Rotax recommended RPM

[Roger Lee]

Hi Tom,

 

Running WOT at 5200-5300 all the time is what cracked crankcases especially in the pre- July 2006 engines and earlier. That was one thing that prompted Rotax to re-design the crankcase. You know you have one of those cases if you only qualify for the 1500 hr. TBO and can't get the 2000 hr. TBO unless you change the crankcase. Early on FD used to set the WOT rpms at 5200 rpm and Rotax and I did too warned them about cracking cases with low rpms. This was a fact (low rpm's cracking cases) before FD started selling the CT2K in the US. That's why FD now sets them all at least 5500+ rpm WOT from the factory. You can cruise at 5200 - 5300 rpm so long as the pitch is such to get over 5500 rpm WOT to reduce the stress on the case and associated parts. I have pictures and have seen others with cracked cases. There have been a few posted on the Rotax-Owner website.

This engine and others depending on the engine have definite design run specs and were built with that in mind. Some engines can run at low rpms without undue stress 24/7 and they are designed and speced to handle that, but too many here forget that some engines including the Rotax 912 series was specifically designed to run at certain rpms. When I worked for Atlantic Richfield we had many engines and turbines that had to be run within a narrow rpm range or damage would occur. Some were as high as 50K HP compressors. We even had to use vibration sensors on them to set them at the proper rpms.

 

Fred,

The over all design of the parts is conventional, but not the design of the run specs and the way these components need to function as a total unit.

Low engine speeds usually mean under 5000 rpm for cruise, but that means long term and extended run times. Not intermittent use like landing approaches or the occasional sight seeing and training. You don't need full throttle continuous operation, only a pitch that reduces the stress which will allow you to get 5500-5800 rpm WOT and allows the engine to not be under torqued, high stress, high vibration and low HP situation for long periods during its life.

Think of a lever on a balance point and a 100 lbs on one end. If I move the balance point away from the 100 lb weight too far and close to the end I need to push down I eventually can't left the weight any longer and I have to strain to try and move it. Too much pitch and not enough torque and HP to turn the prop as it was designed. If I move the focal point closer to the 100 lb weight it gets easier to lift and eventually really easy to lift because I have all the leverage on my side. At that point that's like setting a ground adjustable prop to get 5800 rpm WOT on take off. Lots of leverage and the engine is working easy with lots of HP, but the engine will now over speed at WOT in level flight. Move that leverage point back like adding in a little pitch and you'll find a balance point that makes everything work easy and it;'s useful for all flight characteristics since you can only change the prop on the ground you want that balance.

 

My prop pitch is courser at my 3500'MSL flying than Ed's is at his 10K-13K' MSL ( his pitch is less) More leverage and better balance for his specific situation.

[FredG]

Thanks. Yes, in fact, I understand leverage and mechanical advantage. I also understand that when the propeller pitch is too coarse and WOT RPM is below 5500 in level flight that the crankshaft, connecting rod bearings, main bearings, and cases will be stressed. That was not my point nor my question (please see the last sentence of my previous post).

 

My question is, how is it possible that an engine that can operate continuously at 5500 RPM at WOT in straight and level flight be harmed by continuous partial throttle operation at, say, 4800 RPM in straight and level flight?

 

To use Rogers's analogy, this is akin to putting the fulcrum of the lever at a position that allows safe and comfortable lifting of the 100 pound weight and then reducing the weight without moving the position of the fulcrum. Hard to see how that can harm anything in any engine, including the 912.

 

Fred

[Jim Meade]

Here it is Jim

 

 

Thank you.

 

 

 

 

And what Rotax or FD document is that excerpt from, please? And my next question will be how do we know that the cited recommendations supercede other Rotax and FD published information which contradicts it?

 

 

 

 

 

 

[Ed Cesnalis]

I clearly remember this doc from 2005 or 2006. I see it with a ROAN watermark now I'm not sure if that was the case then.

 

I don't remember the source but it doesn't look like a stand alone document but rather part of something published on the 9 series.

 

Beyond Roger's comments notice that the doc supports the 5,500 speed by saying it is where ignition, carbonation and valve timing all work the best. Sounds like a sweet spot.

[Roger Lee]

To answer your last question the engine doesn't need to be at 80% of max all the time. These numbers are meant to represent long term and extended time like when in cruise which is where you spend more time than any other as a rule during the engine's life and doesn't mean intermittent use.

