How does Rotax do it?

[Tarheelpilot]

"Those who can not remember the past are condemned to repeat it." George Santayana 1906
"Give me Turbines or give me death!" Patrick Henry 1775

Give me a turbine designed for the job. Sure. Give me a modified apu engine. No thanks

 

[Tarheelpilot]

I didn't say everything with 150HP is the same, I said "150HP is 150HP", no matter the source in response to "rev so high yet make so little power". If we're talking about revs needed to make 150HP, I don't see what difference is makes what an engine does to make it happen. Now, if the complaint were about delay in spool up, or peaky power bands, etc. then there's a discussion to be had. But I don't see how something can make "so little power" if we're under the assumption that it is making the same HP as any other engine rated the same. The only caveat would be the weight of the powerplant, or HP/weight in the same airframe.

It’s not just HP. It’s torque as well. 150 hp making less torque means less thrust at the prop. Without those numbers included in the conversation we are not having a real discussion. I don’t know what the numbers are by the way and I’m not saying the Rotax is a lower performing engine. Just saying you haven’t fielded a complete argument.

 

[Tantalum]

Give me a turbine designed for the job. Sure. Give me a modified apu engine. No thanks

There's a certain irony in your statement, as far as I know most APUs are actually modified aircraft engines... so turning "a modified APU engine" back into a jet engine actually returns it to its original purpose. Take for example the Honeywell (formerly Garrett) TPE331 Turboprop https://aerospace.honeywell.com/en/...J7-05c33Zoib8jQbUdmpb6bSFIvVpLqMaAsQsEALw_wcB which later became the Honeywell 331 APU https://aerospace.honeywell.com/en/learn/products/auxiliary-power-units/331-series-apus

And frankly I'd still feel safer behind "some" iteration of a gas turbine than behind any Lyco or Conti.. plus with an APU all the "but hybrid planes make sense!!" fanatics out there should be absolutely jazzed about a turbine's thermal efficiency vs a piston engine (compare Brayton cycle vs Otto cycle) for peak cycle efficiency

 

[Tantalum]

150 hp making less torque

...power is the ultimate measure of the, well, power, an engine makes. With the correct gearing and propeller design (isn't slower more efficient anyway, I believe the PT6 prop RPM is around 1,700 typically, 1,900 redline) you can still get proper torque

 

[Tantalum]

One thing I didn't realize is that turbines have FANTASTIC torque properties.. awesome

 

[Tarheelpilot]

There's a certain irony in your statement, as far as I know most APUs are actually modified aircraft engines... so turning "a modified APU engine" back into a jet engine actually returns it to its original purpose. Take for example the Honeywell (formerly Garrett) TPE331 Turboprop https://aerospace.honeywell.com/en/...J7-05c33Zoib8jQbUdmpb6bSFIvVpLqMaAsQsEALw_wcB which later became the Honeywell 331 APU https://aerospace.honeywell.com/en/learn/products/auxiliary-power-units/331-series-apus

And frankly I'd still feel safer behind "some" iteration of a gas turbine than behind any Lyco or Conti.. plus with an APU all the "but hybrid planes make sense!!" fanatics out there should be absolutely jazzed about a turbine's thermal efficiency vs a piston engine (compare Brayton cycle vs Otto cycle) for peak cycle efficiency

APUs are designed to run at constant rpm all the time. They are not intended to be primary propulsion. Like i sad before. Design a turbine for the intended use and I’m all for it.

regarding the torque conversation yes reduction gearing does Change torque. As I said before the argument for or against less mainstream engines such as the Rotax is pointless without having good data to reference and just throwing around hp numbers don’t mean squat. I already said I don’t have the data and I didn’t express an opinion one way or the other regarding the engine.

