CTSW idle RPM

[FlyingMonkey]

I'm planning to take my CT to Tim Dawson at Optimum Improvement on Saturday to have the idle reset and carbs balanced. My current idle RPM when the engine is warmed up is about 1900rpm; Tim suggested starting at 1650rpm, and if that didn't seem smooth enough to bump up to 1700rpm.

 

1650 sounds a little low, should I try it there or just have him skip right to 1700? I'm hoping the lower rpm will let the airplane come down better at idle and improve my landings, especially with 15 flaps.

 

Also, if anybody has used this shop before, I'd love to hear any feedback on them. They seem competent and said all the right things, and they were referred to me by my instructor who has a CTSW on leaseback to a flight school here.

[mocfly]

I would skip right to 1800.

[FastEddieB]

I've used Tim Dawson - if he's still at Dahlonega, that's right over a ridge or two from Copperhill.

 

Anyway, he helped me with a coolant leak, compression check, carb balance, that sort of thing. He seemed quite competent and he's who I would use locally for ROTAX work.

 

As far as the idle goes, I'd err on the side of lower rather than higher - when set low you can regularly still use a higher setting most of the time, but the slower option is handy for landing shorter if you ever need to.

[coppercity]

1650 to 1750 is what I have used for several years (1800 hrs). I routinely land on a 1100ft runway at density altitudes approaching 9500' no issues.

[FlyingMonkey]

Thanks guys...

 

I agree lower is better for idle, as long as you don't get into roughness or risk a stoppage if you cut the throttle to idle quickly. At my current 1900+ idle, the airplane doesn't want to slow down quickly at lower speeds, and it gets hard to generate fast sink when needed for landings. Also with no brakes on level ground the airplane will taxi at 22 knots, about twice as fast as I generally like an idle taxi to be. I can always throttle up to go faster, but it's hard to go slower than idle.

 

Chris, why do you like 1800? That seems on the high side for me.

[procharger]

Just reset mine to 1650 does help get the plane to slow down. No lower or gear box starts to rattle or chatter some.

[FlyingMonkey]

Thanks procharger.

 

[Roger Lee]

Many recommend 1800 because that is what is usually taught in Rotax school. They use a standard number because it's easier when dealing with a mass of people.The range could be (with some understanding) from 1650 and up to 1900 depending on the plane. I usually use 1700-1720 for most LSA. If you use 1650 just know you do not want to sit on the ground and let this idle there. Keep the rpm up to at least 1750 as a minimum and higher is better. The 1650 for the approach will allow you to land a tad better, but you won't see that 1650 because the air flow will keep the rpm up. Since you have it at 1900 it is even higher on the approach with air flow help.

if you have the old Marc brakes then the plane may want to roll at 1800+ rpm, but if you have the Matco's with the tapered roller bearing that have more friction then that isn't always a problem. I usually set most peoples rpm at 1700-1720 If you are just sitting and waiting to take off or in a line then it is easier on the engine for vibration and not let it carbon the plugs if you idle at 2000. First start of the day letting the engine warm should never be below 2000 rpm. You're better off between 2100-2300 rpm. There is no load on the engine her, but lets it run smoother until all metals are at minimum operating temp.

 

Just remember when people talk about the Rotax 912 it usually has a range of acceptable parameters, but that is not always 100% true.

 

___________________________________________________________________________________________________

p.s.

Throttle doesn't control your speed on approach and landing the stick does. If you're at 1700 or at 1800 pulling the stick back further will slow you down that's why you can land easily at 2700 rpm. Of course if you are shooting for the shortest possible landing then the lowest idle is better. Too many get hung up on rpm and let that become a mental crutch. Just pull the stick back to get what you want for speed. You can land all day at full stall and 2800 rpm. I can land all day long in 1000' with 2800 rpm and any flap setting. Before you say no I have already done all the test because I was challenged by our friends in the UK that use 300m grass strips many times and said you needed to be down to 1550 rpm to land.

