How can I make an informed decision choosing between similar engines?

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fatsportpilot
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How can I make an informed decision choosing between similar engines?

Postby fatsportpilot » Sat May 02, 2020 4:36 pm

When I see something like ULPower UL350iS which is a fraction of the price of a Rotax and nearly as powerful as the Rotax 915iS (130 hp instead of 141 hp) and only a little heavier it really tempts me. I see 915iS for $40,000 and UL350iS for $20,000. But there's very little DETAILED information that I can locate that will let me make an informed decision! There's so much more to understanding how they handle redundancy and differing failure modes that aren't always obvious (like this example here for Rotax ECU power on the Rotax forum).

I see things like ULPower not having great redundancy and that an internal short of the battery would kill the plane without the 2x battery and 2x ECU version (which is a little more expensive). But then again the engine is a lot more simple than the Rotax with direct drive and wet sump oiling. The Rotax engines really stand out at being more complicated than usual (in their favor, they do have a very high TBO).

But it's not enough to compare weight, fuel use, horsepower, and price. Lots of subtle differences might have big reliability impacts. The fuel-injected Rotax engines use electrically isolated generators (on the same stator but still isolated) for the ECU and for the rest of the plane. I'm not 100% on why they do this but my GUESS is to protect from any kind of power surge or something that would get through the fuse and voltage regulator and fry the ECU. ULPower doesn't have that and the same electrical system feeds the plane, battery, and ECU.

I would have also looked at Continental and Jabiru but I'm not too interested in a carbureted design.

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Re: How can I make an informed decision choosing between similar engines?

Postby 3Dreaming » Sat May 02, 2020 4:47 pm

I think you should take a look at the history and track record of both before making a decision.

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Re: How can I make an informed decision choosing between similar engines?

Postby fatsportpilot » Sat May 02, 2020 4:53 pm

3Dreaming wrote:I think you should take a look at the history and track record of both before making a decision.

What I can find is that they're both pretty reliable. Neither of them have a stain on their history like Jabiru but there aren't that many ULPower engines out there to tell but there are a lot of Rotax. They aren't rare at all but they aren't common enough to be able to find a trend.

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Re: How can I make an informed decision choosing between similar engines?

Postby Warmi » Sat May 02, 2020 5:03 pm

fatsportpilot wrote:
3Dreaming wrote:I think you should take a look at the history and track record of both before making a decision.

What I can find is that they're both pretty reliable. Neither of them have a stain on their history like Jabiru but there aren't that many ULPower engines out there to tell but there are a lot of Rotax. They aren't rare at all but they aren't common enough to be able to find a trend.



Well, there is your answer :D
It may be very reliable or may have some hidden flaw that is yet to be uncovered ( given relatively low numbers of these engines being flown , at least as compared to Rotax engines ) - do you want to take a chance that you gonna be the one who gets to uncover it ? :D

Also, UL350is is about 21k vs Rotax 915is being at 38k - but it is not exactly apples to apples comparisons either.

I guess it all comes down to a very personal choice - how comfortable are you flying behind a relatively unproven design.
Flying Sting S4 ( N184WA ) out of Illinois

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Re: How can I make an informed decision choosing between similar engines?

Postby fatsportpilot » Sat May 02, 2020 5:17 pm

Warmi wrote:I guess it all comes down to a very personal choice - how comfortable are you flying behind a relatively unproven design.

That's why I'm trying to look for design decisions and technical information to understand how they can fail and what redundancies they have. If I feel I have a lot of information about them then I would feel a lot more comfortable with a cheaper, but powerful and unproven engine.

It's a possibility that there just isn't that kind of information out there and in that case, I'll just go with Rotax.

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Re: How can I make an informed decision choosing between similar engines?

Postby Scooper » Sun May 03, 2020 10:12 pm

You may want to check out the UL Power subforum on Zenith Aircraft Builders and Flyers. It's pretty active.

http://www.zenith.aero/forum/categories/ul-power-forum/listForCategory?categoryId=2606393%3ACategory%3A607665&page=1
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Re: How can I make an informed decision choosing between similar engines?

