Big Single for Motor Glider
 

I’m in need of some help converting a 141cc Tecumseh 2 stroke snow thrower engine for use in a 5+ meter motor glider. This a large single cylinder engine with a 5.5hp rating. Induction is via piston porting which will be modified to except a 20mm diaphragm (pumper) carburetor, instead of the original float & bowl. Since the replacement carburetor has a higher flow rating some minor exhaust porting work will be necessary. Currently the engine exhaust porting consists of 3 x ½” holes which I suspect was done to restrict the power output and max rpm’s. The engine does have an internal 6K governor with linkage to carburetor. Even with the governor disconnected the exhaust porting/OEM carburetor limits rpm’s to roughly 6500 rpm.
Similar B&S engines of the same displacement are rated at 6.5hp so I feel 7hp is achievable without incurring engine damage. The original cast iron magneto flywheel weighed 8+ lbs the same as the engine without it. The heavy flywheel will be replaced by a propeller and need not worry about shock loads. Of course the engine case is in need of trimming the fat along with removing the ignition coil.
I plan on converting ignition to CDI for better starting and running. Besides the above you assistance is requested to help with making an aircraft engine.

1. Since 6000-6500 rpm is common for airplane gassers my thought is to keep the governor but, enlarger the exhaust and intake porting to develop more torque along with +1.5 more horsepower. Limiting rpm’s should protect the engine from over-revving. What are your thought on this? Recall motor gliders engines rarely run at 100%.
2. The engine has a stout 1” x 4” output shaft. I would like to use this for the propeller and mount the CDI pick-up on the magneto shaft. This would keep the pick-up out of harm’s way.
3. Since the power increase will create more loading on bearings I would consider only using API-TC rated synthetic oil at the recommended 40:1. Would it be prudent to lower the ratio to 32:1?
4. The motor glider TOW is estimated 40lbs does power to weight apply in models like real aircraft? It seems 10-15:1 is common in light propeller aircraft. In models wing loading is common and power to weigh not a concern just engine displacement.
5. Of course likely I missed many important issues so please feel free to mention them.

Many thanks, ED

Please post pics as you do it :cool:

I doubt the governor is required, just pick a suitable propeller. Main bearings are balls and not needles I assume?

Interesting project. Got any pictures?

Find out what the rpm for peak power for that engine is, and try to prop for that rpm on the ground. I'm betting it wil be designed to make its peak power at lower rpms than you think. Rpms will increase some during flight. What we do on ultralight airplanes is fit the largest diameter that will fit, and using an adjustable pitch prop, then adjust the pitch until the engine runs the static rpms that produce peak power. That results in the best overall performance. If you want more climb you can cheat the pitch down a little, or if you want to gain a little in cruise at the expense of climb, you can cheat the pitch a touch higher. Adjustable pitch props are available for model engines, but man, hang on to your wallet!!

With that size engine an oil ratio of 40:1 should be fine, but running at 32:1 shouldn't hurt it.

Good luck with your project,
AV8TOR