 

[Tantalum]

APUs are designed to run at constant rpm all the time. They are not intended to be primary propulsion. Like i sad before. Design a turbine for the intended use and I’m all for it.

and actually, my quip about hybrid aircraft was not entirely tongue-in-cheek.. with the efficiency of a turbine, especially at higher altitudes, and the storage capacity of a battery, there might be some potential case for a plane that uses battery to get to cruise and an APU for cruise range and battery top off.. with electric propellers, you wouldn't even need the propellers turning while you pick up IFR clearance etc

less mainstream engines such as the Rotax

Sure. And I'm not advocating that everybody goes and only put the rotax in their planes.. but it's cool to see that somebody has a viable product making modern engines and pushing the envelope

 

[Tarheelpilot]

and actually, my quip about hybrid aircraft was not entirely tongue-in-cheek.. with the efficiency of a turbine, especially at higher altitudes, and the storage capacity of a battery, there might be some potential case for a plane that uses battery to get to cruise and an APU for cruise range and battery top off.. with electric propellers, you wouldn't even need the propellers turning while you pick up IFR clearance etc


Sure. And I'm not advocating that everybody goes and only put the rotax in their planes.. but it's cool to see that somebody has a viable product making modern engines and pushing the envelope

Hybrid... one day. When batteries weigh a lot less than they do now.

I don’t see Rotax pushing any envelopes. I’ve intermittently flown behind a rotax and unless I was building a kitfox or something similar in size and performance goals they don’t interest me at all.

 

[DaleB]

less mainstream engines such as the Rotax

Kinda funny, really. It seems that right now, for new aircraft in the sub-150 HP range, the Rotax 912 series (912 UL, ULS, iS, 914, and so on) pretty much are the mainstream. I wonder how many new O-200s have been installed in the past few years since the demise of the SkyScratcher?

 

[Tarheelpilot]

Kinda funny, really. It seems that right now, for new aircraft in the sub-150 HP range, the Rotax 912 series (912 UL, ULS, iS, 914, and so on) pretty much are the mainstream. I wonder how many new O-200s have been installed in the past few years since the demise of the SkyScratcher?

What’s your point. They dominate the market for little engine going in little airplanes primarily used for one or two people to not be on the ground... doing pretty much nothing else. They don’t haul freight. They don’t carry sick people to doctors. The can get you and one of your buddies to a pancake breakfast.

 

[Sac Arrow]

Gearing eliminates any difference between piston and turbine torque differences. Gas turbines are used successfully in tanks and smaller ships. They perform great, but just aren't efficient in those applications. In fact they are very inefficient. That is why most modern cargo and tanker ships use massive direct drive two stroke diesel engines.

Turbine engines, and jets, operate most efficiently at high altitudes, where volumetric efficiency is not important.

 

[hindsight2020]

The T-38's GE J85 series engines were originally designed for use on decoys, essentially de facto cruise missiles. So the notion engines not initially designed for "throttling" operations make bad primary propulsion engines is not necessarily a given.

Fun factoid, the original replacement of the T-33, what was to become the T-38, was originally planned to be powered by F-4 engines! That would have been a monster. Basically an F-20. I dig it. Instead we got two puny cruise missile engines. Oh well. Makes good energy management trainer. Not really a 5th gen emulator.

The T-7A has 404 power. That thing is a monster, and should be able to turn better with more wing area. If Boeing doesn't completely eff up the fielding and production that is.....

 

[Dana]

Gearing eliminates any difference between piston and turbine torque differences...

...or between high revving engines like Rotaxes and slow turning old engines like Lycomings and Continentals of the same HP.

The difference is that the faster turning engine will usually be lighter for the same HP.

 

[Sac Arrow]

...or between high revving engines like Rotaxes and slow turning old engines like Lycomings and Continentals of the same HP.

The difference is that the faster turning engine will usually be lighter for the same HP.

Yes.

 

[Warmi]

What’s your point. They dominate the market for little engine going in little airplanes primarily used for one or two people to not be on the ground... doing pretty much nothing else. They don’t haul freight. They don’t carry sick people to doctors. The can get you and one of your buddies to a pancake breakfast.

