Engine selection (Rotax)

[sfGnome]

I’m seeking the wisdom of the elders (and youngers, for that matter), but most of all, I DON’T want to start a religious war!

 

I’m looking at various aircraft that can be registered in the soon-to-be(!!??) 760kg Class G. Some are designed around Rotax engines, which I’m completely comfortable with. I’ve flown behind the 100hp 912ULS fully loaded to the 600kg limit plenty of times and never felt like it was not powerful enough (although that’s possibly because I’ve never flown anything more powerful). The 915 and 916 are out of my budget, so I’m left with the choice of either the 912is (fuel injected) or 914 (turbo). 

I’m not a back country, huntin’ n fishin’ type bloke, so I’m not looking for STOL performance, and I like to fly high (more efficient, smoother, and I just like the view from up there - I’d go into space if someone gave me the gazillion dollars), so… the question for you all is, “is it better to have a more powerful engine that also works better at higher altitudes, or a lower power one that doesn’t have carbies with all the problems that they entail”? More importantly, why? Any other reasons for the selection that can be thrown into the mix are welcome too. 
 

Lighting blue touchpaper and standing back… 😛

[facthunter]

I'm a big believer in aircooled and hopefully being injected and direct drive. Lyc 0-235 is 115 hp simple and reliable and doesn't need an oil cooler. Should get you to 10,000ft without a supercharger.  Maybe Gen 4 Jabiru  6 cyl.  Looks to be a big improve on previous versions. Light and smooth.. Nev

[dlegg]

760 kg perfect for RV 9, if you want to fly high. Lyc 150 hp perfect for that.

[facthunter]

I haven't got details of how the Later Jab Flywheel is mounted yet.  Nev

[dlegg]

[facthunter]

Doesn't look to have changed much. Prefer the starter drive just behind the prop.   Nev

[skippydiesel]

2 hours ago, sfGnome said:

I’m seeking the wisdom of the elders (and youngers, for that matter), but most of all, I DON’T want to start a religious war!

 

I’m looking at various aircraft that can be registered in the soon-to-be(!!??) 760kg Class G. Some are designed around Rotax engines, which I’m completely comfortable with. I’ve flown behind the 100hp 912ULS fully loaded to the 600kg limit plenty of times and never felt like it was not powerful enough (although that’s possibly because I’ve never flown anything more powerful). The 915 and 916 are out of my budget, so I’m left with the choice of either the 912is (fuel injected) or 914 (turbo). 

I’m not a back country, huntin’ n fishin’ type bloke, so I’m not looking for STOL performance, and I like to fly high (more efficient, smoother, and I just like the view from up there - I’d go into space if someone gave me the gazillion dollars), so… the question for you all is, “is it better to have a more powerful engine that also works better at higher altitudes, or a lower power one that doesn’t have carbies with all the problems that they entail”? More importantly, why? Any other reasons for the selection that can be thrown into the mix are welcome too. 
 

Lighting blue touchpaper and standing back… 😛

It seems to me that you should start with a more detailed "mission statement"

 

Some thoughts/headings;

• Why do you want /need 780kg ie what is your projected load?
• Are you committed to RAA or is GA an option?
• What duration (fuel capacity/consumption)? 
• Projected flight levels (turbo or not)? - could even go to cabin pressurisation?
• Performance type - STOL - Blend - High Speed Cruiser?
• Stature of pilot/passenger(s) ie will you comfortably fit in the cockpit?
• If GA - is a night rating/capable aircraft required?
• Nose/tail wheel/ retractable?
• CS prop?
• BUDGET$$$$$$ (new or preloved?)?

 

 

 

[skippydiesel]

2 hours ago, sfGnome said:

 “is it better to have a more powerful engine that also works better at higher altitudes, or a lower power one that doesn’t have carbies with all the problems that they entail”? More importantly, why? Any other reasons for the selection that can be thrown into the mix are welcome too. 
 

Lighting blue touchpaper and standing back… 😛

 

I think you misunderstand the effect of altitude on a IC engine.

 

No matter the power at sea level, as the aircraft climbs, air density diminishes/air gets thinner. The effect is to reduce aerofoil (inc prop) & engine efficiency. The engine can be maintained at or near sea level efficiency, by adding a booster (turbo or supercharger) but even the best probably run out of puff eventually (Nev will know the actual altitude for most boosted engines). I have not been there (above 10kft) but I imagine a CS prop would be needed to delay aerofoil efficiency loses with altitude. Don't forget that your wings are also impacted by the thinner air.

 

As for carbs V Injected;

• Carburettors  are simpler, lower initial cost and for the most part trouble free but can be subject to icing, which can, under most VFR type flying, be managed. Standard Rotax carbs have no mixture control.
• Injected systems are far more complex, likely heavier, have a much  higher initial cost, more fuel efficient and less prone to carb ice. If a modern system, they generally require less pilot input, as a computer manages the engine for you.

 

I note you have mentioned Rotax 912 ULS, iS & 914 (turbo) - you should know that, in Australia, the ULS is by far the most common of the three, for good reasons.

