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Polini Thor 250 Engine Review — The best ultralight engine we’ve seen

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There’s a general lack of information on state of the art ultralight aircraft engines. And there’s a minefield of mis-information on what’s been happening with small two stroke engines in ultralight aircraft. Many believe that two-strokes always seize and many also believe that the only engine to use is a Rotax.

A lot of that is chewed up baloney.

I can’t clarify every issue that’s out there — and I want to establish some positive vibes for a few of the engine manufacturers that still stand strong today — and I want to precisely distill what I’ve found with the Polini Thor 250 into this small collection of words and photos.

First, the background vibes on the current crop of engines. They are good engines. Each has positives and negatives.

— Hirth is making an exceptional 50HP engine — the F23. It is what I refer to as a stump-puller. It has incredible low end torque and is ideal for ultralights with a lot of drag and a need for acceleration and power. It yanks our Belite Sealite floatplane out of the water, even in hot and high and calm conditions. It makes runway length an optional requirement. It is reliable; it is available with electric start, fuel and oil injection, and it is a great value for the money. We’ve sold a fair number of them; but it is too much engine for many of our customers. And it uses a lot of gas (think a mininum of 3.5 GPH; 4 might be more realistic.)

— 1/2VWs continue to be the ‘go-to’ engine for pilots who want four strokes. They don’t have a lot of bottom end torque, but they do have a great reputation for easy to service, reliable operation with a low fuel burn. They are heavy, and ultralight customers fight the tradeoff between weight, legal FAR part 103 operations, and the classic boxer look (which these engines share the Hirth F23). They are a great choice if you are comfortable with the weight and can stand a little longer takeoff roll.

— Compact Radial Engines (CRE) continues to provide the MZ-201 (baby brother to the MZ-202 helicopter workhorse engine.) I’ve always enjoyed the E-ticket thrill ride associated with an MZ-201 takeoff. (Any early Disney theme park fans in reader-land???) The MZ-201 is economical; relatively smooth running; and a jolt in the butt when you hit the throttle.

But for the average Joe, these engines are just a little marginal. Consider:

— The F23 from Hirth is too much power and too high fuel consumption.
— The 1/2 VW is very heavy, and not a great performer in climb.
— The CRE MZ-201 is a little harder to cool, fairly smooth, but not very up to speed on current engine technology.
— And if you’ve noticed that I haven’t mentioned Rotax, it’s because they haven’t pursued ultralight engines in years; they’ve discontinued engines; and they have found a gold mine in ‘912’ and ‘914’ class engines. That’s where their heart is. Leave them alone. Let them be.

If I was to dream about a very small aircraft engine, I’d make up a wish list with improvements on all of the above engines. The wish list would be this:

— The engine must produce an honest 35+ HP.
— It must be very lightweight.
— It must be from a major engine manufacturer. No startups, sorry.
— It must be smoooooooooooth. Like a 4 cylinder GA engine.
— Dual ignition. It’s got to have it.
— Reliability. I’ve got to be comfortable flying cross country in it.
— Fuel economy. I want to be able to loiter at 2 GPH, so that my 5 gallon fuel tank will give me 2 1/2 hours of fun. (Or 2 hours with a 30 minute reserve, to be a little more conservative.)
— It needs to look good.

I found that engine. It was from a major engine company called Polini, made in Italy to Italian standards (think recent vintage Ferrari), and it met every requirement I’d listed.

I was at the Aero-Expo in Friedrichshafen Germany, wandering around the exhibits of the various companies. I walked by a booth and was stopped by a row of engines which looked like they were the right size to be ultralight aircraft engines. One in particular caught my eye: it had dual ignition, a water jacket over the cylinder (water cooled!), gear reduction, a very small size, and the information showed that it developed 36.5 HP (27300 Watts). The indicated weight was less than 50 pounds! And it had an internally counterbalance shaft, for smooth running!

After a few months of reflection, I ordered one of the engines as a big test. Would the weight indications be accurate? Would the horsepower be real?

The engine arrived; we pulled it out of the box; and it looked like this:


It had been packed into a shipping box, whilst mounted on a steel frame. Sweet. I’d never seen anything like that before.

