- Giblet Plus!
- Sep 14, 2004
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The motors from motorcycles that most easily convert to airplane use are the bmw boxer twins, the honda flat fours and sixes, and harleys. All of those motors are separate crank and gearbox engines. And most of them run conventional car speeds. So you're not really finding more power.
I was reading this post and got really curious about the bmw boxer twins. Here's something I found:
quote:Hi,
The use of the BMW motorcycle engines is well documented, and well proven. Although not common in the US, these engines fly in many aircraft in Europe and have a devoted following.
Of the available engines, the new generation (ie R1200 series) engines are far superior to earlier incarnations. Most importantly, the R1200 series have balanced chrank shafts, which almost entirely does away with the typical rocking vibration common to opposed twins. These new engines run from low to high revs with barely discernable vibration. Ideal for aircraft.
Secondly, one should go for the "S" variant of the R1200 if possible since these engines in stock form produce 122hp. Earlier engines produced far less, and were heavier, to boot.
I would also strongly recommend using as much of the original BMW parts as possible. For example, don't mess with anything unless absolutely vital. Keep the engine stock. Don't fiddle with the engine. If you want 122hp, then essentially you need to transplant the engine from the bike to your plane and go fly. There is no need to adjust anything, add anything or replace anything. If you can buy a relatively new engine, then that is (almost) all you need to do. IE There is little or no "conversion" to do.
One of the major reasons new "aircraft conversions" give so much trouble is that builders INSIST on trying to do things to the engines. The R1200 has fuel injection and dual plugs. Leave as-is.
There are four things you WILL have to do before you can go flying behind your BMW, however...
You will need to fit a new exhaust. The BMW engine is mounted back-to-front in the plane (ie gearbox side forward. In the bike it is the other way round). This will require a new exhaust. However, a word of caution. Don't go fiddling with the exhaust trying to improve it. Keep the header pipes the same length. Lead the two pipes into a single pipe (just like on the bike). Keep the catalytic converter. Lead the exhaust into the stock muffler. You will end up with a quiet, efficient, manageable and above all, reliable exhaust system. So you lose a few HP because of the catalytic converter. Get over it. 122hp is plenty. So the exhaust is quite long, and relatively heavy. Big deal. There is no particular reason why the exhausts of aircraft need to be 6 inches long, poke out of the bottom of the cowling and roar loud enough to wake up every conservationist in the tri-county region. In my installation, I'll keep as much of the original exhaust as possible, and have the silencer also. It will exit the plane behind the cockpit.
You will need to create your own air intake box. The one on the bike is custom designed to fit in the bike frame. Keep the same volume, but make your own.
You will need to fit a PSRU. Here you have a number of choices. Because the BMW offers such a clean surface on which to mount the redrive, it is a relatively simple operation by anyone who knows what they are doing. The older BMW1100 engines accepted the Rotax C box, I believe. I prefer to go with one of Neil Hintz' Autoflight gearboxes. These are used world-wide in everything from trikes, to gyros to fixed wing. He has a universal mount drive which can easily be matched to the R1200. It is reliable, has an excellent reputation (no reported failures - ever - I believe) and is competitively priced.
The above three "conversion" items are pretty easy, and should present minimal problems, since you are basically keeping sizes and lengths identical to the bike. The PSRU is a bolt-on option. But the fourth "modification" is more problematic. The R1200 is computer controlled. Problem is, the ECU expects a zillion inputs which are specific to the bike. They are out of place (or non-existant) on an aircraft. The only way round this is to fit an aftermarket ECU. Having said this, the task is far from being as daunting as it sounds. I am using a Link ECU. This is a Kiwi-designed unit, and widely used on aircraft here and in Australia. These units are (almost) self-tuning. But for the final tweaks and adjustments, any competent tuning shop can oblige. I intend to make my ECU map available for other users, so that even this part of getting your aircraft flying behind the R1200 will be a "bolt on" option.
Regards,
Duncan
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Jul 13, 2013 16:50
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