• The iS is rated at the same 100hp but delivers better torque (for faster TO acceleration/shorter ground run and better CO), better fuel econamy and much higher acquisition cost. No experience, however I imagine a higher service/running cost, due to special filters/pumps etc .It is generally accepted that unless you are doing high (?) annual flight hours eg flying school, the additional cost acquisition does not justify the expense.
• 914 - very popular in the USA/Canada. Again a much more expensive/complicated/heavier engine for only 15 additional Hp. Popular in countries with high mountain ranges, as this engine will maintain its rated Hp to somewher around 14 Kft  (check with Rotax for actual figures) meaning it can probably go to  18-20 Kft. For the same airframe the additional Hp will get you shorter ground run, faster/steeper climb out. From my (limited) reading of Rotax Owner Forum they also have a significantly higher service requirement.

[dlegg]

Personally I think the carbie/magneto and coil ignition is the way to go, once running you are not relying on an electrical system to keep running. Electronic ignition and fuel injection is certainly a more efficient way to manage the engine but comes with its own requirements and associated maintenance and much higher install cost. How far do you want to fly, how much weight and really how much do you want to spend?

[sfGnome]

Thanks for the comments thus far. Keep ‘em coming! 😀 I will note that I’m only talking about engines - not the aircraft that they’ll go in. I’ve spent the last few years going through all the mission requirements and have narrowed the aircraft choice down to two (and no, I’m not going to drift my own thread by disclosing what those two are 😛).

[skippydiesel]

9 minutes ago, sfGnome said:

Thanks for the comments thus far. Keep ‘em coming! 😀 I will note that I’m only talking about engines - not the aircraft that they’ll go in. I’ve spent the last few years going through all the mission requirements and have narrowed the aircraft choice down to two (and no, I’m not going to drift my own thread by disclosing what those two are 😛).

Ah yes BUT the type of aircraft should be your mission objectives and the type of engine required to motivate the airframe to meet the objective(s) Not really practical to separate or select engine first & then airframe.

 

EG

 

Check out Rotax  912 ULS powered, two seat aircraft examples;

 

 http://worldrecordplane.com/  - high speed, economical,  two seater tail dragger (Cruise 175 knots)

 

https://www.foxbat.com.au/a22ls-foxbat - low speed STOL (Cruise 100 knots)

 

https://www.atecplanes.com.au/ - compromise performance (Cruise 134 knots)

 

 

 

 

 

[RFguy]

go with a big bore 912(low compression)  and a afterthought turbocharger.  Or if you are scared of heights and heat , just a 912ULS with big bore kit.

Jab Gen4 is OK, also ,  but not really cheaper than a rotax per hour.  so you might as well buy a medium lifed rotax if you have the cash.

If you have not much cash, you can get a 400-800 hour Gen2/3 Jabiru for peanuts  and rebuild it to new for 5k. 

 

BUT ! the aircraft nose weight requirement will drives things, also. Lycoming O-235 would be my first choice if nose weight was required.

 

 

Edited October 26, 2023 by RFguy

[skippydiesel]

13 minutes ago, RFguy said:

 

 

BUT ! the aircraft nose weight requirement will drives things, also. Lycoming O-235 would be my first choice if nose weight was required.

Which one (100-135 hp)?????

13 minutes ago, RFguy said:

 

 

 

[sfGnome]

1 hour ago, skippydiesel said:

Ah yes BUT the type of aircraft should be your mission objectives and the type of engine required to motivate the airframe to meet the objective(s) Not really practical to separate or select engine first & then airframe.

Definitely airframe first, which dictates the engine options. I’m an engineer, and I know enough to know that I DON’T know enough to start fiddling with what the designers settled on. One of my two selections specifies a particular engine, so no choice there. The other specifies the range of Rotax engines, hence my initial question.

[sfGnome]

22 minutes ago, RFguy said:

go with a big bore 912(low compression)  and a afterthought turbocharger.  Or if you are scared of heights and heat , just a 912ULS with big bore kit.

All I know about Rotax engines is what I can see on their website, so what is this big bore 912? an aftermarket mod?? Where do I go to learn about these (what I assume to be) non factory options? Thanks

[RFguy]

It's not unusual to find a 1000 to 1999 hour rotax you can afford which has plenty of good hours on it and put a big bore kit on, (yes aftermarket)   There are a couple of major players that have variations. Mark Kyle  might chime in.....

An inflight adjustable prop can also get you a bit more ability.... worth quite a bit of HP for TO and climb I think.

Read the heavy maintenance manual of the series, also (both the MML and the MMH )  to get some engine background.  If a rotax is specified then a Lyco  might be too heavy to go with a Lyco. depends.... There is plenty to learn, find your way around the internet, lots of enthusiasts doing big bores, turbos etc.  Much to learn for sure.  Of course, as soon as you go away from the out of box crated item, you are a test pilot, which doesnt bother me for certain uses, but just saying.

