Which Engine Type?

[Badland-F5 Pilot]

So I'm looking at engines to hang off the front of my eventual F-5 Fujita. The Polini engines are pretty amazing.

Polini.jpg (28.28 KiB) Viewed 2149 times

However I'm liking the flat/horizontally opposed/pancake, or whatever you wish to call them type of engines. I had a flat 4 in a Subaru Brat I owned when I was very young. That engine simply would not die no matter how badly I beat it up. I hear the same said a lot about the VW conversions, both 4 cylinder and 2 cylinder. These engines also seem to be a lot smoother than a single or 2 cylinder inline. Another issue that I have with vertical engines and this especially applies to tail draggers or more appropriately, tail wheeled aircraft is that the nose gets in the way of visibility. The taller the engine, the higher that nose has to be in the field of vision out the front of the plane. The bigger problem is not just liking a horizontally opposed engine, it's finding one that is small enough for an ultralight! There are several, but the only one that seems to be within a reasonable price is the Hirth F-23

http://www.recpower.com/F-23%202%20cycle%2050hp.htm.

The F-23 is a 2 stroke, 2 cylinder, horizontally opposed engine. Max HP is 50 and weight is (including reduction unit, electric start, and exhaust) is only 78Lbs. That's about .64 hp per pound of weight. It is a 2 stroke, and that scares some people. I've rode a lot of dirt bikes with 2 stroke engines when I was young. There were issues, but I can only recall those issues were because I was lazy and didn't maintain the engine like I should have.

On the flip side there are the VW conversions.

These are 4 stroke and half VW engine so 2 cylinders. We know that 4 strokes are more reliable than 2 strokes, but how much? Add in that the half VW with just the engine and starter, no reduction gear needed, but no exhaust is 83Lbs, not much more than the Hirth. Except, that's 83Lbs for about 35~37 HP! That comes out to if on the high side of that HP range to be .45 hp per pound. That's a large difference. There are some other similar engines available with better power and lighter weight, but after inquiry I found them to be just under 3 times the price of the Hirth F-23.

There is a single stroke (yes that's right), 22Lbs, 50HP, two cylinder, 4 combustion chamber, horizontally opposed, 16,000 RPM engine designed by Ampere.

AMPSS-2B.jpg (11.02 KiB) Viewed 2149 times

The engine is the SS-GEN1 Mk2 AMP SS-2B 460 CC A. Here's the URL - https://www.ampereinc.ca/single-stroke-engine This would be an amazing engine to hang off the front of a Badland ultralight. The problem - there's little information about this engine, no videos of it running on YouTube, and I've been unsuccessful in contacting the company for more information. The company has been at airshows, even a few years at Oshkosh. The entire line of engines looks very promising. However, I'm not holding my breath on this one. I'd like to see more information available and actually see one running, at least in a video on YouTube. So for now, the Hirth F-23 is my engine of choice.

What's your idea of an ideal ultralight engine?

[Allen Sutphin]

A 2 stroke is not unreliable just because its a 2 stroke. Consider that the average 2 stroke has less than a dozen moving parts compared to a four stroke. Gets fresh oil with every stroke. Runs like a turbine above half throttle, and instant power when throttle is applied. Routine maintence is the secret to realibility. And most important, two strokes rarely fail. THEY ARE MURDERED BY THEIR OWNERS!!!!!

[Badland-F5 Pilot]

Agreed. My two stroke motorcycles rarely had any issues. Most of the time it was because I didn't take the time to what I should do to keep it running smooth and starting easy. The power band has even improved and is much wider today than it was when I was riding my motorcycles. The same that you said for the two strokes reminds me a lot of what my CFI use to always say to me - planes like to fly, it takes a pilot in most cases to crash them.

[mrpilotron]

If you can afford the weight and the CG works out, the 2 cylinder Hirth looks like a fantastic option. The extra cost for the fuel injected version still looks like a dollar store bargain compared to what a new Rotax costs in the 2 seaters that everybody is building. You may need to have a really short engine mount to keep the CG, especially with that titanium frame that weighs next to nothing. You may also consider mounting the battery farther back to offset the weight. The tail has a lot more moment than the engine so even a small weight added back there makes a significant difference.

I am struggling with the same choice too. I really don't care about making weight because I will register it EAB and put a tail number on it if it ends up too heavy. That also means a 1/2 VW is in the running if I decide I really want a 4-stroke. The factory didn't chose the Polini series just by flipping a coin. Those engines are fairly well proven and there is a significant dealer network supporting them. Of course, since Chris is an authorized Polini Thor dealer, it makes things really easy to buy his complete pre-engineered power system and be done with it. I still need to get up there and test-fly one before making my final decision.