Technique not rpm is the key. For me the extra rpm just adds prop wash over the tail for a much more solid control instead of a more mushy control feel and a need for more input on the controls.

 

But to each his own.

[FlyingMonkey]

Roger, I'm not sure what you are recommending...should I have the rpm adjusted? To what rpm? Or is your point that it's all just personal preference?

[coppercity]

Thanks Roger, we dont let it idle below 1800 on the ground but having the slightly lower setting on approach can help if you end up high. If your at 55-60 knots you wont see below 2200 until you touchdown and have slowed below 35 knots. By the time you taxi off, a slight bump up the throttle gets it above 1800 again.

[Roger Lee]

I would recommend around 1700-1730, but it can vary with personal preference. If a person can't land in this rpm setup then they need some help.

[N89WD]

9WD is at 1900, smoother idle, no box chatter. Have no problems. It may be I stabilize my approaches a little sooner to get the plane set up.

[Roger Lee]

To me rpm isn't the REAL issue, technique is. Setup approach and early recognition and adjustments in your approach and landings are the key. Speed can always be adjusted for. If your technique isn't good or you don't recognize you need to make an adjustment then any rpm can prove to be bothersome.

[Jim Meade]

The Operators Manual says 1400 rpm is the minimum, and the reason for a minimum is to prevent gear box damage. I've heard, but haven't found, that there is a service document (SB?) discussing idle setting. Can someone point me to it?

 

The reason given to avoid a high idle is because it can make the engine hard to start.

 

Is there some Rotax sponsored publication that discusses why the 1650-1700 rpm range is preferred to 1400?

[Roger Lee]

The manual is written for three engines, 912UL, 912ULS and the 914. 1400 is better suited to the 9:1 compression on the 912UL and not 10.8:1 on the 912ULS. You are supposed to read all service documents (about the last 13 years should suffice) and all the 5 manuals then apply what goes to each engine. You can not work or understand a Rotax engine without doing this or having all this at your disposal. Rotax would prefer a higher idle for smooth running on the 912UL too, but it doesn't need to be quite as high as the 912ULS since the 912UL has a lower compression and usually no slipper clutch. High idle would be 2000 or more. Idle rpm is not specific, but like many Rotax items, it's a range. There is leeway here and it becomes personal preference.

 

Rotax school is where you can get not easy to find answers. Not everything unfortunately is in the manual like any MFG, it's not all inclusive and we have some translation issues from overseas. Even when taking test in the Rotax schools not everything is taught. You usually spend a night or two going through all the manuals looking for answers.

[FlyingMonkey]

Roger, can you say why you would not sit on the ground at 1650? Does it not circulate sufficient oil or coolant or something?

[Roger Lee]

With a compression ratio of 10.8:1 it beats the gearbox because of its 30 degree gear travel on the dogs and sends unwanted vibrations throughout the engine. It is more or less lugging the engine. The 912UL at 9:1 can handle this because of the lower compression and no slipper clutch.

Look at it like a souped up car. If you jack the compression way up by a bore and stroke and add a new cam and try to idle too low it wants to shake itself to death and gets close to stalling out the engine.

 

Will your 912ULS at down at 1650, yes, but it isn't good and has never been taught that way in any Rotax school when talking about the 912ULS 100 hp. If you could pull the throttle back that far, but you're coyote enough to idle up where it should be then it wouldn't be a big deal, but not everyone thinks like that. When an Mfg teaches a class they try and make things simple because many won't get or will forget the fine print and do the wrong thing so Rotax picked 1800 for idle. Not too low and not too high and no fear of being too low and have an engine quit at a poor time. It has quit on some on an approach set too low. Liability, liability, liability so some class teachings could have more than one answer and many times do which a number of older 912 mechanics and owners have learned, but one safe answer is given. That's why sometimes you see more than one answer here or other forums. It may be a single answer in a class, but actually may have a range of answers if you understand the engine and its design parameters.

