Rotax recommended RPM

[Duski Don]

G'day from Down under,

 

I have an in flight adjustable 3 blade Neuform prop on my 2006 CTsw.

 

At full coarse setting (position 7) I get a WOT RPM OF 4670.

 

Reading other posts I gather this is not desirable.

 

At setting #5 and WOT I get 5280 RPM.

 

At lesser settings the WOT RPM increases.

 

At 5500 RPM my glass panel display goes into the yellow zone and at 5600 RPM into alarm.

 

I assume someone who set up the glass display figured this was correct for my aeroplane.

 

Any feedback, comments or clarification welcomed>

 

Duski Don (Briggs)

[Ed Cesnalis]

DD,

 

I have no experience with in-flight adjustable props but I am up early so let me ask you some questions.

 

5,800 RPM is permissible for 3 minutes, doesn't the alarm prevent you from comfortably using this setting for take off and climb out?

 

The 912 was designed to run its whole life at 5,500RPM, isn't your yellow alarm discouraging you from using this RPM?

 

4,670RPM is below recommended cruise RPM, would you not be better off just to throttle back and keep you RPM up?

 

Who has a CTSW with in-flight adjustable prop and flat settings? What RPM do you realize? How does it work out?

[Roger Lee]

Hi Don,

It's been a while. I hope all is well down under?

 

The in flight adjustable prop can be vacuum pump, but most are hydraulic pump run. You can find these in the parts manual if you want to look them up. Running your engine RPM at WOT in the 4000's is absolutely asking for a cracked crankcase and other broken internal parts and in time at that WOT rpm you will damage your engine. Not to mention you get the worst performance ever. Poor climb, poor cruise and poor fuel economy. Ideally for an in flight adjustable prop it would be set up to get 5700-5800 rpm WOT in climb. Then in cruise you should have a setting to get around 5700+/- WOT, but then pull the throttle back to somewhere between 5100-5500 rpm depending on what speed, fuel economy or special circumstances you may be flying. I would say most people cruise between 5100-5400 RPM on the average. The 5500-5800 rpm run time is published at 5 minutes and that is what 98% of the pilots adhere to, but it can safely run longer as this has a huge liability safety factor built in and unless you take the Rotax school with the right person you wouldn't know this little tidbit fact. Don't set the prop pitch to only see 5200 WOT rpm and run there either. The prop is still too over loaded at this setting. Make sure the prop WOT is a minimum of 5500 WOT and would be better served up to 5700-5800 and then throttle back to the 5100-5500 rpm. 5500 rpm is where the yellow should start on your instruments.

[Jim Meade]

Don,

 

Have you checked the Neuform manual on your prop recently? It has some interesting information in it. From the manual, it sounds like your prop may be set off of spec. The manual has some pretty definite recommendations about setting cruise RPM and max engine rpm.

 

It sounds like you are interested in the manual control (H) discussion portion of the manual.

 

Note the maximum prop rpm is given as 2600 for one minute. There are numerous references to engine rpm in various conditions.

 

The reference below was published 28 April 2010.

 

From what I can tell, this propeller was selected by FD. In your position, I'd be sending an inquiry directly to FD to see if they wanted you to conform to the Neuform manual or if they had other instructions.

 

http://www.neuform-composites.de/_downloads/pdf/operating-manual-v3-r2-20100428.pdf

 

 

[Tom Baker]

Roger, I think your using ground adjustable thinking. He should be setting manifold pressure with the throttle and RPM with the prop. However the low pitch high RPM setting should give him no more than 5700-5800 on climbout and he will need to reduce it to 5500 after a few minutes. When he levels off he needs to increase the pitch for cruise because the climb pitch would give to high of an RPM in full throttle level flight. Now I understand from his description that it is not a constant speed prop, but the point of having an adjustable prop is to get the best performance for all flight conditions. Without digging in the manuals or looking at the SB I din't know what an ideal MP/RPM setting should be.

[Jacques]

that's what the Operator manual says

 

 

run the engine accordingly and try to stay ABOVE 5000

 

http://legacy.rotax-owner.com/SI_TB_INFO/returndoc2.asp?PATH=http://legacy.rotax-owner.com/si_tb_info/serviceletter/sl-912-016.pdf&DOCID=SL-912-016&S_TYPE=SS

[Ed Cesnalis]

that's what the Operator manual says

 

 

run the engine accordingly and try to stay ABOVE 5000

 

http://legacy.rotax-...2-016&S_TYPE=SS

 

The take off and max cruise settings make sense but the 75%, 65% and 55% settings don't make sense to me. How can you keep your RPM above 5,000 when using settings as low as 4,300???