[Badland-F5 Pilot]

Peer here on the site (LA F2 Flyer) on his CG it came out a bit further back than he’d like, so as you mention this actually may come forward too far. I like the idea of placing the battery further back and pulling the mounts a little closer to get that ideal center CG point. There will also be an emergency chute behind me so that should also pull the CG a bit more back from the front. Good to know about the moment for the tail. This is my first build so the more help I can get, the better! Thanks.

I’m in the that stage where I’m just not sure if I should go with the heavier but a little more reliable ½ VW 4 stroke of if the F-23 with fuel and oil injection is the way to go. I like the idea of the lower RPM’s, and being able to hang the prop right off the crank. However that flat F-23 sure does look appealing and is easy to maintain. Peer and I have discussed this many times. We both have motorcycle backgrounds. I was pretty much born on 2 stroke dirt bikes, so I have a good idea of how to maintain them and keep them running.

Choices --- so many choices.

[mrpilotron]

I own and have owned many 2-stroke engines over the last 40 years. One thing I've learned is that you cannot judge one by the others! I have zero actual experience with any of the 3 engines in consideration so I must lean on the experience of others. While no engine is ever 100% perfect and reliable, there are some better than others. I read comments by a Hirth dealer who had sold, serviced, and flown hundreds of engines that the Hirth F-33 single cylinder engine is junk and he refused to sell any more of them. That same guy also said the F-23 boxer engine is one of the best engines he ever used and enthusiastically endorses it. I found this polar-opposite opinion to be confusing but since he discourages sales of an engine that would profit him, I tend to give his endorsement a bit more value.

The internet is full of opinions in the various forums, but sometimes the people posting can blow a small problem into a major catastrophe since they are mad. They also tend to paint with an overly broad brush: "My 2-cycle engine failed so 2-cycle engines are unreliable!" We know where that can lead...

[Badland-F5 Pilot]

I have to very much agree with all you've said. I've really not had that many issues with 2 strokes, and I've also had issues with some 4 strokes. A great number of these issues to me seem to be owner caused. On that same note though, I do like to hear about engines that can take some abuse and still perform well. Typically though, those are heavy and not good for flight use. It's sort of like reading the customer reviews on Amazon. People giving 1 star because UPS/FedEx delivered and the packing box was damaged - which has absolutely nothing to do with the actual product that was delivered. Got to love those posts.

[Allen Sutphin]

Most two strokes don't fail, they are murdered by their owners!

[mrpilotron]

People giving 1 star because UPS/FedEx delivered and the packing box was damaged - which has absolutely nothing to do with the actual product that was delivered.

This. When I see those reviews, I just want to reach through the computer and strangle someone. I also have seen 5-star reviews with text that sounds like a 1-star. Take EVERYTHING you read on the internet with a grain of salt. Happy customers rarely write reviews. Unhappy ones rarely shut up!

I am leaning toward the Polini 303, but it wouldn't take much to convince me to use a 1/2 VW. Whatever I choose, it will be new, properly installed, properly used, and properly maintained. What that entails remains to be seen, but I have a strong vested interest in giving my engine the best possible chance of success. I don't want to be the customer who won't shut up.

[Badland-F5 Pilot]

Not only the customer who won't shut up but also the customer that can no longer speak at all because he's permanently horizontal from an engine failure! Having my two stroke Yamaha enduro fail while riding is a lot less likely to get me killed than losing a Hirth, Polini, or any other at 8,000 feet. Though it's very possible and likely that I'll land safely, there's usually only one chance to do so. On the bike, I could walk home if needed.

I'm one of the few that does post positive reviews as well as negative reviews, but I'm fair about both. I try my best to keep any emotion out of my reviews and provide honest feedback. People also have a real problem with properly understanding the products strengths and weaknesses which are largely based on country of manufacture. I have a Chinese ATV. The engine is a 4 stroke and a copy of a Honda engine. I know for a fact it's not built to the quality standards of Honda. I'd be a fool to think it would be. Yet I take good care of it. Have made some modifications (28mm to 32mm carb), better air filter system, smaller front and larger rear gear, stronger chain...etc. You get the picture. It has worked very well and I don't push it like I would a Honda, Suzuki, or other. I know it won't take the stress, yet it will last a long time with proper care. Polini and HIrth both have really good reputations for quality. I'm sure there can be problems with them. There are always a few lemons in mass manufacturing, but it's pretty rare. I haven't had any discussions with Hirth yet, still forming questions before I send those questions off to customer support. I have had a few conversations through email with Polini. While I still believe the engines they manufacture are very good, their customer support isn't so great to deal with. They are very short in responses, leave a lot of the requested information out of the response, and in one case got a little rude in their response.