We have two important issues here, internal vibration levels and more importantly the torque/HP required to turn the engine. If you have an engine that has little resistance generated against it then the load stress will be less and it can usually run at a more varied rpm range for the life of the engine. If the force needed to run the engine and needed torque value is high because of the type of loading then it will need higher rpms + HP to over come that load and turn without undue stress and vibration. If you read the manuals you will see that peak torque isn't reached until 5000 rpm and that's based on the prop being able to reach 5800 rpm WOT. As that torque goes past the 5000 rpm mark it will start to decrease some, but the HP to turn under those conditions of load continue to climb. Many engines have rpms that a Mfg warn to stay away from because of vibration and the Mfg did not build it to run there. Undue stress and vibration on a Rotax has not only cracked cases, but popped heads of of valves and other valve damage, bent rods, caused case and head leaks, caused piston damage.

It's probably the same for Continental and Lycoming. They give an rpm range to run for the life of the engine because it was designed that way and experience has proven that any thing else will over time cause issues and running too high or too low isn't good and sooner or later they end up with big bills to pay. We have all heard of these instances over the years. I hear about Rotax ones all the time because I specialize in the Rotax.

You may not hear about these as an owner just because of limited exposure, but it doesn't mean it isn't there. Every mechanic on this site and all the A&P's with 20+ years that attended a Rotax school went in thinking what can they teach me I don't already know about an internal combustion engine and how can it be any different, but they all walk out realizing there is a lot of info they didn't even know existed and they were shown first hand why it's different and why it is the way it is.

Why not go to a Rotax school? Each one is an eye opener and teaches you more into the inner workings. There are three. Service ( not too deep, just plugs, oil and carb sync, just the basics for service. In Line maint and Heavy Maint. you start getting deeper into the engine and the why's. Taking the first two schools (4 days total) would probably answer every question you will ever have as an owner.

[Tom Baker]

Roger, I'm not saying to run it at 5200 and WOT. I'm saying a pitch setting for that and running 5000 RPM or 75% power. Fuel burn will be reduced by about 25%, and he will still be with Rotax limits.

[Roger Lee]

Hi tom,

 

Actually I have run those test in real life. Here is, but one example. I do a lot of testing and research for my education and Mfg's that never make this forum due to their confidentiality and then write it up for their submission.

 

I had to pick up a CT at another close by airport. I took my plane. My plane was pitched to get 5600 rpm WOT flat and level. The other plane was pitched to get 5200 WOT flat and level right from FD. First thing was In the other plane I could not keep up with the guy flying my plane. He out climbed me something terrible and distanced himself from me so bad he had the throttle way back for me to catch up. Then I was flying at 5200 rpm WOT and my plane was down around 4650 rpm to even the speed. My fuel burn at 5200 and 5000 rpm was 1.5 - 2.0 gals worse than my plane. Fuel burn increases when the pitch starts to get too course regardless of the lower throttle setting after that. The engine is just working harder at that setting. The reason is the Rotax I was flying was over pitched on the prop so I had to use more throttle just to keep up which cause me a lot more fuel burn and I couldn't cruise as fast. It didn't have the HP and torque to work at an easy pace. I get this very question at least 3 times a week on the phone. You can try this test yourself and it will yield the same results. I have tried different pitch combos between 5000-5800 and different rpms for cruise at those settings. It the same in all of those lower rpm WOT pitch settings. I explained this to the owner and he let me change his prop to get around 5600WOT. He came back after a test flight and said he thought he had a new engine and he'd never go back to lower pitched rpms again. I have never in all my years had a single person want to go back after an prop adjustment when you took them from a lower WOT rpm like 5100-5300 rpm up to around 5600 rpm WOT. Charlie tango can tell you his experience with this very thing 5 years ago from a discussion very much like this one. His was even worse due to his high elevation airport.

[Roger Lee]

Hi Fred,

 

I was in the shower and thought of a good analogy with as you said a conventional engine. I don't know how old you are, but back when I was younger we had a 3 speed on the column manual shift cars.(4 speed sticks and manual shifts today are included) Nothing special about 50-70 year old car engine. In each auto operations manual it had operating speeds (mph) or even a tach for a given gear selection. Some thing like this: 0-15 mph - 1st gear, 15-40 mph - 2nd gear and 40+ mph 3rd gear.

