# HELP WITH SNOWMOBILE ENGINE!

### Help Support Homebuilt Aircraft & Kit Plane Forum:

##### Well-Known Member
Hello all, I have just recently purchased a polaris colt 250 twin engine. It starts on the first pull everytime and it sounds great. My question is, Where can i find a belt reduction drive for it and more importantly how can i establish things like redline rpm, Does anyone know anything about these engine? It has a tapered drive shaft and dual mikuni carbs. If you know anything about snowmobile engines on ultralights let me know!:grin:

#### mstull

##### R.I.P.

Using snowmobile based engines on U/Ls is common... probably most common, since Rotax engines are snowmobile based. Kawasaki snowmobile engines are also used. I'm not specifically familiar with your 250. But you'll need a sleek, aerodynamically efficient plane to be able to climb with that small engine. Most U/Ls use engines with a displacement of 340 cc or larger. A 250 cc engine might produce around 30 Hp.

I think Compact Radial Engines makes belt reduction drives for different engines. It would be very expensive if you have to have one machined from scratch. Try to find out what brand of engine it is. Polaris and other snowmobile brands bought engines for their snowmobiles from different manufacturers over the years. If you know the exact year and model, a snowmobile parts outfit will know the brand. Or you might find a brand name cast into the engine somewhere. There's a good chance it's a Kawasaki.

A 250 cc twin would probably have a red line of at least 7,000 RPM. Again, with the exact year and model, you might be able to find an owners manual for the snowmobile, that might have a red line. But if the snowmobile didn't have a tach, the red line might not be given. The tapered drive shaft is common.

Snowmobile based engines tend to be a little on the heavy side for their power.

#### PTAirco

##### Well-Known Member
This outfit sells some very simple, v-belt reduction drives that are adaptable to many different types of engines:

New and used engines

Scroll all the way down.

#### mstull

##### R.I.P.
The Kawasaki 340 engine I bought came with one of those reduction drives. It was ridiculously heavy, and relatively mechanically inefficient. It was so heavy, I deemed it unsuitable for U/L aircraft. I forget what it weighed... somewhere around 10#. The ones from Compact Radial Engines weigh around 3#.

##### Well-Known Member
I cant believe a belt drive cost $600+! It would seem silly to pay that much when the engine itself was only$130. Maybe its the right way to go though..... My goal was to cut back on build time by using foam ribs. I know this sounds like an idiot move but i think foam ribs could be quite strong with the right technique. Does any have any experience with a direct drive setup? And would you have any advice for optimizing thrust and fuel consumption?

#### Dana

Staff member
$600 for a good redrive isn't unreasonable... but it would seem unreasonable for the crude drive shown on the Vortec website. Direct drive can be used, but it's quite inefficient on small 2-stroke engines due to the high rpm and small prop size, so it can only be used if you have power to spare... which you won't with this engine. Optimizing thrust and fuel consumption is a matter of choosing (or designing) the right propeller. Foam ribs are really a subject for a different thread than one on engines, but they have been used, quite succesfully... in aircraft designed for them (the Lazair is a good example). What you need to do, if you haven't already, is to get out and look at a bunch of different ultralights in person, before you start designing your own. -Dana When I was young I was told that anyone could be President. I'm beginning to believe it. #### radioinred ##### Well-Known Member Thanks PTairco, New and Used Engines seems like the way to go. Just odered a catalog this morning... The gentleman on the phone was extremely helpful. We talked for about 20 mins. and he had excellent advice for me. #### Canuck Bob ##### Well-Known Member Air Trikes in Montreal sell plans for a reduction and he offers good support as well as a number of engine packages and another high hp PSRU. You will find it on this page. Air Trikes: Engines and Conversion Kits. #### mstull ##### R.I.P. Yes, I successfully ran a 2 stroke engine direct drive on 3 of my designs. It takes over 600 cc of engine displacement to have enough torque to turn a big enough prop direct drive to get enough thrust to climb. How big a prop you can turn depends on torque. Reduction drives multiply engine torque by the reduction ratio. Larger diameter props generate MUCH more thrust. Look on Barnstormers.com for a used reduction drive. I have one off a Hirth F-33, that I'd sell real reasonable. I'm not sure if or how you might adapt it to your engine. That's the same problem with any reduction drive that wasn't designed for that engine. It might not be adaptable at all. Or it might cost$hundreds in custom machine work to adapt it.

