Cruising Speed and Best RPM for CTSW

[Jim Meade]

 

Rotax like any company with an engine can not write down every circumstance a user may use their engine or even how it may be installed. So they have to draw the line in the sand some place. That means some users will fall below that line and some above that line. It's just a guide. This is why school is so important to know the difference between a guide line and a hard rule.

 

It would be very helpful if, when you disagree with the Rotax manuals, you would point that out. I assume you would disagree with the manuals because of something you learned in a Rotax school or through your own experience. In either case, if you are recommending that we follow your advice and not the Rotax manual it would be beneficial if we knew what we were doing and why that is different. I would want to annotate my manual so I didn't forget.

 

I wonder why Rotax would teach something in their own school but not put the change in the manual? I don't understand that. I'd think their lawyers would tell them to be consistent.

 

 

[Bobby CAU]

its TRUE, why ROTAX made up something different with the reality?.

what is manual says in ROTAX, practically different ...

 

regards,

Bob CAU

[Roger Lee]

Hi Bobby,

 

It was really nice talking to you on Facetime the other day.

The numbers are actually close, but what most don't take into consideration is the many ways Rotax is setup around the world and this can be huge. They can only write one manual. Then there are just some personal run preferences to figure in. All the different props i.e. in flight adjustable -ground adjustable and fixed pitch props, poor manual discussions on altitude influences, Rotax classes being more current than the manuals, minor translation differences with language, three different engines talked about in the same manuals, different interpretations of what an item means and trying to interpret how all these things apply to you, your engine your different aircraft and then understanding what the engine specs and run parameters mean i.e. temps, pressures, rpm, vibration and what one means in relation to the other.

[Roger Lee]

Not aimed at anyone just a general thought:

If you are a reasonably new Rotax owner then these in depth or specific debates can be confusing especially if your getting incorrect info from Joe your neighbor or another mechanic that doesn't have any more Rotax training than you. We have good techs on this website and knowledgeable techs on the Rotax-Owner website that have worked on Rotax for years and work on them everyday. I wish a few more would check our site a little more frequently. I have stated before that it should be a must to attend some current Rotax training to get all your up to date and current answers straight from the horses mouth so to speak. I would say it would be tough for someone with only 1-2 years around a Rotax and no Rotax schooling to truly debate some of these issues compared to someone that has 10+ years of Rotax experience, several Rotax schools behind them, sees 15-30 Rotax engines a year and keeps current with Rotax in special ways that owners usually don't know about or even has access to.

I will be the first to admit that in the many years I have been around Rotax my learning curve has been steep at times and even with many classes you need years in the field to see and do enough things to make it come together in an understanding and to see how one component affects the other, plus years of experience really help these mechanics when helping others diagnose field problems.

I have learned that the deeper you go into the Rotax engine i.e. Heavy Maint. that you may need to attend that class more than once and actually Rotax requires that a Heavy Maint. rated mechanic that wants to keep his standing with Rotax (an IRC) rating to attend this 3 day school every two years. The reason for this is because you don't see or do all the things included in that class very often unless you work for a distributor so you loose that knowledge base from lack of skill currency so a refresher is needed. Having been there and done that I can see why they are that way.

Like they say: If you don't use it you loose it.

If you take your Rotax to someone that puts himself out there in commitment and attends the update school and the Heavy Maint class every two years and works in the business with Rotax engines as his main job you should feel reasonably good about that person trying to keep up with current teachings and a commitment both mentally and monetarily to doing the right thing for the engine owner.

I know most of the techs on our forum and I know they are here to help whenever possible and offer the most up to date information.

[Safety Officer]

For those who wanted something from Rotax about rpm and vibration here you go.

http://www.rotax-owner.com/media/kunena/attachments/5408/SL-912-010AbnormalVibration.pdf

SL-912-010AbnormalVibration.pdf

[Ed Cesnalis]

Roger,

 

The above service letter instructs on how to identify and correct abnormal vibrations it does not address best RPM.

 

The abnormal vibrations addressed occur at 3,600 to 4,800 RPM so avoiding this range would make sense if you have abnormal vibrations in that range, but what if you do not?

 

One of the reasons for the abnormal vibrations in this range is prop moment of inertia is out of tolerance. How can we translate this moment of inertia tolerance to an acceptable prop pitch range? Where is this moment of inertia published?

 

Assuming that I only have normal vibrations at all ranges do the facts in this service letter infer that this RPM range should be avoided none the less?

 

I do avoid this range on take-off, climb, cruise but not on descents. Is this 3,600-4,800 range to be avoided unless engine is not under load? I guess we know the loading makes the difference and I wonder if Rotax has that theory documented beyond having a moment of inertia tolerance?

[Jim Meade]

The SL is dated 2003 and lists a number of causes for vibration, propeller pitch not listed as one. Useful to see how they set up to test the vibrations.

 

 

[Roger Lee]

Hi Ed,

I think his post is more of an educational FYI.