So what happened when you shifted into 3rd at 15 mph? The car shook and bucked. It was under torqued, lacked the HP at that range and stressed the engine and gearbox. Nobody ran a car like that. They all used more or less the appropriate recommended mph or rpm range if so equipped with a tach for shifting and running. No different for us with our Rotax.

 

 

As a side note:

I have a friend with a CT that just bout a Bombardier Spyder motorcycle. It has a Rotax engine. I was looking at it yesterday and it has recommendations for shifting and running rpms . It even says to apply thermal conducting paste on the spark plugs that are NGK DCPR9E plugs. Ours are DCPR8E plugs and it is also a 100 HP, 998 CC 2 cyl. engine and it can rev to 9500 rpm. The maint manual has many similar things to do as our aircraft engine. Two very different engines with similar run and maint traits .

[Ed Cesnalis]

To continue your analogy.

 

Today I drive an Audi with a high performance engine and it has a 6 speed gearbox. With higher performing motors smaller RPM ranges are desirable. A car cannot run at a fixed RPM it has to run in a range in order to accelerate. An airplane can run at a fixed RPM most of the time and doesn't need to be operating in a range like a car.

 

A high performance aircraft engine can opt to compromise less and design to run best in a narrow range or even a specific speed like 5,500

[FredG]

Roger,

 

Sadly, we are not communicating. You are identifying errors in points that I am not making and answering questions that I am not asking. I have repeatedly said that I was asking about the situation where less than full power was used to run the engine at less than 5000 RPM.

 

Yes, I am old enough to remember column shifts. And, yes, I understand your example about lugging an engine under power. I suspect that we agree that, as power output is held constant and RPM decreases, the power delivered by each power stroke increases, thereby increasing the "stress" on an engine. So, yes, it is clear to me that an engine can be damaged by lugging it at full (or excessive, for that situation) throttle. I am not asking about such a situation.

 

I still see nothing that says that continuous running of a 912 at 4500 RPM, or 4800 RPM (or other similar speeds), at partial power is a risk to the engine.

 

Please see my use of your lever-arm analogy and my suggestion that the situation is analogous to a reduction of the weight to be lifted, rather than a change in the position of the fulcrum.

 

Fred

 

ps. The ROAN document provided by Charle Tango is from http://www.rotax-owner.com/pdf/ROAN%20FAQs.pdf. It is hardly an authoratative technical treatise.

[Jim Meade]

I clearly remember this doc from 2005 or 2006. I see it with a ROAN watermark now I'm not sure if that was the case then.

 

I don't remember the source but it doesn't look like a stand alone document but rather part of something published on the 9 series.

 

Beyond Roger's comments notice that the doc supports the 5,500 speed by saying it is where ignition, carbonation and valve timing all work the best. Sounds like a sweet spot.

 

I found the document you cited on the Rotax Owner Assistance Network, www.rotax-owner.com\ I didn't find a date for it (so it could have been pre-2006 engine change.) It does not give any references for it's assertions. We don't know who wrote it.

 

"Rotax Owner is about "EDUCATION" via our associated independent Rotax Training Organizations (iRTO) which meet or exceed strict Rotax authorization guidelines assuring the most comprehensive technical training available for the Rotax Aircraft Engine world wide"

 

What does this mean? Does Rotax authorize this outfit to speak for it when it comes to operating practices? ROAN claims to be a factory approved website. I don't see where Rotax says that. Rotax has the www.flyrotax.com site that provides information on their engines. There is also the BRP site www.brp-powertrain.com. Neither of them says go to ROAN to learn how to operate the engine.

 

As far as I can see, we have an article written by an anonymous author with no apparent Rotax backing that gives views on how to operate the engine. I do not accept that as authoritative absent more genuine Rotax confirmation.

[Roger Lee]

Hi Fred,

Sorry I'm missing your point and I see what you are saying now. This is one of the topics taught in the Rotax schools. There are SB's that talk about the running at low rpms and that they are not preferable. You'll have to go back and look for them. Any Rotax trained and operating mechanic should be on the same page for this too. All I can tell you is the manuals don't have it all and classes are the only way other than a Rotax trained mechanic to get the info. The Service class should be an absolute must for any Rotax engine owner. Talk to one of these shops: Leading Edge Airfoil, Lockwood Aviation tech, CPS ( Kevin Kane). These are distributors.

