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Rotax recommended RPM


Duski Don

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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

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Hi Tom,

 

I fully understand what you and others are saying, but some are including other engine's and prop setups.

Strictly consider the Rotax 912 a ground adjustable or in flight prop and nothing else.

We should not include constant speed props because non of us have them and It's an entirely different animal and I agree the MP is important there or for twin engines.

I agree 100%.

"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."

This is exactly my point and what the Rotax manual is set up for.

Would you agree that in the 912 with a either a ground adjustable prop or an in flight adjustable that has higher the rpms at 5800 generates more HP?

If you answer yes then you have your answer that MP doesn't play much of a part for us with a 912 and that the final solution rest on rpm which determines HP output. If MP has been so critical to picking an rpm setting why hasn't all the 912's been equipped with them? I would bet that 98% of us look at rpm's for cruise and don't set cruise with MP.

I'm not trying to be a smart ass, just point out that we really don't need or have we ever really needed MP for a 912 with our (CT) set up.

Like ED and I we are usually talking about the same thing, but from two different directions.

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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.

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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.

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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.

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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.

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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.

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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?

 

 

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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:

 

7824958978_f6ebc750ec.jpg

 

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.

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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.

 

Tom is correct and this is what I am arguing.

 

Roger,

 

The biggest reason Don needs MP is to set the throttle for the desired power setting after he selects a cruise prop pitch. He has several to choose from and each of them will produce a different power and economy at the same RPM so RPM alone makes an inadequate target.

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Hi Ed,

 

I fully agree with what you and Tom are saying. What I'm saying is he will set the selector to get let's say 5800 rpm WOT or even 5500 WOT (his choice), but then he will pick an rpm he prefers and it won't have anything to do with best MP. Could he do that, yes and as you pointed out it is the best way, but in his first post he didn't do that and neither do any of us. Here in the US we all fly using the rpm because MP doesn't make any real difference to us because we can't change the pitch while flying. I think we branched off and mixed constant speed, ground adjustable and back to in flight adjustable, setups in the US and setups there and now we're back to a focused point again. We mixed to many different points and it got a little off course and I'll be the first to admit I branched off myself. So I agree with yours and Tom's last post.

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Thanks Jim,

 

I've had my aeroplane for a coupla' years but this issue only came up when a minor glitch with the pitch control was rectified and allowed me to get the full range of pitch control.

Prior to this I had no problem achieving 5800 and usually cruised at 5300.

Sounds like cranking up the cruise RPM a bit wouldn't be a bad idea.

 

I'm about to do my third across Oz flight (in the CT) so all very timely information.

 

Best regards from Down Under,

Duski Don

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Hi Glenn,

 

There was more, but it was turning into a dissertation and my fingers were stating to bleed, besides I'd rather talk to Ed on the phone. biggrin.gif

 

Besides that's how we learn. Trading ideas, thoughts, facts and theory and then one day we land on Mars with a rover. Couldn't be better.wink.gif

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Roger,

 

You are correct to this extent. I would never need to see MP ( as long as I didn't need to cacluate my power setting. ) Because I always fly at WOT due to high altitude and I always fly at 5,500 or as close as I can, all I would need to do is select what ever position is closest to position 1 that does not exceed 5,500.

 

If I can exceed 5,500 I should move towards pos 7.

 

It gets a little more complicated if you fly at partial throttle settings.

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Hi Ed,

 

Good morning.

You said exactly what I meant because unless you have a power setting chart with you and you actually even cared about what power setting you were at , which most don't (CT pilots) then the MP as you said isn't important. For the most part flying in the CT with an in flight adjustable prop who usually cares if you are at 65%, 68% or 70% power at each separate setting. Could you care and need the MP, sure, but in real life that doesn't happen. Yes I know some of the larger aircraft have and use the charts, absolutely, but typically not us. If pilots really cared they would pitch their ground adjustable prop then make up a chart for all the rpm settings and compare it to the MP. Who does that.

 

You and I would find a setting with the switch that gave us 5800 WOT on takeoff and then do the same in level flight and then throttle back to where we were happy at our preferred cruise rpm or in your case find the setting that gave 5500 rpm all the time in level cruise at and use WOT.

 

The bottom line is we as CT pilots don't really care about or need MP to fly just a little knowledge about our specific plane, engine and the correlation with rpm to torque and HP.

 

Now I can sleep at nights. I thought I would never get you to say that.laugh.giflaugh.giflaugh.giflaugh.gif

 

Give me a call and let's catch up.

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G'day Guys,

 

Just when we thought hostilities had ceased another flare up!

 

Roger considering your last post (not that one they play on the bugle) the following question:

 

Is 5500 the minimum WOT RPM that I should pitch my prop to achieve in cruise or should I be aiming higher?