 

My post may get a little side tracked, but related to rpm and vibration in the Safety Officers post.

 

I think what the Safety Officer is just trying to relay are the causes which are listed on the first page. I would guess that most us us have more than one of those causes. It is okay to be in this range(3600-4800) for the short interim like reducing power for landing or a short sight seeing look, but long extended periods like cruise should be avoided. In Bobby's original post he says he cruises at 4600 rpm.

The other thing here is for us as owners to help reduce increased vibrations by addressing the items we have some control over. Like making sure we don't over pitch the prop, check blades during annual's to make sure they are all pitched the same. Although not spelled out at the 5 year rubber change here is a big indicator to change engine mounts. There is no way unless they are just totally shot that an owner can adequately tell if mounts are too hard, too soft, crushed, miss aligned from crushing, split, ect... After 30 hose changes and seeing 30 sets of engine mounts I wouldn't think of not changing them.

 

So for me he was indicating that there is an rpm to avoid for long periods and some here wanted to see something in writing. This gives us ways we can help reduce the unwanted and many times un-felt engine vibration. If you can feel the vibration then something really needs correction. I can't tell you the number of pilots that have come in and by simply doing a good carb sync, setting the prop blades for a better rpm and making sure they are perfect in equal pitch (two very common issues) that when we test fly it they are shocked at how much smoother it is and they thought the old vibration was normal.

[Doug G.]

What is the reasoning for setting the prop at WOT, as opposed to max. torque for instance? It would seem to be most efficient there. And, with a helical prop blade, isn't there only one setting that will put the angle of attack in the best position along the entire blade? When you adjust it, if it is not at that angle, some portion of the blade will be inefficient.

Is this theory of adjustment in a manual somewhere? Or is it only because Rotax says it maxes hp at 5800?

Doug

[Ed Cesnalis]

What is the reasoning for setting the prop at WOT, as opposed to max. torque for instance? ...

 

You could have both, max torque @ WOT. Flatten the pitch a little more and you can have max hp at WOT.

 

I'm looking for max hp because hp is so limited.

 

 

[Doug G.]

Yes, but max torque is where you put the most power to the prop, isn't it? And these props are not a flat pitch, they are helical. I think this is more complex than people are treating it. That is why I would like to find something from FD or Neuform on this.

[Ed Cesnalis]

Yes, but max torque is where you put the most power to the prop, isn't it? And these props are not a flat pitch, they are helical. I think this is more complex than people are treating it. That is why I would like to find something from FD or Neuform on this.

 

No it isn't. Max torque is where you put the most force to the gearbox but if you advance the throttle you can do more work, at a higher RPM with a little less force. This is how you go 125kts

 

Torque is force, power is that force x distance / time. I want max power over max torque because I want to maximize the amount of work my engine does over time vs having the ability to accelerate.

 

@ 5,500 RPM the torque is adequate to produce 75hp (7,500') accelerating from max cruise has little value to me.

 

Our Rotax's are more like a little honda 200hp car engine than a 200hp truck motor. The truck motor has a lot of low end torque for accelerating all that weight from a stop then up to hwy speeds. The light little honda car doesn't need all the torque but can use the hp for high speed / economical cruising.

[Admin2]

I'm along the same lines as Ed (Charlie Tango) on this.

Maximum torque for the 912ULS is at 5000 rpm with a WOT set of 5800 rpm. As the engine goes past 5000 rpm the torque decreases, but the HP and rpm keep increasing. Without enough HP you can't develop your full torque. Over pitching a prop plays a big part in hurting your torque and HP. Under pitching is just wasting both torque and HP. Torque will eventually peak depending on the engine and and delivery setup. Here are the Rotax graphs. Since we don't have dyno's in all our hangars we rely on the Rotax numbers which are based for the 912 on rpms for the end user. (914 turbo is not in the discussion). All the numbers in the manuals are usually based on a 5800 WOT setting and most of the time with an in flight adjustable prop. Most results are not based on ground adjustable.

So for most of us we need to balance our performance characteristics. We strive to achieve a balance between climb, cruise and fuel economy. Then we figure in special circumstances like Charlie Tango may have with his normal cruising at 10'K-13K' MSL or maybe some one has a heavy plane with floats. Then a tad better climb prop with a little more reduced pitch. FD and Neuform aren't going to be able to help you. It is an engine design issue. Rotax has been setting up the engine parameters of HP and torque since the 1930's and with the 912 since 1989. They should have a pretty good grasp on the HP and torque.

912ULS HP and torque.pdf

[chanik]

A more useful formula to remember is that power is directly proportional to Torque x RPM.

[Ed Cesnalis]

A more useful formula to remember is that power is directly proportional to Torque x RPM.

 

http://www.rotaxservice.com/documents/912Sperf.pdf

 

Look at these graphs, above 5,000RPM power increases but torque decreases so I think your statement should be: "A more useful formula to remember is that power is [almost] directly proportional to Torque x RPM. [or throttle setting]"

 

Torque is a component of power.