I really am starting to lean pretty heavy for that 1/2 VW. I like the slower RPM sound as well as direct connection of the prop to the crank, and of course the TBO is pretty nice on them too (not failing to mention, inexpensive compared to other options).

[mrpilotron]

I just watched the engine webinars that were part of the EAA Ultralight Days series. The "top 10 reasons 2-stroke engines fail" discussion was a curious one. If you missed it, go watch it at the EAA web site. My takeaway from it is that the guy presenting it is very vested in Rotax. He is a long-time service tech and dealer for them and gets a little testy when people murder his favorite engines. They require careful setup and don't tolerate poorly chosen props or being operated outside their sweet zone. If you can get them set up just right, they are extremely reliable and last a long time. If you don't, you can go from "just fine" to "I'm a glider" in 2 seconds. He did mumble a 1-second comment that the Hirth F-23 is a fantastic engine, but otherwise nearly ignored anything non-Rotax.

On the other hand, the 1/2 VW presentation was a completely different style and tone. It was presented by a guy who built and flies one, not a professional service tech or dealer. He had a few failures that could have been avoided with prior knowledge of pitfalls and at least one that was just a straight-up bad luck day. He accidentally (and almost invisibly) made a great case for the value of investing in the dual ignition system because his "bad luck" day wouldn't have been nearly so bad with a second spark plug lighting the fires. It seems that the VW engines tolerate just about any dumb choices you can make with prop selection without causing an engine failure. They won't deliver the performance you hoped for, but they also won't make you a glider because of it. They do have a "sweet zone" to operate in, but that zone is much broader than for the 2-strokes which works better for higher speed aircraft due to the significantly different prop loading at high forward airspeed.

There was almost nothing useful in regards to the Polini engines specifically so it remains to be seen how well they perform in this airframe. Setting carburetor jets and prop pitch is likely to be just as important as with the Rotax 2-strokes. I'm sure Chris is able to help with this and make sure it's all dialed in good right away.

[Badland-F5 Pilot]

I've got a relative here at the house so wasn't able to watch the videos. However I am an EAA member so will watch them as well as I'm creating links to each putting that in our Badland MultiMedia section of the forum. Typical for those having an interest of any nature will be bias. I'll be watching the videos as soon as I can. I was really happy to see the EAA put on this series. I've seen the EAA pick up a bit on ultralight coverage, which is nice to see. I fully get that the majority of pilots are GA and flying GA or LSA, but the ultralight growth is taking place once again. Good to hear/know that the presentation for the 1/2 VW powerplant. I do like the idea of two spark plugs per cylnder. It's not that much extra weight compared to the added performance and safety it adds to flight. VW's seem to be nearly as reliable in the air as they are on the ground. What you said about 2 strokes, espeically the Polini makes perfect sense. Since those engines are designed for paramotors which typically are near or at full throtle or idle, use on an airplane makes them more vulnderable to failure since when flying there is certainly a best cruise in that mix. Tune for best cruise and it may not be good on the enigne going full tilt. Go full tilt and it may not be good at cruise setting. Going in for a STOL touch down may be harder on a 2 stroke engine from variable revving to control the landing. Typically someone has gone through these pains and finding out where a specific engine runs the best. Finding them and getting their advice and knowledge is sometimes difficult, but your right. With Chris's primary engine being the Polini, it's a pretty sure bet he's got that all figured out.

[Antoinette]

I enjoy building engines. Right now I am playing with water cooled Wankels. Longevity is my current bug-a-boo. My "Mark IV" has been used in one of my airplanes but is not anywhere near as reliable as my two-stroke piston power plants, which have a TBO of TBDBJUHR (to be determined by Jesus upon his return).

My water cooled rotary is just a tad under half the weight of my best air cooled two-stroke engine producing equal power. That makes it a fifth or so of the weight of an air cooled four-stroke engine producing the same power.

(To be honest, I have never built one of those massively inefficient four-stroke powerplants, what with eight times the number of moving parts and three times the rotational mass of a two-stroke engine per horsepower. Saying that a four-stroke is 33.3% more efficient per hp than a two-stroke is also saying that a two-stroke is 150% more efficient per pound than a four.)