 

Edited October 26, 2023 by RFguy

[facthunter]

It depends on whether you want to fly or fly and Play with engines as well. IF both the engine and the Plane are very experimental you might face an overload of your time and knowledge. Life is short and you don't want to scare the $#1t out of yourself too often or you will chuck it eventually because it's all too much . The weather is increasingly becoming more limiting of sensible flying activity.  Nev

[RFguy]

here's a good example of a busy rotax...

https://www.facebook.com/groups/796324257055294/permalink/7003877179633273/?sale_post_id=7003877179633273#

 

[BurnieM]

The Rotax 912ULS and 912IS of similar vintages are pretty similar in climb and cruise preformance. The IS is more fuel efficient in cruise.

Is the 914 still an option ? Look at the 915 for a fair bit more in cost.

The power requirement/backup issues of the IS seem well known and allowed for and the 912IS has been flying for 12 years.

 

The both versions of the 912 seem ideally suited for either 600 or 760kg.

Are you really looking at high attitudes ? A lot of these aircraft have limits of 13,000ft (ish)

 

The Sling 2 is both an LSA at 600kg and a VLA at 700kg and was flown around the world in 2009 at 900kg each day for 40 days. It used a 912ULS.

With this aircraft going to class G this will now (soon?) allow you to take 2 people with full tanks.

 

If you want to fly both engines then Fly Illawarra have an Evektor Sportstar with a 912ULS and a Evektor Harmony with a 912IS.

Edited October 26, 2023 by BurnieM

[skippydiesel]

sfGnome - without  budget & mission information from you, its all just talking fluff (being polite)

[sfGnome]

16 hours ago, RFguy said:

It's not unusual to find a 1000 to 1999 hour rotax you can afford which has plenty of good hours on it and put a big bore kit on, (yes aftermarket)   There are a couple of major players that have variations. Mark Kyle  might chime in.....

 

Thanks Glenn. I’ve dug around a bit and I can see why you are suggesting a big bore kit. However, when I read your conversation with Mark on this subject (https://www.recreationalflying.com/forums/topic/37555-rotax-912-minimum-things-that-need-to-be-done-for-specific-hours-up), I ran screaming from the room. 🫣😝 I have expertise in all sorts of odd subjects, but engines aren’t one of them, so I the absence of knowledge, equipment and/or knowledgeable mates close by, I think that I have to stick to shop bought. I’ve budgeted for a new motor, so that aspect doesn’t worry me.

 

[sfGnome]

10 hours ago, BurnieM said:

The Sling 2 is both an LSA at 600kg and a VLA at 700kg and was flown around the world in 2009 at 900kg each day for 40 days. It used a 912ULS.

Good point. The airframe designer in question recommends the 912 for 600kg, and the 914 for 760kg (but they demonstrate with a 916!), so I guess it comes down to how spritely we want it to climb. I’ll bet the sling at 900kg lumbered a fair bit. 

[sfGnome]

2 hours ago, skippydiesel said:

sfGnome - without  budget & mission information from you, its all just talking fluff (being polite)

Skip. I’ve said that I can stretch to a 914, that I’m not a STOL jockey, I like to fly high (high in this instance is <10k ft) and that the airframe in question can take to full range of Rotax engines (and only those engines). All I’m asking is, in the opinion of all our friends here, is it better to go for more power with the downsides of ice/blockages/tuning (914), or no carbie and better fuel figures with the downside of less power (912is). Also, are there other pros and cons that I haven’t listed? That’s all.

[facthunter]

It won't hurt you to go to the Vans Aircraft site.  Nev

[skippydiesel]

12 minutes ago, sfGnome said:

Skip. I’ve said that I can stretch to a 914, that I’m not a STOL jockey, I like to fly high (high in this instance is <10k ft) and that the airframe in question can take to full range of Rotax engines (and only those engines). All I’m asking is, in the opinion of all our friends here, is it better to go for more power with the downsides of ice/blockages/tuning (914), or no carbie and better fuel figures with the downside of less power (912is). Also, are there other pros and cons that I haven’t listed? That’s all.

There are some aircraft with STOL like characteristics than also have high speed cruise eg Pipistrelle Virus SW and ATEC Faeta - both have a sub 30 Kn stall and mid to high 130 knot cruise, on Rotax 9 engines. The Faeta offers 80 / 100 / 115 Hp engine options - I think the Virus 80-100 Hp (but there has been at lest one 115  Hp variant). Low stall speed is a great safety feature - you are more likely to walk away from a crash, gives you more landing ground options (outclimb obstacles and will glide further in an engine out scenario) - down side is usually sensitive to turbulence. I would also speculate both these aircraft should have good altitude handling, curtesy of longer wings & larger control surfaces, than most in their class.

 

The 914, as with most turbos, will give you slightly better TO/Climb performance curtesy of the extra few gee gee's but where it really shines is in delivering rated power to a much higher altitude, than the non boosted variants = higher ground speed..

 

Nev will know, I wold speculate that to get the best from the 914, at altitude, you may have to consider a CS prop (more $$$)

 

Also bear in mind, the extra power comes at the cost of fuel consumption/hr, offset by higher ground speed, which may impact on range ie you will need to do some realistic benefit analysis.

 

Are you looking at a new (factory/homebuilt) aircraft or considering retrofitting an existing one, with your engine of choice?