Which is what 90% of so called GA activity is all about - getting up there by yourself or with your buddy for no particular reason - so what’s your point again ?

 

[Tarheelpilot]

Which is what 90% of so called GA activity is all about - getting up there by yourself or with your buddy for no particular reason - so what’s your point again ?

My point is for anything other than pleasure flying low and slow in something that doesn’t require a medical to fly rotax doesn’t offer an alternative to engines designed over 60 years ago. Where did you get 90% of ga is goofing off in a two seat that’s barely bigger than an ultralight ?

 

[DaleB]

What’s your point. They dominate the market for little engine going in little airplanes primarily used for one or two people to not be on the ground... doing pretty much nothing else. They don’t haul freight. They don’t carry sick people to doctors. The can get you and one of your buddies to a pancake breakfast.

So, if you’re not doing any kind of flying that is a candidate for a sub-150 HP engine, why are you whining about how Rotax engines do or do not do what they do?

I mean, I did recently see a twin here flying aerial mapping with a pair of Rotaxes, but yeah, that’s the exception rather than the rule. But there’s a crapload of them out there flying, and they’ve proven themselves solid and reliable. The fact that Rotax doesn’t make turbines doesn’t negate that. And turbines are what get sick people to doctors and haul freight.

 

[Warmi]

My point is for anything other than pleasure flying low and slow in something that doesn’t require a medical to fly rotax doesn’t offer an alternative to engines designed over 60 years ago. Where did you get 90% of ga is goofing off in a two seat that’s barely bigger than an ultralight ?

No, 90% of GA is pleasure flying ( goofing off ) - regardless if it is a a 120 knots two seater or a 180 knots four seater.
But it doesn’t matter - the mere fact that you mentioned “ doesn’t require a medical” is enough to stop responding to your post .. your only point is that you don’t have a point except to ***** about “ people flying Rotax equipped planes without medicals “ and how that somehow “cheapens” the whole experience for all “real pilots” out there ... whatever.

 

[Tarheelpilot]

No, 90% of GA is pleasure flying ( goofing off ) - regardless if it is a a 120 knots two seater or a 180 knots four seater.
But it doesn’t matter - the mere fact that you mentioned “ doesn’t require a medical” is enough to stop responding to your post .. your only point is that you don’t have a point except to ***** about “ people flying Rotax equipped planes without medicals “ and how that somehow “cheapens” the whole experience for all “real pilots” out there ... whatever.

that is pure fiction

I never said, implied or thought that rotax cheapens Aviation

your 90% claim is at best anecdotal and is no where near factual

I never bitched about people without medicals flying rotax powered aircraft. In fact I hope to retire one day and be one of those people myself. To be very specific I’m hoping I can be another @Capt. Geoffrey Thorpe and roll around goofing off, having fun and goto a lot of pancake breakfast events In my experimental with a rotax.

My opinion that while engaging in those really fun and individually rewarding activities I will not constitute 90% of GA and the engine on my airplane will not be pushing any technical envelopes does not in any way constitute the disdain and arrogance you have accused me of having.

thanks for playing @Warmi but you fail. Do you always resort to personal attacks when people express opinions you don’t like?

 

[Tarheelpilot]

So, if you’re not doing any kind of flying that is a candidate for a sub-150 HP engine, why are you whining about how Rotax engines do or do not do what they do?

I mean, I did recently see a twin here flying aerial mapping with a pair of Rotaxes, but yeah, that’s the exception rather than the rule. But there’s a crapload of them out there flying, and they’ve proven themselves solid and reliable. The fact that Rotax doesn’t make turbines doesn’t negate that. And turbines are what get sick people to doctors and haul freight.

I’m not whining. I’m merely saying that putting out the power a rotax makes, reliably is not that hard to do and the darn things are not special. That is all. Why on earth is this so hard to grasp? I’m not saying rotax is **** ok. They just are not special. Who gives a crap how high their rpm is in relationship to a long stroke big bore engine? Why is that remarkable in any way?