 

I am going to go to Narrogin (where I keep my aeroplane-200 kms away so I may not receive your reply 'til Sunday pm our time.

 

Needless to say the next coupla' days will be spent adjusting pitch, throttle ans recording the results which I will probably bore everyone with early next week

 

Thanks Guys (may the battle continue)

 

Duski Don (Briggs)

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Hi Don,

 

No battle here. Next time I see Ed I'll have to give him a big Ol'e hug. wink.gif We are all friends that were in this discussion. As far as I know there is nothing personal, just input, ideas, education and working out the technicalities and certainly some of this just boils down to personal preference.

If you were someone that has never seen us around each other or know that we talk on the phone I'm sure it may seem like a battle, but I guarantee it isn't. I especially like people that dig for knowledge and education and has an opinion that's how we all learn.

As humans we all fall down, but it's learning top pick ourselves up dust ourselves off and keep going that separates us from other life forms.

 

5500 WOT in flat and level cruise should be the minimum. You could fly at that 5500 rpm too or pitch to get 5700 WOT and throttle back to around 5200 +/- rpm (this would be my preference). That is up to you, this is the personal preference part. There will be a point here where more rpm generates very little speed increase, but more fuel use.

 

I would be interested as I know Tom and Ed would be to what you find in your settings.

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Is 5500 the minimum WOT RPM that I should pitch my prop to achieve in cruise or should I be aiming higher?

 

 

Don, 1st a preliminary question. Rotax published that the 912 was designed to run its whole life at 5,500. Do you wish to be able to comply with that or just fly at high RPM that sometimes approach that?

 

Next what is the highest altitude for which you will ever want max climb power?

 

For me I have 14,000+' ridges and higher peaks to deal with so I might want my #1 position able to achieve 5,800 at 15,000'. Now to your question above, the #1 setting for high altitude take-off or high altitude full-power climb would also be useful at 5,500 RPM for low power setting cruise at a low or at least much lower altitude. So there is no need to have a cruise setting ( like position 7 ) any flatter than 5,500 at your lowest cruise altitude ( perhaps near sea level )

 

When you need flatter cruise settings for altitude or flatter cruise settings for low power slow and low or low power economy cruise you can use a climb setting.

 

( I just saw Roger's post, we essentially agree I just threw altitude into it )

 

 

 

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Thanks Roger and Ed,

 

No worries Roger I can assure you all my comments are tongue in cheek and align with my,and most other Australians,

tendency to "humorous sarcasm" (which is never intended to offend)

 

Back to business - all taken on board and as stated in previous post I'll be trialing different settings later today and tomorrow.

 

Kind regards,

Duski

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I don't completely agree with roger on this one. Since we have to deal with just ground adjusable the 5500 rpm WOT works pretty good. In your case while in cruise I think I would use a pitch setting that would give more like 5200-5300 WOT, since you will not be lugging the engine in climb with this setting. I'd at least give it a try to see what the number come out to be.

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Don, 1st a preliminary question. Rotax published that the 912 was designed to run its whole life at 5,500. Do you wish to be able to comply with that or just fly at high RPM that sometimes approach that?

 

 

Where does it say that the engine SHOULD run it's whole life at 5,500 rpm? The impression given on this forum is that the only speed to run the engine is 5,500. I've seen other documents that imply a range of rpms is acceptable. I've cited some of them earlier in this thread.

 

 

Can someone cite the Rotax or Flight Design documentation, please?

 

 

 

 

 

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Where does it say that the engine SHOULD run it's whole life at 5,500 rpm? The impression given on this forum is that the only speed to run the engine is 5,500. I've seen other documents that imply a range of rpms is acceptable. I've cited some of them earlier in this thread.

 

 

Can someone cite the Rotax or Flight Design documentation, please?

 

 

Here it is Jim

 

 

 

 

 

post-6-0-06737500-1345774169_thumb.png

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Does anyone know what ROAN means by "low engine speed"? I am well aware that the 912 is neither a chainsaw nor a Continental. Still, it is a four cylinder, four stroke, pushrod engine with two side draft CV carburetors. Pretty conventional stuff, in the world of piston internal combustion engines. At low power output, engines of this type tolerate operation at speeds well below redline. For example, for the 912, cruise at 5000 rpm is 86% of redline RPM and 90% of maximum continuous allowable RPM. Such operation of any other four cylinder, four stroke engine would be perfectly ordinary and acceptable. (I am not talking about full throttle here (i.e., climb) but level flight.) Is the 912 so different from all other similarly configured engines that continuous operation, with less than full throttle, at engine speeds of 80 to 90% of maximum allowable speed is detrimental? (And, yes, I am talking about a plane with a ground adjustable prop set for 5600 RPM WOT in straight and level flight).

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