If one uses a quality tank and fuel line, the fuel mix "machts nicht." Pump gas is an ever changing mongrel... Whatever the refinery has on stock that can be mixed to come close* to regulations on a given day. It varies load to load... Personally, I distill and refine organic mass into ethanol. 100% ethanol is not problematic... No fuel is stored on the aircraft, no space is provided for condensate, and water is drained pre-flight.

Ethanol has a different burn profile than petrol. So what? Time the engine to the fuel you use. Different petrol formulations have differing burn profiles as well... Any engine can be optimized to burn a specific fuel, it just takes a small bit of math and a little bit of effort.



*close meaning 87 could be 83 if required considering the completely impotent fiduciary add-on costs and likelihood of their imposition due to non compliance versus the real world cost of having large quantities of unsalable refinery output in gigantic storage tanks.

[Antoinette]

"Not only the customer who won't shut up but also the customer that can no longer speak at all because he's permanently horizontal from an engine failure!"

I would submit that no minimally competent and sober pilot ever died from an engine failure. Your engine quits-- congratulations, you are now a glider pilot, they don't die upon landing, do they?

"Having my two stroke Yamaha enduro fail while riding is a lot less likely to get me killed than losing a Hirth, Polini, or any other at 8,000 feet."

OMG, if you are at 8000 AGL and the motor stops... Even with a 7:1 or 8:1 glide ratio a total noob has a freaking eternity to find a nice place to land. (I tend to fly at 1500 or 2000 feet AGL.) And really... one shouldn't be flying over the world's largest piranha farm while wearing A1 or Heinz57 steak sauce anyway... one should be cognisant of the fact that mechanical devices fail and one should have a plan on what to do if that should happen.

[Terry Croup aka Baron Von]

I am late to the party, here. I am pretty sold on the F23. First it is air cooled, meaning less to go wrong like a coolant leak, etc. Second, opposed runs smooth. Smooth is less to go wrong. Third, the F23 can be throttled back in cruise to like 40 or less HP, so less wear and tear, and fourth, the F23 looks like an airplane engine. Many times in aviation what looks right IS right.

I also read about an F23 driver who said he thought his engine was running rough and producing less power on a flight, so he raised the throttle a little and limped home to a safe landing… then he found what was wrong (I forget what it actually was,) but it turns out what made it seem like less power that day was that only one piston was producing power. He further said it felt like the F23 did well on one cylinder and if it weren’t for the reduced power he felt, he was safe while flying. That story really sold me.

The F23 will require a chute to be 103. Chris said he could figure out the Weight/Balance. I like the whole idea.

Good luck everyone.Terry

[Badland-F5 Pilot]

I am late to the party, here. I am pretty sold on the F23. First it is air cooled, meaning less to go wrong like a coolant leak, etc. Second, opposed runs smooth. Smooth is less to go wrong. Third, the F23 can be throttled back in cruise to like 40 or less HP, so less wear and tear, and fourth, the F23 looks like an airplane engine. Many times in aviation what looks right IS right.

I also read about an F23 driver who said he thought his engine was running rough and producing less power on a flight, so he raised the throttle a little and limped home to a safe landing… then he found what was wrong (I forget what it actually was,) but it turns out what made it seem like less power that day was that only one piston was producing power. He further said it felt like the F23 did well on one cylinder and if it weren’t for the reduced power he felt, he was safe while flying. That story really sold me.

The F23 will require a chute to be 103. Chris said he could figure out the Weight/Balance. I like the whole idea.

Good luck everyone.Terry

I found the same pros' for the F-23, not failing to mention its 1000 hour TBO. Not many engines have that and that's way more than I'd ever need, but nice to know it's there, meaning the engine's wear and tear is very low over its life. There is a drawback to air cooled over liquid cooled, at least, IMHO. Water cooled very rarely run into the problem of temperature shock. The liquid cooled engine changes temp slowly and smoothly. An air cooled engine can be damaged by quick changes in air temp. This is now rare for both, but it is a higher possibility with an air cooled engine. Good on the simplicity, but slightly less reliable. With the F23 though, I don't see this to be much of a concern - it's a tough engine from everything I've read and one of our own here has used the engine, and loved it. He may chime in. If Scott's Thump Air reaches the 45hp he says he's close to, then there's not much difference between the Thump Air and F23 for power, just 5hp. However, the Thump Air is currently only rated at 300 hours TBO, but, it's also about half the price of the F23.

Decisions, decisions!

[Terry Croup aka Baron Von]

You rascal you!