Postby MrMorden » Tue May 05, 2020 7:26 am

These are not quite apples-to-apples engines. The 915iS is a turbocharged engine and will make full power (141hp) to 15,000ft. So for $20k you have a 130hp NA engine, and for $40k you have a TC 141hp engine. Which is better? I dunno, what do you want to do with it?

If you are a low-and-slow flyer who stays below 5000ft MSL, honestly the 915iS might be wasted on you. IMO the extra $20k isn't worth the 11hp. But if you live at high altitude, or are building an airplane to cruise at higher altitude, then the 915iS offers a LOT of additional power/capability. You get what you pay for, but only if you match the engine to the application.
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Re: How can I make an informed decision choosing between similar engines?

Postby drseti » Tue May 05, 2020 7:48 am

Andy is right. Before you acquire any airplane, engine, prop, or accessory, you must always consider the mission.

But when comparing engines specifically, don't forget to compare weight, TBO, dimensions, parts and service support, available mods, fuel burn, your own level of training and experience, smoothness, oil consumption... the list goes on. It's about far more than just price and HP.
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Re: How can I make an informed decision choosing between similar engines?

Postby nub_pilot » Tue May 05, 2020 4:28 pm

TL:DR
I know very little about engines and LSA's as I just got into learning and still a student. With that I have read a lot of articles about the Rotax engines and have read that the 915is would require a constant speed prop in order to maximize the HP and efficiency. The is a no-go for an LSA.

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Re: How can I make an informed decision choosing between similar engines?

Postby Warmi » Tue May 05, 2020 4:48 pm

nub_pilot wrote:TL:DR
I know very little about engines and LSA's as I just got into learning and still a student. With that I have read a lot of articles about the Rotax engines and have read that the 915is would require a constant speed prop in order to maximize the HP and efficiency. The is a no-go for an LSA.


Every engine requires a constant prop if you want to maximize efficiency ...
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Re: How can I make an informed decision choosing between similar engines?

Postby drseti » Tue May 05, 2020 4:58 pm

Warmi wrote:Every engine requires a constant prop if you want to maximize efficiency ...


True. The 915 performs well with a fixed pitch prop, and even better with a constant speed! The only real drawback for an LSA is that it's a really heavy engine (at least, when compared to the good ol' 912ULS). Since all LSAs are weight limited, it may not be the engine you want. Even John Rathmell III, the Bristell dealer in Lancaster PA, said in an AOPA interview regarding the 915-powered Bristell, "it's too heavy."

(Trey, if you're lurking here, thanks for your frankness and honesty.)
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snaproll
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Re: How can I make an informed decision choosing between similar engines?

Postby snaproll » Thu May 07, 2020 1:13 pm

Over my 50+ years of flying, I have been behind Continentals, Franklins, Lycomings (including the GO-145), Volkswagens, Corvairs, P&W, Jacobs, Kinners, Rangers, Menascos, 36 HP Aeronicas, Hirth, Cuyuna, taxied a LeRhone (owner wanted me to get the full Castor Oil and tail-skid experience), and Rotax. The choice of engines depends on the application, weight per horsepower ratio, desired outcome and performance. All engines have their glitches, some more than others, but good practice is staying with what is proven reliable and has a supply chain to back it up. Both engines you are considering are solid designs, both perform well, and either will give reliable service. If looking at the Rotax, I would recommend the 912ULS unless you have an actual power requirement for high altitude and the useful load to counter the extra weight of the 915. I will defer to Paul Shuch regarding Rotax question based on his extensive knowledge and experience, plus he’s one sharp Professor.
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Re: How can I make an informed decision choosing between similar engines?

Postby fatsportpilot » Thu May 07, 2020 8:11 pm

Scooper wrote:You may want to check out the UL Power subforum on Zenith Aircraft Builders and Flyers. It's pretty active.

http://www.zenith.aero/forum/categories/ul-power-forum/listForCategory?categoryId=2606393%3ACategory%3A607665&page=1

I actually just found that and it's an awesome resource!


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