraft motor, especially if you
figure in hospital bills (or funeral expenses) into the cost of a
motorcycle conversion.

The term "aircraft motor" includes, for your purposes, Rotax motors and
the like. Some of these are certficated (meaning expensive) and some
are for us kitbuilders (same motor, but cheaper). There are all kinds
of good aircraft motors (2 and 4 stroke) that can be had for several
thousand bucks. Cheap. Safe. Easy.

Please don't interpret the following info as bragging -- I offer it
just to establish some credentials to add some credibility to my
comments. I have an A&P, two Aero degrees, CFI/II/MEI, been riding
motorcycles for 46 years, retired USAF fighter WSO, build and fly
gyrocopters (powered by Rotax, Hirth, Subaru), and have also seriously
thought about using a motorcycle motor for an aircraft.

What horsepower range will your aircraft need? What kind of flying
will you be doing?

How much building experience do you have? How much help will you have?
What shop capability do you have?

Anyway, my suggestion is to use a motor designed for aircraft. It'll
be so much safer, and, unless you hit on some magic combination that no
one else has found, it will perform better and be cheaper in the long
run.

E-mail me at mricxxxflo.DeleteThis@dr.com (remove the xxx) if you want to chat
about more details. Or, we can chat some more here.

Good luck.