 

Given that the 912 was designed to run its whole life at 5,500RPM why wouldn't I want 5,500 for 75%, 65% and 55%???

 

If my thinking hold's water i would want my 7 positions set like this:

 

1)Take off/ initial climb: Flat enough to realize 5,800 on take off at my highest field elevation ( 7,100' fo rme )

2)Max Cruise altitude: ( my coarsest setting ) flat enough to realize 5,500 WOT level flight at highest cruise altitude

3)Max Cruise best perfroance: pitch for 5,500 at 7,500 DA WOT

4)Economy Cruise: pitch for 5,500, normal cruise altitude; 90% throttle

5)Economy Cruise: pitch for 5,500, normal cruise altitude; 80% throttle

6)Economy Cruise: pitch for 5,500, normal cruise altitude; 70% throttle

7)Low and Slow: pitch for 5,500, normal low and slow altitude; desired throttle

 

This makes more sense to me, please feel free to tell me why I'm wrong.

[FastEddieB]

 

Given that the 912 was designed to run its whole life at 5,500RPM why wouldn't I want 5,500 for 75%, 65% and 55%???

 

 

I envy you your flight adjustable prop.

 

Regarding your question, some plane/engine combos may have a "sweet spot". You might find a different (lower) rpm may be quieter or smoother, or both or neither. You may also find that different rpm settings may result in different airspeeds for the same fuel flow, affecting range.

 

No harm in experimenting!

 

As an aside, I don't think controllable propellers are kosher for Light Sport in the U.S.

 

"A fixed or ground-adjustable propeller if a powered aircraft other than a powered glider."

[Roger Lee]

Okay Don see what you started, your in trouble when I come over there.

 

Don has an in flight adjustable prop. RPM recommendations are not much different for a ground adjustable verses an in flight except that he gets to pick the best rpm for any circumstance at any time, where here in the US we can't. With a ground adjustable we have to select a pitch that will have balance between climb, cruise and fuel. Rotax school's recommend 5800 rpm on take off for in flight adjustable. Using a ground adjustable that wouldn't work for us at all as we would go way over 5800 at WOT in level flight. For a ground adjustable prop the manifold pressure is just about worthless to us as far as setting our pitch. It will always be 1" - 1.5" less than ambient because we are a normal aspirated engine. Most Rotax 912's don't even have manifold pressure gauges and it has just been in the recent years with the digital instruments that we have it more wide spread.

 

Charlie Tango (aka ED) has the right idea. We (other CT owners on a different forum) had this debate about this very thing back in 2007 and ED found out first hand what prop pitch can do for you and when it is set up for your average altitude. The numbers in the book are performance numbers only and are not engine run numbers down into the lower rpms. You have it right ED.

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Hi Tom,

 

He doesn't have a turbo 914 so the manifold pressure won't help much and it's not like the 914 and will always be below ambient.

 

If he sets his prop to achieve the proper engine rpm then the MP won't matter and he won't care what the manifold pressure is. He has a hydraulic run in flight prop with a normally aspirated engine. He should set his prop up for climb and then cruise. 5800 WOT in climb is 5800 WOT in climb no matter what and it doesn't matter what the MP will be. I know there are a few other selections on his panel for settings, but climb and cruise are his important two. Pitch for climb for the airport elevation. That would be to achieve 5800 rpm WOT in climb. Then at cruise set the prop up to get the same max rpm and pull back to what ever he wants to cruise at like ED said maybe 5500. He should never set the prop up to only get 5300 and below at WOT for a cruise setting. Engine before about July 2006 has had cracked crankcases from doing that. That is why they warn about low and or over pitched prop rpms. What owners also have to remember is the Rotax was designed with a specific rpm range to run in mind.

 

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The guys at my field with CT's and other LSA have watched me re-pitch over a hundred LSA 912 run props. I set them all up for that person's average altitude to get 5550-5700 WOT rpm flat and level. Average altitude prop setup is important. No use setting a prop for a person's sea level home base when he fly's at 10'K all the time and never even near sea level. I don't think there is a single person that has flown the new pitch setting and has ever wanted to go back. It would be nice if we all had in flight adjustable props because that would be the best of all worlds, but in the US it probably won't happen. We even have enough room under the CT cowl to put a setup in.