 

Power is torque times a distance / time ratio. Power continues to increase above 5,000RPM in spite of torque decreasing because that decrease is offset by the distance / time ratio increase ( RPM )

 

 

 

[chanik]

http://www.rotaxservice.com/documents/912Sperf.pdf

 

Look at these graphs, above 5,000RPM power increases but torque decreases so I think your statement should be: "A more useful formula to remember is that power is [almost] directly proportional to Torque x RPM. [or throttle setting]"

 

Torque is a component of power.

 

Power is torque times a distance / time ratio. Power continues to increase above 5,000RPM in spite of torque decreasing because that decrease is offset by the distance / time ratio increase ( RPM )

 

That distance is the perpendiclur distance to a force at right angles to the rotational shaft, which for a distributed resisting load force acting along the axis of the prop is largely unmeasurable.

 

Torque x rotational frequency is exactly equal to power if you use the correct units. With RPM you just need to convert to omega, multiply by 2*PI/60. Then power is in Newton-meters/sec AKA Watts

[Ed Cesnalis]

That distance is the perpendiclur distance to a force at right angles to the rotational shaft, which for a distributed resisting load force acting along the axis of the prop is largely unmeasurable.

 

Torque x rotational frequency is exactly equal to power if you use the correct units. With RPM you just need to convert to omega, multiply by 2*PI/60. Then power is in Newton-meters/sec AKA Watts

Agreed but you said:

 

 

A more useful formula to remember is that power is directly proportional to Torque x RPM.

Using torque as a starting point to determine power isn't more useful to me because the torque curve makes it complex. Its far simpler to know that power corresponds well to my throttle setting or my RPM reading.

[Doug G.]

Isn't it true that a helical prop has only one place where it is most efficient no matter what the rotational speed? This is no standard airfoil.

If this is true, you can only de-tune it if you do not set it to the angle it was designed for. Either the tip or the root end up at the wrong angle. The best efficiency for the prop can be decreased, but not increased, no matter what the rotational speed, right?

[Admin2]

I did some reading on helical props. I don't believe we have helical props. They look nothing like the ones I saw or the diagrams.

[Doug G.]

I also find it interesting that when my LS was assembled the prop was adjusted by the A & P in accordance with the Nueform maintenance manual, not the Rotax manual.

[Admin2]

The problem was Neuform isn't Rotax. Their props like Rotax engines go on many different planes and they all need to be a little different depending on the overall design. Your striving for engine and plane performance the prop is only the facilitator and it gets set where the engine and plane need to be and no where the prop (Neuform) wants to be.

[chanik]

In the broad sense, all modern propellors are helical in basic design which just means the blade angle of attack is ratiometric with radius away from the hub axis. The useful part of the power = T x RPM x 3.14/30 is that you can see that the HP part of the rotax graph is really just the Y axis torque times the X axis RPM

[Ed Cesnalis]

Isn't it true that a helical prop has only one place where it is most efficient no matter what the rotational speed? This is no standard airfoil.

If this is true, you can only de-tune it if you do not set it to the angle it was designed for. Either the tip or the root end up at the wrong angle. The best efficiency for the prop can be decreased, but not increased, no matter what the rotational speed, right?

 

That may be true but de-optimizing engine performance or reducing its output sounds like a poor trade-off in order to optimize prop efficiency. Better to match the prop and adjust engine to optimum RPM.

[Doug G.]

Ok, I really am not trying to be a pain here. What I really want to know is the theory behind choosing max rpm for setting the prop. If it were not for the Rotax limit it looks like the hp would continue to increase beyond 5800. Why is 5800 the magic number if I normally fly at 5000?p The Neuform manual gives a template and says nothing about rpm.

[Ed Cesnalis]

Ok, I really am not trying to be a pain here. What I really want to know is the theory behind choosing max rpm for setting the prop. If it were not for the Rotax limit it looks like the hp would continue to increase beyond 5800. Why is 5800 the magic number if I normally fly at 5000?p The Neuform manual gives a template and says nothing about rpm.

 

Here is my theory, first 5,800 is the logical max RPM because it permits the realization of 98hp or if set up for altitude maximum take off power for that altitude is available. Now my CT climbs so well and 5,800 is only available for 5 minutes of climbing that I am willing to give up 5,800 in favor of 5,500 to optimize for cruise over climb. My cruise altitude is 10,000' so I pitch for 5,500 @ 10,000 and I can get more power and speed than at any other pitch.

 

No doubt that hp would continue to climb but Rotax has to put the redline somewhere, it is their call.

 

 

5,800 or 5,500 represent safe amounts of loading. If you present too large a load you could be at WOT at 5,000 RPM but the Rotax would wear due to vibration damage and your fuel burn would be high. If you did set your prop to 5,000 RPM max then you should limit your throttle and you would have an essentially crippled CT.

 

Is the Neuform prop used only on Rotax 912s? If your starting point is the prop manual in determining RPM then your next decision would be what engine best matches my prop.