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?

[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."

[N89WD]

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

[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.

[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).

[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.

 

 

 

 

 

[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.

[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.

[johnr]

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.

Hi Charlie,

I cannot agree with your statement That "the higher the MAP the harder the engine is working" I believe the opposite is in fact the case. Forget aircraft for a moment & consider a car (auto) with a manual gearbox. The dumb driver fails to change down going uphill the engine is labouring, the manifold pressure has dropped this is not good. So we tell the dumb driver to change down a gear he changes down the revs rise the MAP rises, his engine is not working so hard. We'r not finished yet, dumb driver leaves car in low gear & comes to a down hill section his revs rise way over the safe limit & so does his MAP. The egine can do this because its not working hard high RPM (high MAP)

An engine is not working hard at high rpm with a high MAP & if the MAP remains near static at lower RPM then its not working hard either.

I will take this opportunity to also disagree with Roger (my Guru). I see no problem with a lower RPM as long as the engine is not over working. I will quote Flight Design

 

"Aircraft Operating Instructions" AU 010 01000 Rev 7 29.April 2008. stating normal cruise is 4200---5200, cruise at 75% 5200.

 

I am only getting to know the 100hp Rotax, but it is really sweet at 4800--5000rpm & is showing (preliminary) 4800rpm 16 litres per hr, 5000rpm 17.5 LPH.

 

The Rotax 914 fitted in my Jodel D11 runs so sweetly at 4200 MAP 29, & was returning 10/10.5 LPH at a steady 80kn. (Slower than I would normally travel at but in a group of 4 with the slowest having a cruise of 80Kn ( 8000Km round trip)

As a matter of interest a vacumn retard was fitted to many early cars, this retarded the timing when the engine was labouring over riding the centrifugal (rpm timing).

 

I had the pleasure of meeting Duski at Narrogin last weekend. Hi Duski.

 

Best Regards

John

[Ed Cesnalis]

Rog,

 

In this case we are not saying the same thing there is disagreement here.

 

Back to Don's original question.

His selector for the prop is based on RPM. That's all he really has control over.

 

 

He has control over both RPM and MP. He can control both with the throttle as well as [shift] into 6 other RPM ranges by changing prop settings.

 

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.

 

Doesn't Rotax do the equivalant of giving MP by qualifying WOT and altitude? Torque would not peak at 5,000 if your pitch permitted you to achieve 10,000.

[Ed Cesnalis]

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

 

It is not possible to save low altitude performance, the performance will deteriorate with DA increase and Don has nothing at his disposal to prevent this. You would need a 914.

 

 

 

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.

 

 

The Rotax publication isn't very helpful, it says to avoid RPM lower than 5,000 and then lists 4,300 as the RPM setting for 55%. The method you describe above can probably find 55% at or near 5,500 but you would need to consider MP to determine which is the best prop setting for this.

 

You could find the prop setting that gives your 5,500 at your desired throttle setting but the throttle settings are not calibrated so you would be guessing where if you used MP to determine the optimal pitch for your desired throttle or power setting you can be precise.

[Ed Cesnalis]

In the end it still comes down to max rpm and or cruise rpm.

 

 

Not at all, with 7 settings it should come down to 7 scenarios. You could have a sea-level full power take-off / climb at 5,800 which would be a different pitch, different mp but same RPM as a 7,000' ~78% power take-off / climb.

 

 

You could have multiple cruise settings for a given altitude all at 5,500 RPM but with 100% throttle, 90% throttle, 80% throttle, ...etc. In the end it comes down to prop pitch and RPM which will determine MP.

 

 

The reason you would want to rely on MP to determine power setting is because it works at any DA! You could determine power setting by working with pitch - RPM - DA but that is far more complicated.

 

 

RPM for us (without too deep an analogy) is HP.

 

Not with an in-flight adjustable prop.

[Ed Cesnalis]

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.

 

 

This works for best speed only. For Economy cruise settings the throttle will be partially closed and there can be multiple pitch settings that will achieve that goal. If you only look for an RPM range you can find it but at what power setting will you be? You want to consider MP to determine if you are at the desired pitch setting resulting in the desired power setting.

[Tom Baker]

Roger, For our set ups here in the US RPM works OK. I know you have adjusted 100's of props. If you have a engine that turns 5200 WOT you decrease pitch to increase the RPM. Correct? What happens to your MP with the 2 different set ups at 5200RPM? The decreased pitch has a lower MP because you don't have to open the throttle as far to achieve 5200 RPM. Next time you adjust a prop look at MP at WOT before you adjust and after you adjust. It should be the same for both at WOT. For us in flight we only have control of 1 thing and that is throttle position, so MP or RPM will give us our power output. For Don he has control of 2 things, throttle position and propeller pitch. He needs MP for throttle position and RPM for prop position. One reading can't tell the whole story.

Just like flying we control 2 things power and pitch. If our pitch was set all we would need is RPM. One power setting would allow us to be in level flight. Since we can control our pitch to we need an airspeed too.

[Jim Meade]

The Rotax publication isn't very helpful, it says to avoid RPM lower than 5,000 and then lists 4,300 as the RPM setting for 55%.

 

Which Rotax publication are we referencing? The SL says don't run less than 5200 at WOT. It says nothing about running at less than 5200 at less than WOT, in which case perhaps the table referred to earlier might come in to play?

 

 

Does anyone have a POH for an airplane with a Rotax and a CP or in-flight adjustable prop? I'd like to see it.

 

 

 

And, again (3rd time I've asked) does the Neufrom manual apply or not?

 

 

[FastEddieB]

Hi Eddie,

 

MP can never be equal in a normally aspirated engine regardless of plumbing while it's running.

 

 

Agreed.

 

If it has to suck in air it can't be equal.

 

Obviously. As stated before, obstructions to the airflow cause lower pressure - the less obstruction (more throttle) the less difference - but always some.

 

Kinda off topic, but here was a flight test in my Cirrus:

 

 

The 2690 was with the power lever all the way forward. Should have been 2700, but that's pretty darn close and would vary somewhat with conditions.

 

Note that LOP there was not much difference as I changed RPM, as long as I kept fuel flow the same. And LOP HP is determined by fuel flow. But it seemed like I had a "sweet spot" at about 2650 rpm on an engine which could and would run all day at 2,700. Different engine/prop combinations might have similar "sweet spots", and it can't hurt to experiment.