[N89WD]

Roger, I'd like to take advantage of the "set up room" and make an appointment at your shop ASAP!

[Roger Lee]

Any time. Come on down there is plenty of room. Just give me a call.

[Ed Cesnalis]

He doesn't have a turbo 914 so the manifold pressure won't help much

 

I would find the MAP readings quite helpful. With 7 different pitches how do you know if you are at a relatively lower or higher power setting?

 

RPM / MAP x conversion = HP

 

If my target is an economy cruise at 65% power I should be able to find 65% at a few prop settings so which is best? Which ever gets me closest to 5,500RPM without going over when resulting in 65%.

 

Or I'm cruising at 10,000' and want best speed, Then I want the setting that results in up to 5,500RPM at WOT and I can use MAP to calculate what HP or % I'm realizing.

[Roger Lee]

Hi ED,You can't change the MP only the rpm setting, so you set it for the best rpm at or above 5500 for that altitude.

[Jim Meade]

The SL says continuous use of less than 5200 rpm at WOT should be avoided, especially above 248°F and at/below 3500' MSL.

 

Obviously, there is no restriction against running less than 5200 at less than WOT, that is, at less than maximum possible manifold pressure. The way to do that is to reduce the throttle setting. It is hard for me to believe that Rotax would put out the SL and the chart if they did not mean you to comply by pulling back on the throttle.

 

Would anyone like to comment on whether the Neuform manual is appropriate for this application? If not, why not?

 

My CP settings would address:

 

Take off, cruise climb, max cruise at preferred altitude, max cruise at highest likely altitude, comfort cruise at preferred altitude (perhaps 5100-5200 rpm), economy cruise at preferred altitude (reduce MP and rpm), max economy cruise (lower MP and rpm - for case where last drop of fuel is needed).

 

As an alternative, I might try for a long distance cruise altitude (8K) and a short range cruise altitude (4K).

[Roger Lee]

Some of you guys are confusing max throttle prop pitch setup with cruise rpms and trying to control it with MP. You can cruise all day at 5100 if you want so long as the prop pitch is set to get 5500-5800 max WOT. It's the max rpm that dictates what stress you put on the engine at lower rpms. It's internal vibration that low rpm settings cause with low WOT rpms and an over pitched prop. They teach in Rotax school that the MP is almost useless with the ground adjustable prop to use as any type of control or setting.

 

5200 rpm cruise is terrible if you can only get 5200 rpm WOT. 5200 rpm is good if the prop pitch will achieve 5500-5800 WOT.

 

Don't mix all these together or over think this. You have control on two settings with a ground adjustable prop. The WOT prop pitch setting which will affect all throttle settings and engine stress from idle to WOT and then the cruise rpm with a reduced throttle. It makes absolutely no difference what the MP, you only care what the rpm is, the MP is what it is.

[Ed Cesnalis]

...

The WOT prop pitch setting ...

 

that is 2 settings: 1) throttle * 2)pitch

 

 

 

[Ed Cesnalis]

Hi ED,You can't change the MP only the rpm setting, so you set it for the best rpm at or above 5500 for that altitude.

 

Manifold Pressure gauge reads Atmosphric Pressure. So where ever you are , when the engine is not running, this gauge will read your current Atomosphric pressure.

Now fire up your engine and close the throttle to idle. Now this gauge is reading how far open your throttle plate is and allowing Atomosphric pressure in. So at idle you are reading real low on this Mainfold pressure gauge.

Now as you open the throttle plate it opens up the intake to more Atomosphric Pressure and the gauge reading starts to go up.

Now open her up all the way. You will notice that the reading does not reach outside Atomsphric pressure. This is becuase the engine needs some of this pressure to run (normally aspired). It will be close to out side pressure but not quit there.

So you can use this gauge to measure how much load you are putting on the engine, or how much you are making it work. The lower this Manifold Pressure Gauge is reading the less you are working this engine. The higher this gauge reads the harder you are working this engine. You control this in your CT with the throttle.

[FastEddieB]

You will notice that the reading does not reach outside Atomsphric pressure. This is becuase the engine needs some of this pressure to run (normally aspired). It will be close to out side pressure but not quite there.