[FredG]

I am hoping to take the Rotax classes. I'm sure they are very useful.

fg

[Duski Don]

G'day Guys,

 

Good to see that the 'robust discussion'(see Roger I've resisted using 'battle')is continuing.

 

My pitch is adjusted by a hydraulic system activated by a lever in the cockpit which can be locked into 8 different positions. Position 1 being full fine and 8 being full coarse.

 

Went and flew my aeroplane on Friday and found the following:

 

I can achieve 5800 RPM on take off and climb in the No. 2 setting I didn't bother with recording the MP as the Rotax manual doesn't indicate a preferred number it just says WOT.

 

In cruise on No.5 setting and altitude 3500' I can achieve 5800+ RPM with WOT.

At 5400 RPM I get 24.3 MP.

At altitude 5500' & 5400 RPM my MP is 23.1

At altitude 5500' & 5500 RPM my MP is 23.6

 

In cruise on No.6 setting and altitude 3500' I can achieve 5540 RPM with WOT.

 

Am I not getting it or am I right in saying you can't do much about MP if the main aim is to pitch the prop to achieve around 5800 RPM WOT?

In my case the coarsest setting in cruise that gives me 5800+ RPM is No.5 on the control lever.

 

Any and all feedback welcome.

 

Thanks,

Duski Don (Briggs)

[Roger Lee]

Hi Don,

 

Thanks you just demonstrated my point for the Rotax and what Rotax says about the MP use with the Rotax in our specific situation. That the MP for you or a CT with an in flight adjustable prop really isn't necessary. If you want to use it go for it, but you would need to have a chart to give you all the numbers.

 

Don's Quote:

"I can achieve 5800 RPM on take off and climb in the No. 2 setting I didn't bother with recording the MP as the Rotax manual doesn't indicate a preferred number it just says WOT.

In cruise on No.5 setting and altitude 3500' I can achieve 5800+ RPM with WOT.

At 5400 RPM I get 24.3" MP.

At altitude 5500' & 5400 RPM my MP is 23.1"

At altitude 5500' & 5500 RPM my MP is 23.6 "

 

In cruise on No.6 setting and altitude 3500' I can achieve 5540 RPM with WOT.

Am I not getting it or am I right in saying you can't do much about MP if the main aim is to pitch the prop to achieve around 5800 RPM WOT?

In my case the coarsest setting in cruise that gives me 5800+ RPM is No.5 on the control lever". (end quote)

You can see from your figures there was only about 1" of MP difference, so one easy thing to do is to have a fuel flow meter and a good air speed gauge and cross check these two when you got the best fuel economy verses speed. At the higher rpms you will use more fuel, but don't get the extra speed you may think so throttling back a tad will give better fuel economy with a very small loss in speed.

In take off you only need to pitch for the 5800 WOT in climb. You nailed this one. Who cares about the MP.

In cruise pick a number on the pitch selector at the altitude you are at to get at least 5500-5800 rpm WOT. If you like to fly WOT then use the position for 5500 WOT in flat and level WOT cruise. If you want to unload the prop a little and get a little better fuel economy then pitch it for around 5600-5700 WOT then pull the throttle back to between 5100-5300 and that's a personal choice too. To say I want to cruise at 60, 65, 68, 70% power will take a chart or some figuring and it can be ever changing with different settings. I don't know anyone that wants to fly by percentages, but you can find them in the manual, but then you would need to fly exactly the Rotax numbers and rpm to have the same % numbers. Some of the numbers in the manual are paper numbers and not actual in flight tested numbers and they are based on things being just right.

Just like you pointed out Rotax actually basis there pitch and cruise settings on the finial WOT setting and the HP to rpm %, torque, fuel use and speed numbers will change with different WOT pitch settings.

The whole point of this was that although MP can be useful under certain circumstances it isn't necessary to fly with the Rotax engine because they want you to be able to achieve 5500-5800 rpm WOT and you can figure that out just by flipping the selector switch and looking at the WOT rpm. I haven't heard of a single person in another country that I have talked to with an in flight adjustable prop that uses MP to fly or set his prop pitch selector, as you stated it's all rpm based then toss in a little personal preference.

 

[Duski Don]

Thanks Roger,

 

I'm off across the country in a coupla' weeks so I'll have plenty of time to monitor speed versus fuel flow etc.

 

Thanks again to you and all the guys that responded to my post.