[Jim Meade]

You know, Roger, I asked you a simple question - where is the reference on 1650-1700 rpm and I get talked down to like a school boy about how I need to read all the manuals and the SB and how I need to be able to pick out the pertinent aspects of each engine from the manual.

 

In the first place, before I typed that question, I had done at least a title search and in a few cases a document search of the service, line and heavy maintenance manuals and the SI, SE and SB for the 912 on the Rotax web site. So, I had at least attempted to do what you implied I had not done. Plus, I've been to the service and line maintenance schools, so the only school I haven't attended yet is heavy maintenance and I'm not so sure it entails idle settings, a topic that would seem more in line with service or line maintenance.

 

My experience with Rotax schools is, again, directly contrary to what you continue to say. I've never heard a word that is not straight out of the manual. I assume if I go to heavy maintenance school Eric will take a few minutes to talk to me about the intricacies of idle setting.

 

Your discussion of compression ratios is more in line with what I'd hoped I'd hear. I know someone who is competent on motorcycle and car engines and I'm going to ask him what he thinks about that and how it might affect the gear box.

 

Let me ask you specifically to give me the reference to the Rotax publication that says 1400 is too low on the 912 ULS. Keep in mind I made an honest effort to find it last night and it seems you have this information readily at hand so I hope it's not too much of an imposition to ask for it.

[C ICEY]

If anyone has ever really listened to a 912ULS idling at 1400, then they will know not to idle it there......... or else their hearing is BAD> perhaps they could place a rod on the gearbox and then their forehead on the rod. leave it there for a while.

 

Roger's explanation is great IMO.

 

beyond that, the ONLY place one might consider the LOW IDLE is if you are on floats. Prepare then to work on the gearbox. jmho gleaned from my ears and respectable mechanics.

[Roger Lee]

Jim,

I'm not talking down to you. This is why I really prefer phones over written text because of interpretation and tone misinterpretation. i know you're having a trust issue with me and I don't know how to fix that because everything is not in writing and that isn't my doing. All I can do is give you information from the documentation when that exist, from several Rotax classes over the years (and I learn new things I didn't learn other times from each class because they aren't discussed in every class and it depends on who teaches them) and from years of experience. That's all I got. I know you want everything in writing, but that isn't the way this particular system is setup and I can't change it. I'm one lowly guy in the system trying to help my friends.

You have always wanted documentation. The only way to get that education is through the documentation that currently exist and this is where it all starts. When I first got out of my first of many Rotax schools (one will never get you all the answers) I read all the manuals cover to cover and then went after the bulletins. I did exactly as I have suggested to you and nothing less. Even then I didn't fully grasp everything until I had more years of seeing a hundred engines. Then when I would work on some of my first engines with the manuals right there with me. When I ran into a tough spot I called an expert with more time than me. I don't know what to tell you, but this is where and how to get the information. It may take years if you do it by yourself, but that can be shortened if you rely on some others that have already done it. Even someone knew like the folks at CPS, it will take years for them seeing, hearing and teaching to get caught up. It isn't a short process. i still go back and read at times to brush up on things you rarely see. Eric is very in-tuned to liability since he teaches and represents Rotax. Many times he will only give an answer to cover that and not open up about anything that could be taken out of context or cause him or Rotax any liability. How do I know this, is because when I went to my first Rotax classes I had done some research ahead of time. He flat stated his answers were to cover Rotax. After I got to know him over the years things changed and we had some good talks that were different and more in depth about certain subjects. Some things taught strictly in class are not always the bottom line. There are ranges and exceptions, but those won't be taught to the general public.

 

Not everything you get about Rotax is etched in stone. Things taught in class may be simplified to help narrow down the learning process and cover for liability, but many things still have a range of parameters.