 

I think a better way to look at it is that the drop is caused by the engine having to suck air past obstructions (air filter, throttle plate, bends in the intake system, etc.). Theoretically, if those obstructions could be eliminated, MP would equal outside pressure.

 

You can see this by applying just enough boost to compensate for the drag in the intake system. Voila! Mp=Atmosperic Pressure.

 

BTW, I agree that with a flight adjustable prop on a ROTAX, you have two ways to set power: MP with throttle and RPM with the prop control. The same power may be achieved with different combinations, some more efficient than others.

 

A different animal, but I ran some tests like this on my Cirrus. I'll try to find and post the data later.

[Duski Don]

G'day Guys,

 

Talk about hornets' nests.

This is the best reaction I've had since i told my beloved I was thinking about a Lear jet.

 

Seriously thanks for all the info and feedback.

 

I think I've got it and will adjust my pitch control as required to achieve 5800 WOT in either climb of cruise configuration.

 

Best regards from Down under,

 

Duski Don

 

PS: Roger, we have a very good remedy for getting out of trouble (you may have heard of it) It's called cold beer.

[Jim Meade]

 

 

I think I've got it and will adjust my pitch control as required to achieve 5800 WOT in either climb of cruise configuration.

 

 

Don,

 

 

Rotax and FD documents would call for you to operate at NMT 5800 for NMT 5 minutes, typically in climb to develop max horsepower, and NMT 5500 continuous in any attitude.

 

 

 

 

 

[Roger Lee]

Hi ED,

 

I remember 5 years ago you and I had the rpm setup discussion. Your original pitch at your high airport elevation and cruise altitudes were too course. You never once mentioned or did we talk about MP, it was all about rpm and you flattened your pitch to make and I quote "Woodstock Comes Alive". I know you don't fly MP you fly at 5500 rpm all the time. On the Rotax Owner forum we talk about rpm and not MP. I also remember when we were both at the Page Fly -In and over the Grand Canyon. You had your prop set up for that 10.5K - 11.5K altitude and could still get 5500+ up there and I was a low lander and could only get 5150 up that high. We both went full throttle and you walked away from me. I know you were grinning ear to ear. For the 912 normally aspirated engine rpm is everything.

Which also brings me to the point of you and Erin should be back at Page Fly-In this year. We have lots of catching up to do.

 

 

The MP does move with throttle, that's a given, but we don't fly by our MP pressure we fly by our rpm, other wise we would always be talking about what MP to cruise or climb at and not rpm, or what MP to set your static at. Rotax would have a MP in the books to set your flight characteristics for and not rpm. The only time MP is important in a 912 is if you are running twin 912's (there is a new Rotax twin Tecnam at my field or if you disconnected the carb cross over and had an individual MP for each carb and separate throttle cable. The last scenario is no different than syncing the carbs on the ground except you would sync them in the air. We never talk about MP in school other than just mentioning it is always 1"- 1.5" below ambient. You set the Rotax 912 pitch up for rpm not MP and you fly by the rpm. I haven't heard a single person here or really ever on any forum ask, what MP do you guys fly at cruise or what you MP is on climb, what's the best prop pitch for a specific MP. It's all about rpm for the 912. It's a normally aspirated engine. The digital world has brought us a lot more info and it's learning how these not only affect the entire engine, but how one reading may affect another single reading and what it means. Until the digital instruments came along your rarely ever saw a MP gauge on a Rotax. So then how have these guys been flying with a Rotax 912 for the last 30 years without an MP.

 

 

Bottom line MP doesn't mean much to us as far as normal everyday flying and setting prop pitch. MP will always be different depending on the day. There is only one scenario I can think of the MP is a diagnostic tool while flying for the 912.

[Roger Lee]

Hi Eddie,

 

MP can never be equal in a normally aspirated engine regardless of plumbing while it's running. If it has to suck in air it can't be equal. It takes something like a turbo to equal or boost MP to atmospheric or above.

[Ed Cesnalis]

Hi ED,

 

I remember 5 years ago you and I had the rpm setup discussion. Your original pitch at your high airport elevation and cruise altitudes were too course. You never once mentioned or did we talk about MP, it was all about rpm and you flattened your pitch to make and I quote "Woodstock Comes Alive". I know you don't fly MP you fly at 5500 rpm all the time. On the Rotax Owner forum we talk about rpm and not MP.

