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

 

The flying brick (mine is a '91 K-100RS) isn't particularly light -
but I think it's still a great choice (it's the only motorcycle engine
I know of that I'd be tempted to fly behind). I believe I have the
engine weight in a manual somewhere, but don't think it included all
the accessories (fuel injection, radiator, etc.). There's a host of
technical info available at...

http://www.ibmwr.org/ktech.shtml

Mark Hickey

 

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.

 

I have looked into this too.
One problem with aero-conversions that no-one else has mentioned
is the vibrations set up between the crank or reducton box and the
prop.
They can cause failure at the crank, gearbox, or prop hub. This is not

something you want to discover on your first or second flight.
Analysing vibration modes is not an exercise for the amateur and even
the pros are sometimes surprised.
(You may also find that your 75% cruise will cost you about 9 GPH.,
which is what you get from a Cessna 152 at about the same power
output.)
MadDog
"Experience is the greatest of teachers, but she's expensive! "

 

Cheap VWs are heavy for their power, and you'd need two of them to get
the power you said you wanted. If you want a lot of power, you can use
hot rod VW stuff, and fly for several hours before you get to practice
your engine-out drill. If you want a lot of reliable power from a VW,
you'll spend a LOT. Even then, you'll have a power to weight ratio
about as good as a C-85. VW's have to turn 3200 rpm or more to make
any reasonable power,.. so you either have to use a redrive (more
weight and $1,000s) or spin a prop so fast it then gets very
inefficient.

If you absolutely must use some sort of non-aircraft motor, then use a
Subaru. The EA-81 can make 80 hp all day at 4gph. Typically they'll
go for hundreds of hours with very little maintenance. I know several
guys that use them in gyros -- they require much less TLC than a
Lycoming or Continental, they use less gas, and they are really cheap.
For some reason they have the ability to move the heat away from the
cylinder head like most other auto-conversions can't. The only downside
is that they are a little heavy for their power unless you get one of
the highly-modified-for-aircraft versions like these:
http://www.eggenfellneraircraft.com
http://www.stratus2000.homestead.com/

Believe it or not, the Corvair conversion is getting more and more
popular because it is cheap and reliable -- a tough combination for an
adequate powerplant. Being six cylinders, they are smoooooth as well.

Check out this engine site.
http://www.wanttaja.com/avlinks/engines.htm It's got more links to
more aircraft engine sights than democrats have losers. It even has
some places dealing with motorcycle conversions.

What kind of kitplane are you thinking about?

 

Maybe a Chevy 454 crankshaft vibration damper would do the trick, while
adding
beneficial flywheel effect to the lighter motorcycle crankshaft... ;-)

 

Hey Mel,

Thank you for your input so far

I'm looking at building a Super Pulsar 100 or Pulsar 150
(www.pulsaraircraft.com)... The company has a good reputation on
rec.aviation.homebuilt... the Pulsar 150 has been around for about 10
years (formerly as the KIS) and has a large number of flying examples.
The SP100 is a new design and the first customer builds are starting to
come online right now... obviously, the lack of time on the airframe is
a bit disconcerting, but every builder I've contacted has nothing but a
glowing impression of the company, and the SP100 is an adaptation of
the Pulsar XP - a sucessful kit with a large number of flying
examples...

One of the particular bonuses of the SP100 is its cabin size... I'm
6'4, 240lbs.

I've already spoken to the factory about this - they're basic take is
that if I match the power/weight characteristics of their reccommended
engines, it should work. The mentioned that several SP100 builders are
already putting Corvair engines into their ships, and they're
considering 'officially' supporting that engine.

At this point, the Corvair is definitely a very appealing route... I'm
still trying to 'feel around' and see what other options I might have,
however.

-Scott

 

No cheap fix. You need a aviation
class engine.

 

Scott scribbled;

 

...and BMW was in the aircraft engine business untill 11/11/1918 when the
treaty of Versailles forced them to find new direction which is why their
logo depicts a spinning prop. Vtwins are essentially the by-product of
radial aircraft engines, two adjacent cylinders wheras the Bavarians chose
to use two opposing cyl. All that shit happened 80 years ago and I'd still
prefer to entrust my life with newer technology, especially in an airplane.

 

people has been using VW aircooled engines for years as airplane motors
and now they have aftermarket blocks and every other part you need and
you can get upto 200 HP out of one of those engines so the HP rating is
there and they weigh under 200 lbs. and they are the simplest to
maintain and quite affordable to rebuild too when the time comes,just
look around the VW clubs and other kit plane builders and some are using
this engine and according to one I talked to years ago it looks like an
airplane and is very balanced since it is a horizontal arrangement,and
back in the late 60's and early 70's they used to swap out the BMW
engines with the VW to give it a lil bit extra kick
good luck on your hunt and safe flying