There are a lot of piston engines working in commercial services. That is not just for turbine equipment.

 

[Capt. Geoffrey Thorpe]

To be very specific I’m hoping I can be another @Capt. Geoffrey Thorpe

That is a first. Ever.

 

[KSCessnaDriver]

I’m not whining. I’m merely saying that putting out the power a rotax makes, reliably is not that hard to do and the darn things are not special. That is all. Why on earth is this so hard to grasp? I’m not saying rotax is **** ok. They just are not special. Who gives a crap how high their rpm is in relationship to a long stroke big bore engine? Why is that remarkable in any way?

There are a lot of piston engines working in commercial services. That is not just for turbine equipment.

And there are Rotax engines working in commercial service, what's your point.

 

[Tarheelpilot]

And there are Rotax engines working in commercial service, what's your point.

The entire premise of this thread was that rotax was somehow special and there was amazement at how they do what they do.

My point is there is nothing special about the rotax. No mysteries of engineering were applied to create rotax. That’s my point.

 

[Tarheelpilot]

At the risk of thread drift...

Isn’t the whole point of this community to provide a place for us to have conversations about aviation related topics? A place to express our opinions and have discussion?

so I am perusing the threads and see this one. My reaction to the conversation is “rotax engines are fine but nothing special. Nothing mysterious or amazing about them. So I say just that... rotax is nothing special. I throw in the idea that if we are going to discuss engine performance that torque needs to be part of the conversation.

the response to my OPINION has been so welcoming and warm. Some of you people straight up suck.

 

[Tarheelpilot]

That is a first. Ever.

I hold you in a high regard based on what you have shared about yourself here over the years.

 

[DaleB]

At the risk of thread drift...

Isn’t the whole point of this community to provide a place for us to have conversations about aviation related topics? A place to express our opinions and have discussion?

so I am perusing the threads and see this one. My reaction to the conversation is “rotax engines are fine but nothing special. Nothing mysterious or amazing about them. So I say just that... rotax is nothing special. I throw in the idea that if we are going to discuss engine performance that torque needs to be part of the conversation.

the response to my OPINION has been so welcoming and warm. Some of you people straight up suck.

OK, I apologize if I came across as a dick in my last response. And, I may very well be guilty of conflating some of your responses with other peoples'.

To your point, though, my opinion is that right now -- Rotax actually kind of is something special, at least in some ways, and in the context of their actual space in the market. That fact is borne out by the fact that essentially no new commercial designs in the past 10 years or so that could have used the only "old school airplane engine" competition that's still available as a new engine (unless I'm mistaken), the O-200. Certainly none that were successful. I think a major reason for the utter failure of the SkyCatcher was their selection of the O-200 instead of the lighter Rotax.

Rotax has been producing efficient, light weight engines for aircraft for about 35 years now, so it's not like they're something new and untested. They produce performance as good as an O-200 with less than half the displacement. They've managed to design and produce a solid, reliable gearbox that lets them use more modern methods of producing power than other manufacturers. There's no problem with carb ice, no need to fiddle around with mixture, no magnetos to wear out, no $40 apiece spark plugs, and they're perfectly happy running pump gas I can buy for less than $3 a gallon. The down side is -- they're really, really proud of those engines, and they cost a bundle. That fact is giving some other manufacturers an opening to start trying to compete. D-motor, Aero Momentum, and others are playing catch-up, but things are looking promising.

To compare them to larger piston engines and turbines -- well, there is no comparison, because they're completely different powerplants designed for completely different jobs. That would be like complaining that the engine in my pickup is a POS because it's not a diesel locomotive, and therefore can't pull a coal train through the Rockies.

 

[Capt. Geoffrey Thorpe]

 

[will44s]

Playing devils advocate. People keep mentioning the inefficiency of gas turbines down low. Does that have to be the case? Why would electric power plants use them. Seems with the quantity of fuel consumed they'd pick the most efficient option.