 

 

The MP does move with throttle, that's a given, but we don't fly by our MP pressure we fly by our rpm, other wise we would always be talking about what MP to cruise or climb at and not rpm, or what MP to set your static at. Rotax would have a MP in the books to set your flight characteristics for and not rpm. The only time MP is important in a 912 is if you are running twin 912's (there is a new Rotax twin Tecnam at my field or if you disconnected the carb cross over and had an individual MP for each carb and separate throttle cable. The last scenario is no different than syncing the carbs on the ground except you would sync them in the air. We never talk about MP in school other than just mentioning it is always 1"- 1.5" below ambient. You set the Rotax 912 pitch up for rpm not MP and you fly by the rpm. I haven't heard a single person here or really ever on any forum ask, what MP do you guys fly at cruise or what you MP is on climb, what's the best prop pitch for a specific MP. It's all about rpm for the 912. It's a normally aspirated engine. The digital world has brought us a lot more info and it's learning how these not only affect the entire engine, but how one reading may affect another single reading and what it means. Until the digital instruments came along your rarely ever saw a MP gauge on a Rotax. So then how have these guys been flying with a Rotax 912 for the last 30 years without an MP.

 

 

Bottom line MP doesn't mean much to us as far as normal everyday flying and setting prop pitch. MP will always be different depending on the day. There is only one scenario I can think of the MP is a diagnostic tool while flying for the 912.

 

Roger,

 

MP isn't useful to a pilot with a fixed pitch prop.

 

When the prop pitch varies either by in flight adjustment or by a constant speed prop adjusting to maintain the constant speed then MP is useful to the pilot to determine the power setting but only when both MP and RPM are considered.

 

MP becomes quite useful when you can adjust your prop in flight and RPM alone becomes far less useful because it can no longer be relied on by the pilot to determine power at that altitude. You now have 7 different pitches to choose from 5,000RPM ( or any RPM ) will result in 7 different horse power settings.

 

MP becomes even more useful with a constant speed prop.

[Roger Lee]

Hi Ed,

 

(Just so the others know, Ed and I are friends and we aren't bickering as someone said a long time ago and we like to have good discussions. That's how we both learn and usually figure out we are saying the same thing just from two different directions. That's why it's fun.)

 

I fully agree on the fixed pitch which for the most part is our ground adjustable on most LSA and the constant speed.

This is where the discussion got side lined and mixed with other engine setups.

 

Back to Don's original question.

His selector for the prop is based on RPM. That's all he really has control over. Yes when he picks a specific rpm the MP changes, but he is still looking to set just his rpm to achieve 5500+ to 5800 RPM either at WOT or full cruise rpm (that part is up to the user). Rotax gives RPM targets and not MP like the constant speed group. Torque for our engine peaks at 5000 rpm and goes down the higher the rpm, but HP increases up to 5800 rpm. So what he is basically trying to do as altitude increases is save some of the HP and torque and not let it deteriate more than it has to. We loose approximately 3% HP for every 1K' of altitude and if you are over pitch with a ground adjustable that becomes worse real fast. So he is selecting an rpm setting that controls the prop pitch to keep 5500+ rpm. It won't matter what the MP is. He can watch it change, but he's looking for the rpm. The Rotax publications for in flight adjustable props Rotax says adjust to 5800 rpm then MP doesn't really play a role. In the end it still comes down to max rpm and or cruise rpm. RPM for us (without too deep an analogy) is HP. Don get's the best HP and torque settings over us because he gets to manipulate the pitch at any altitude to help compensate for losses where we are stuck with one setting. Like you said he has 7 prop pitch selections to choose from. All he has to do is flatten or coarsen the pitch for a given altitude to achieve his Rotax rpm goal of 5500-5800 rpm. You could do this for the life of the engine and never look at an MP gauge.

[Tom Baker]

Roger, MP is the only true indicator of power output. When you move the throttle lever in the airplane you are changing the position of the throttle valve, which only controls the MP. With a non-inflight adjustable prop your adjustment of MP has a dirrect connection with engine RPM. Since the pitch of the prop can not be adjusted in flight RPM makes a good substitute for MP. With a constant speed prop the pitch change is automatic to maintain the RPM regardless of throttle setting. This is the same for normally aspirated as well as boosted engines. In the case of a prop where you can select the different pitches in flight MP will be very important in finding which pitch position and throttle setting will give the power output you want. The MP and pitch setting will determine the RPM. After the RPM is determined then he can use that RPM with that pitch setting on future flights.