 

Like I said the manuals are written for all three engines, but in some cases are poor in the separation of specifics between the three. Some times they are very specific. The perfect example is your reference to the idle rpm. They made it one idle rpm at 1400, but that doesn't work for the 912ULS. You can test it yourself. Set your idle to 1400 rpm and see what happens.

 

I have a class in Nassau next week with Eric and I'll try and remember to ask why they don't separate some info better. That said he doesn't write the manuals either, they come out of and interpreted from Austria. This is also why they have bulletins and manual revision over time to help correct interpretations. I don't have all the answers to the why's, I just try my best to help the few.

 

A simple example of manuals not being correct was the idle stop screw direction of turn for the idle set. It said counterclockwise for many years and should have said clockwise to increase the rpm. It just got corrected this year. Some people went berserk and said it would cause people to crash. If you couldn't see or tell you were decreasing the idle rpm when balancing the carb then you really had no business doing it. That one was so obvious it is like asking which way do I depress the gas peddle to make a car move.

 

 

Your question:

"Let me ask you specifically to give me the reference to the Rotax publication that says 1400 is too low on the 912 ULS. Keep in mind I made an honest effort to find it last night and it seems you have this information readily at hand so I hope it's not too much of an imposition to ask for it."

 

 

 

Answer:

I can't because the documentation isn't separated or correct enough to be specific to that question as it is written in the manual.

 

Sometimes I personally wish I could be more specific or point at specific documentation, it would make my life much easier and people may not have as many questions, including me.

[sandpiper]

Just because an idle is set at, say 1600, doesn't mean you let it run at that speed. I set mine in that vicinity so that at 60K on approach, with throttle closed it will indicate 2000 RPM +/-. Higher than that and I get too much float and find it more difficult to land short. Once on the ground I keep it at 1900-2000.

 

Use the brakes, short bursts, to control speed. You will not wear out your brakes if you have Matco. I have done this for the 3-4 years I have had Matco with very little wear.

[Jim]

My engine was set to idle at 1400 when I took delivery of the airplane. During the transition training, an aggressive low speed stall was demonstrated and the engine stopped. I like the idle at 1700-1800. And I always keep the ground rpm high enough that I can't hear the gearbox complain.

[Jim Meade]

The lack of specific Rotax documentation drives me crazy. The idea that Rotax only teaches some thing to cover liability I understand thought I don't like it. My question to Roger is why would he take the chance to publicly discuss procedures that Rotax seems won't back him up on.

 

For those with a low idle setting, when the prop is windmilling on descent, as on final, can you hear any gear box chatter? Or does this only happen when the prop is under load, even though at idle it is a low load?

 

I'm interested in some more discussion of the relationship between gear box damage, compression and idle speed. If Rotax is willing to say that 1400 is OK for an 80 hp low compression engine, then there should be some scientific way to determine an equivalent rpm for a high compression engine. I am not saying I am opposed to a little bit of personal preference in the exact setting, but I think it is a fully valid question to ask Rotax to specify an rpm or rpm range that is appropriate for the high compression engine and I should not have to rely on a closed-door discussion on the topic the occurred "after class".

 

Roger is exactly right. I want it in writing.

[Roger Lee]

"My question to Roger is why would he take the chance to publicly discuss procedures that Rotax seems won't back him up."

 

Because I like to pay forward for those that helped me in the past and I just like to help people. Many people on several forums have become my friends over the last 10 years and it would be hard to turn my back on them. The things I tell people aren't isolated items, they are generally well accepted in the Rotax community depending on your educational and experience level.

 

 

My quote for the day and maybe a good one to live by:

Help others as you would want to be helped and come to the aid of others when they can not aid themselves.

 

I guess after 30 years as a firefighter and always helping people at a low point in their life some habits die hard.

[Ian]

....... These German companies are small, and even Rotax a division of a much larger company still runs as a Euro entity independent of the bigger BRP in Canada and so does pretty much what it wants and how it wants.....

 

Just for accuracy Rotax is Austrian, not German but as you say now owned by Bombardier Recreational Products.