It had the following goodies in the box:

— a starting battery
— machined aluminum engine mounts, with rubber bushings
— a pre-wired electrical panel, with an engine start switch and dual mag controls (more on that later)
— a radiator (more on that as well)
— all the hoses, bolts, washers & nuts
— propeller hub plate


Very impressive. Very, very impressive.

The engine installation was straight forward; a few weeks later, we had it installed on our ProCub Lite demonstrator airplane. We chose to machine up a couple of extruded rectangular cross bars for the engine mount; if we weren’t into making so many lightening holes in our engine mount it would have gone even quicker. The photo belowIMAG0279-2-1 shows the engine mount, along with the impressive Polini fit & finish, all on our ProCub airplane.

One day while I was hanging around at the shop, I decided to test the strength of our Polini motor mount by resting nearly 250 pounds of engine and human blubber on the mount. I formally documented the results in the following picture:


And I should note a few things about the process of getting the engine mounted on our airplane. Machined aluminum standoffs were included, along with rubber bushings. We just bolted those to our engine mount. You can see them in one of the photos. (Albeit with a caution: ALWAYS install a heavy safety wire or strap in case a bushing breaks. And change your bushings out every year, when you annual your airplane. [You DO annual your ultralight, don’t you?])

The electrical harness came with the engine. We bolted the control box below our existing panel on the left hand side.

We didn’t use the battery which Polini had packaged. Being Lead-Acid, it was too heavy and archaic for such a svelte airplane or engine. We upgraded to a Lithium starter battery, packing more cranking amps and less weight.

I wanted a conventional throttle (push to apply power), so I engineered a reversing connection on the throttle cable. Push to increase power; pull to go back to idle.

The propeller we chose was a Powerfin 3 blade, “B” type with machined aluminum hub. The diameter is 54″ on this beautiful propeller. It has proven to be a perfect companion to the Polini.

The radiator was mounted below the engine, in the airstream. Polini supplied the radiator, and first flights proved it was effective, in fact, way too effective. I was unable to get water temperatures over about 120 degrees fahrenheit, which is below their
mininum recommended operating temperature. So we downsized the radiator to a tight little motorcycle radiator which I’d found on Amazon. We just used one of the pair; the item is a left and a right. With some welding, both would be usable, but the side without a cap was usable out of the box.


The radiator set may be purchased here:

Another important detail, unique to this engine, was the ability to monitor the water temperature. I designed a water temperature gauge combined with an EGT gauge, and came up with a new Belite instrument. The water temp gauge is matched to the unique probe which Polini uses, and I carefully calibrated it using a boiling pot of water in my kitchen. Not many guys get to drop a probe in the hot water, then take electrical readings off the voltmeter, all with the approval of significant others. Thanks, Kathy!

The panel in our ProCub included this new instrument. You can see it in the photo below.

water gauge panel

This instrument also spits out an alarm signal, so that any over-temp on the water temperature may be used to trigger an alarm light, such as that found on our Multi-Function Instrument. (The MFI has a row of three indicator alarms on the top; another is used for a low fuel indication.)

In nearly 50 hours of operation, I’ve only seen the temperature gauge climb off the top of the scale once. That was after we’d installed the new radiator, and hadn’t bled air bubbles out of the water pump after the coolant fill. Since water wasn’t moving, the engine water temperature quickly shot off the top side. I shut the engine down, and the problem was quickly diagnosed.

Speaking of coolant, we’re using 50/50 mix, and the engine takes about a half gallon of coolant to fill. That’s four pounds of water.

The weight of the engine including radiator, hoses, exhaust, etc., ready to mount, is about 54 pounds. This doesn’t include coolant.

Polini’s technical specs on this engine are here:

It’s time to take the Polini Thor 250 flying. Here’s the observations.

After filling the tank with 50:1 premix gas, engine start is easy. I’ve gotten in the habit of giving a few squirts on the miniature primer bulb (included from Polini!) until a few drops of gas drip out the overflow line. Hit the starter; the engine is running.

At low idle, the propeller may not engage — it has a centrifugal clutch. Very unusual.

The first observation is that the vibration level is very low — almost GA standards low. I’ve compared it to the vibration level in a four cylinder airplane — it’s that low. It’s a mind-blower if you’re expecting the vibration that competitive single cylinder or even twin cylinder engines provide.

With application of power, acceleration is best described as brisk, and our Belite ProCub is soon off the ground and climbing out a fun angle, even at gross weight.