 

Duski Don

[Roger Lee]

Hi Don,

 

Have a good trip and keep us up to date. No battles here just friends exchanging educated thoughts that help keep the old timer cobwebs away.

 

Try these two settings. Pitch to get 5500 WOT in cruise and fly there at WOT and check the speed and fuel use. Then try one and pitch for around 5700 WOT and then reduce throttle to 5100 then 5200 then 5300 and last at 5500 rpm in cruise. I'm curious as to how these will compare in speed and fuel usage. It may boil down to just a personal preference between fuel usage verses speed.

 

 

Have fun!

[Ed Cesnalis]

Don,

 

A few more thoughts.

 

1. Position 1 which is too flat for takeoff at your altitude1 becomes interesting and useful in 3 scenarios:
   • Higher altitude take offs and climbs
   • Continuance of low altitude initial climb if RPM degrade within 5 minutes
   • Higher RPM Economy Cruise
     • This is where you want to use MP to determine power. You will be able to get the same power setting but at a lower RPM if you use a coarser position and you can compare them for comfort and economy.

[*]In cruise on No.6 setting and altitude 3500' I can achieve 5540 RPM with WOT. - this sounds like your best speed setting for 4,000'[*]Your best speed setting is the one that will give you 5,500 WOT @ 7,500

When I consider all the settings I think of my 18 speed bike with 3 sprockets in front and 6 in the back. To find the perfect gear for a given grade I first have to select the front sprocket the gear I want could be on any of them.

[Duski Don]

Thanks Charlie T,

 

I'll try all those settings in a coupla' weeks when I do the big trip(again)

 

Best regards,

Duski Don

[Jim Meade]

Don,

 

The Operators Manual For Rotax Engine Type 912 Series, has the power setting information you are interested in. Operating limits, which tell you how many rpm you can turn, are in Chapter 2. The charts that show torque, power, fuel consumption, manifold pressure for variable speed prop and at non-standard conditions. PP 5-5 to 5-7.

 

Note that while SL-912-016, dated 28 July 2009 RECOMMENDS that continuous use of engine speed below 5200 rpm with WOT should be avoided, the above mentioned operators manual, dated Apri 01, 2010, permits WOT across the range of rpms. Obviously, if Rotax wanted us to never operate per the SL they would not issued a later instruction that permitted it. So, the SL is advisory, it seems to me.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

[chanik]

Don,

 

The Operators Manual For Rotax Engine Type 912 Series, has the power setting information you are interested in. Operating limits, which tell you how many rpm you can turn, are in Chapter 2. The charts that show torque, power, fuel consumption, manifold pressure for variable speed prop and at non-standard conditions. PP 5-5 to 5-7.

 

Note that while SL-912-016, dated 28 July 2009 RECOMMENDS that continuous use of engine speed below 5200 rpm with WOT should be avoided, the above mentioned operators manual, dated Apri 01, 2010, permits WOT across the range of rpms. Obviously, if Rotax wanted us to never operate per the SL they would not issued a later instruction that permitted it. So, the SL is advisory, it seems to me.

Even that they deleted from later rev's of SL-912-016

http://www.rotax-owner.com/pdf/SI-912-016-R3.pdf

http://www.rotax-aircraft-engines.com/portaldata/5/dokus/d04899.pdf

I avoid turning above 5200RPM because of those performance curves. The graphs Jim refers to clearly show a plateau in Torque at 5100-5200. Power still goes up since RPM is going up, but there is an inflection there with worse fuel efficiency vs power above that point.

[Jim Meade]

Kurt,

 

You cited a Service Instruction (SI) while the rpm issue is addressed in a Service Letter (SL). I should have been more clear in my post. Both have the same numbers. Confusing, to me.

 

http://www.flyrotax.com/portaldata/5/dokus/d04645.pdf

 

 

[chanik]

Wicked confusing, as we would say back in Boston.

[Duski Don]

G'day Guys,

 

Done the big trip across Oz.

 

Haven't got all the recorded data at hand.

 

i just made sure whatever height etc I was at that I could achieve around 5800RPM with WOT and then adjusted the throttle to get whatever fuel flow/TAS combination I wished to run at.

 

Had no dramas and got an average around 112 TAS & 18+ litres per hour.

 

Thanks again for everyones' interest.

 

Best regards from Oz,

Duski Don