There have been several good discussions of 2 stroke engine choices in the 2 Stroke Engines forum, if you'll scan back through the pages. If you're planning to use it on a true, legal U/L, all the existing choices have drawbacks, or even serious flaws, even though they're expensive. Pick your poison. And there's a lack of good engines in the 28 to 35 Hp range that most true U/Ls need to fly well.

Most of the twin cylinder engines are kinda heavy. But most of the single cylinder engines vibrate too much and/or have too little power.

##### Well-Known Member
Ok little bit a vague question here but ill shoot.....I'm looking at buying a 2:1 belt drive for the engine, it reds at about 7k or so. With 27 horses, what size of prop should i consider? Ive been looking at the ultraprop and it seems to be more efficient than most wood props out there. Does a 54X18 seem right?

#### mstull

##### R.I.P.

Actually, the Ultra is the least efficient prop out there. Its blades have no twist, so the tips have too much pitch while the roots have too little. Buying a ground adjustable prop takes some of the risk out of prop buying.

I like the Powerfin because it is very light, and is the only brand that offers small chord blades (E-blades) that are suitable for low power engines. I'd go with a 2 E bladed prop in the largest diameter they offer, which I think is 51". And yes, around 2.0 to 1 reduction would probably be good with that prop. Your engine might turn a 3 E-blade prop with more reduction (around 2.5 to 1), but I think you'd get more thrust with the 2 blade on a 2.0 reduction. You can adjust the pitch to find the most thrust, and even trim the diameter if necessary. Stuart Gort, Sr. at Powerfin is one of the top prop experts in this country. Call and get his recommendation.

Most all the prop web sites either have application charts or will make suggestions on the phone. The power and RPM of your engine is very similar to the Hirth F-33, and Rotax 277, when you look in the charts. If you decide on a wood prop, be aware that you get what you pay for. I've tried cheap, mid-priced, and higher priced props. The difference in quality and/or efficiency are significant. I really like the Tennessee brand.

I've also found that you can get more thrust from larger diameter, lower pitch props on an U/L. Achieving that with a ground adjustable prop isn't always possible, since the twist of the blades is optimized for a higher pitch. On some planes, prop diameter is limited by the airframe or earth, so you can't choose a large diameter, low pitched prop. In that case, a Powerfin might be perfect. If diameter isn't limited, I really like 22" pitch, using the largest diameter, at that pitch, that the engine will turn up to red line in climb. You can trim the diameter of a wood prop too.

I'd guess around a 52" diameter by 22" wood prop (with narrow blades) would be about right for your engine with a 2.0 reduction. Most reductions are greater than that though, closer to 2.5. With a 2.5 reduction, you might turn a 54" by 22". I'm turning a 51" by 22" with a 2.45 reduction on my 25 Hp engine.

If you do order a wood prop, be sure to ask for an efficient prop. An efficient prop has a "thin airfoil" (as wood props go), "with the airfoil surfaces continuing straight back to a sharp trailing edge." You'll have to ask for that to get it. It's only the outer 1/2 or 1/3 of the blades that really matter on that. A prop that has blunt, rounded trailing edges is much louder, and less of your horsepower will be converted into thrust.

Newton says that the thrust you get is equal to the mass of air that is accelerated multiplied by how much you accelerate it. But larger diameter props are much more efficient than small diameter ones. So you'll get more thrust from a larger diameter lower pitched prop, than a smaller diameter higher pitched one. Again, only go with a smaller diameter if the larger diameter won't fit on your plane.

At U/L speeds, using 3 blades instead of 2 is usually disadvantageous. The main reason is that with 2 blades, you can choose a larger diameter. The other reason is at U/L air speeds, a 2 blade prop still accelerates all the air flowing through the swept disc. Adding another blade mostly just adds drag that robs Hp. Also 3 blades cost more than 2.

#### BBerson

##### Light Plane Philosopher
HBA Supporter
Mark,
What is the tip chord of the narrow blade prop you are using?
I agree with you about large diameter and narrow chord for low horsepower.
With some sort of tip reinforcement, perhaps the tip could be made thinner than could be done with wood alone.
BB

#### mstull

##### R.I.P.
BB,

It's around 2" or so. I ordered the squared off tips.