With the improved radiator, temperatures have never exceeded 155 degrees, even in full power climb. They’ve generally run around 135 or 140 degrees.

Did I mention the smoothness? The engine gives no vibration to the airframe in a ProCub while in cruise.

Engine RPMS peak at around 7800 RPM (and Polini recommends 7500 RPM as red line). At full power, the engine makes the ProCub scoot — in experimental configuration, the airplane will move along at 75mph IAS. FAR Part 103 prohibits setting the prop angle so that this is achievable. In other words, keeping increasing propeller pitch on the ground until RPMs drop and speed drops. Cruising is comfortable anywhere between 6200 RPM and full power. Loitering is possible at 6000 or perhaps even 5800 RPM, depending on your weight, airframe drag and propeller pitch settings.

Fuel consumption at low cruise is less than 2.1 gallons per hour — setting a new standard for 2 stroke engines. Our ProCub can fly with the fuel sipping Polini and 5 gallons of gas for nearly 2 1/2 hours. Sorry, fuel consumption at full throttle has not been determined.

I’ve found the engine very comfortable to fly behind. The proof of this was my 1000 kilometer (620 miles) flight from Wichita to Oshkosh earlier this year. A video flight report from that trip was published here:

The smoothness of the engine is amazing (that’s at least the third time I’ve said that; I won’t bring it up again) and I like the power of the engine as well. I’ve flown my ProCub in some rough air; the engine powered me out of situations that would have been scary for a lower horsepower engine. Sometimes, there’s no substitute for power.

I’ve run the engine mostly on 91 octane auto gas (with the 50:1 mix, of course) and I’ve also run a couple tanks of 100LL. It can be done, but it makes me nervous long term because of plug fouling issues and the small gaps in the plugs. Polini has indicated that the use of up to 10% ethanol is OK. Just make sure the octane rating is correct.

All in all, the engine has provided the best flight experience I’ve had in a Belite.

Now, let’s talk honestly about the problems. There have been five problems with the engine, none of which were really the fault of the engine…

1) We blew the seams on the welded muffler which Polini provided.
2) An ignition coil failed. That’s a good reason to only fly behind airplanes with dual ignition.
3) An ignition electrical connection failed. Same as above.
4) The starter motor failed.
5) We found some bad soldering in the electrical box which Polini supplied.

The blown muffler was dramatic in flight but a non-event in reality. I was in cruise; the engine sound suddenly changed to obscenely wrong; RPM dropped 3 to 4 hundred; I looked around for a suitable field for a precautionary landing. Since the engine was still running, I decided to limp to home field, with a continuous eye for the off-field landing. Given the evidence in flight, it strongly suggested a muffler problem, and that is what it was.

This got Polini’s attention. They responded by redesigning and shipping a new muffler to Belite. We’ve never even installed it — we repaired the existing muffler to a higher standard, rewelding practically every seam, and it’s never given us any more problems. But I’m thankful that Polini improved the muffler for the next customer.

The failure of the ignition coil seemed to be random badness. The coil had a short from the step up side over to the primary. It’s a common coil from a well known manufacturer, and I know their products are in other engines in the ultralight market.

The electrical connector was one of several ‘push-on’ connections that got some corrosion in it. Easily fixed.

The starter motor was more random badness, but I’ve been wondering if it isn’t because we’re using a hot lithium battery for starting the engine. The copper braids to the commutator brushes broke. In any case, the starter motor is readily available everywhere and is used on many different products, such as snow machines, ATVs and so forth. You can even get the starter at NAPA!

We found some bad soldering in the electrical box. This was an irritant to my 35 years of electrical design and management experience, but I’m very confident Polini will never ship a marginal soldering connection to us, after I sent them a carefully worded email with pictorial evidence of the bad soldering. We found the soldering issues while changing the box from push-button ignition test to more conventional aircraft toggle switches.

So all the problems boil down to a bad muffler, subsequently redesigned, and some electrical unhappiness which has been resolved. General engine operation through the last 47:30 of use has been just awesome.

So is this the best ultralight engine ever? Well, I haven’t flown behind everything, and I’d still love to fly behind a wankel someday… but it is clearly the best engine we’ve found for our aircraft.

Caveat: Belite is a dealer for Polini.

Terry Strip 1

02 Feb, 15

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