A couple more thoughts. First, I would definitely have the reduction drive in hand before ordering the prop, since prop size is significantly different for different reduction ratios. And the reduction ratio isn't often exactly what they claim. You'll have to measure it with the reduction drive installed and belt tight. Rotate the prop pulley exactly one revolution very slowly, and count how many revolutions the engine pulley turns.

Biplanes are inherently very inefficient. First because of the proximity of the two wings, you lose about 20% of lift compared to a monoplane, without losing any drag. Second because of having double the tip losses, you'll lose lift and add drag. Third because of the added drag of all the exposed wires and struts.

I'm flying my biplane in my little picture on the left.

This snowmobile engine isn't going to have enough power to make a biplane climb, especially at the gross weight you propose, unless... There is a way to get much more thrust from a small engine: If you run a much greater reduction drive, like 3 to 1, or even 4 to 1, so you can turn a MUCH larger diameter prop and get much more thrust. If you look at Culver's Big Twin engine... That engine has low power and high weight. Even though the engine turns a perfectly reasonable RPM to run direct drive, they put a reduction drive on it to turn a huge prop to get excellent thrust.

One problem with getting that much reduction on your engine... The crankshaft pulley would have to be tiny. Very small pulleys don't have enough contact area with the belt/s. So the belt tends to slip and wear too much. To avoid this problem, you'd need custom pulleys where the prop pulley is extra large. Obviously, you'll need enough room for the very large diameter prop to make this work too. With a 3 to 1 or greater reduction, you could turn 56" diameter or larger Powerfin B blades.

Without that high reduction and huge prop, you'll need a more powerful engine, which will probably be heavier. Before you get too far with you airframe design, you'll need to resolve your engine, reduction, and prop issues. It's often wise to design an U/L around a certain engine, so you can plan for its weight, thrust, and vibration.

If you choose a low thrust engine/reduction/prop combination, like your 250 cc engine and 2.0 reduction, you'll need a very aerodynamically efficient plane, like a motor-glider. If you choose a heavy engine, you'll need an very light airframe to make the weight limit. If you choose a very light engine, you can put more of the weight into making a strong, aerodynamic airframe. Pick your poison.

Remember, you can gain more performance with lighter weight than with any other modification. Try to find ways to design your plane lighter. Even seemingly insignificant parts really add up to significant weight. So try to save weight everywhere with a very simple design.

Last edited:

#### xj35s

##### Well-Known Member
Mark, I'm a little confused (as always) with this disccusion. I didn't read (miss it?) where an airplane was mentioned. I saw foam ribs. I'm runnng a Rotax 277 single cylinder with a 60" 39 pitch, ground adjusable EVO prop. My koala is close to 280lbs.

With two cylinders won't that 250 engine have a little less torque but more HP?

#### mstull

##### R.I.P.
XJ,

The scrimmed foam ribs are pictured in the thread "New Design", which is my present U/L. You just laminate one ply of extremely light fiberglass on both sides of a sheet of styrofoam, then cut the ribs out with a band saw. It's very quick and easy. Be sure to remove the protective plastic from both sides of the styrofoam before scrimming it.

You must have a very high reduction on your 277 to turn such a huge prop. That's my suggestion for Radio, if he wants to use that 250 cc snowmobile engine. Engine torque is multiplied by the reduction ratio, so the reduction ratio will make more difference than engine torque. What is your reduction ratio? That would be useful information for Radio.

I think I see what you're getting at. But engine torque is measured as an average through an entire revolution. Even though the cylinders of a twin fire alternately, they add together by the end of a revolution. Your 277 may have a tad more torque than Radio's 250, depending on how the two engines are tuned. But the twin may have a higher red line to generate the same, or even a tad more horsepower.

Red line is usually limited by peak piston speed. Making an engine with more smaller cylinders, rather than fewer larger cylinders, brings the piston speed down for a given total displacement, which should allow a higher red line, which generally will result in more horsepower. More cylinders generally make for a heavier but much smoother engine. The alternate firing cylinders of a twin make it much easier on belt drives.

Horsepower = Torque x RPM.

Exhaust tuning can make a huge difference in torque (and thus Hp) on a 2-stroke. Radio hasn't told us what kind of exhaust he has. And we don't know its red line yet.