912ULS Idle Speed

[NC Bill]

My engine is currently turning 1650rpm at idle.

 

I warm up at 2000-2200. Taxi around at 1800-2200.

 

The only time the engine idles is on landing because I pull the throttle back and do not monitor RPMs. Of course I could. I just don't out of habit.

 

Any thoughts on whether I should have the idle speed raised or left where it is.

[Jeremy MacGregor]

Most engines including my own start and Idle around 1600-1700rpm. This I usually when the engine internal temperature is between 50-75F . After oil pressure indication has reached 29+psi Rotax wants you to adjust your throttle to 2500rpm for the warm up process till the engine reaches 122F on hour oil temp at this time if I pull back my throttle to Idle I will be at 1850rpm which is exactly where I want it. This is the happy medium between not having to much float on final and not stressing your gearbox to much. There have been many forum post about this over the years both on "er" and "ier" :-) and it seems like the general opinion is 1800-1850 when warm. This is also what FD (not that it matters since they don't make the engine) and Rotax recommend.

[John]

The latest CTLS AOI specifies approx. 1500 for the idle speed (this is with the engine at operating temp). This is to allow for proper approach speeds & landing distances. This is comparible to similar LSA's.

 

For ground ops, simply increase the throttle to the proper idle speed. All CT's feature a yellow arc from 1400-1800 to discourage operations in this area.

 

John

[Roger Lee]

I'm sorry, but still have to disagree and this one is one of my soap boxes.

This is still an area that most of us (owners and mechanics) believe FD is wrong. Just like when FD came out and set the engine WOT rpm at 5200 rpm. Myself, Rotax and others complained about overloading the engine prop combination. I must have changed 60-75 prop settings myself. It needed to be up to at least 5500 WOT and now that's where FD sets them from the factory. This is wrong, too. Rotax does not recommend for the 100 hp 10.5:1 compression engine to operate or set the idle rpm at 1500. In every Rotax or other school I have been to in last 7 years it has been recommended right around 1800 for idle to protect the gear box. When you feel the 912ULS chugging then you are beating up the gear box and sending unwanted vibration to other parts of the engine and mounts. Rotax thought this type of vibration was so bad they now have the "Soft Start" module. That vibration no matter when or where it comes from is not good.

I'm sorry, but 1800 rpm has nothing to do with proper approach speeds or landing, if it does then the pilot has very poor skills. All those can be done at 2800 just as easy as 1800 or even 1500. The stick controls the speed and even though the throttle controls altitude, as they say, pulling back the stick and dropping the speed as low as you want will make you descend. I can pull the stick back to slow down for my speed if need be. I can put the CT in 1000' at 2800 rpm all the time. If a pilot in any CT can't put his CT down at 1800 rpm as he could at 1500 then he needs a whole lot more practice or a new CFI. Watching the group at the Columbia Fly-In do spot landings proves this out because these guys could hit a mark at any rpm and none were at 1500. The Rotax 912UL 80 hp with a 9:1 compression can idle to the 1500 without hammering the gearbox and associated parts. Eric Tucker has never in any school I have attended of his has ever recommended 1500 for the 912ULS. I have had many come to my shop with a new CT with the idle from the factory set down to 1500 and have complained about loosing an engine at those rpms. No one can really advocate to just let the pilot control the throttle to keep the rpms over 1500 so he won't loose an engine because being human means we will screw that up at some point and make an error in judgement and cut the throttle too far back at the wrong moment and oops we have a serious problem.

Why would anyone want the liability of causing anyone to be that close to an engine loss?

I personally don't know of but one shop that sets the idle on a 912ULS at 1500. Plus I wouldn't want to have to defend it in court on an engine out crash because the rpm was that low or it might have dipped a little lower and quit because it was too close to an engine loss and caused a crash. In court you would have to use Rotax to show 1500 was acceptable for the 912ULS and since Eric Tucker has been teaching 1800 in all his classes for years with the 912ULS that would leave someone in a bind. The industry standard is 1800 which would be what someone would be judged against. Someone would have to meet the standard of care or practice against other mechanics taught by Rotax and they have been advised and set their idle rpms closer to 1800. The Rotax manuals have too many misprints, i.e. the Operator manual says the 912ULS has an 11:1 compression ratio, it's wrong. Just one of the many misprints. So the bottom line is that the 1500 rpm shop would be judged in court by what is taught in class (for many years) and what other mechanics use as a bench mark for the standard of care. It would be a battle that I believe the 1500 idle rpm shop would loose and it would cost a fortune just to defend itself.

It is far better to error on good information and the good judgement side of the higher idle rpm than to cause an engine loss for a customer or cause any long term vibration problems.

 

 

For me,

Better safe during flight and not cause long term vibration damage than sorry.

[sandpiper]

Keep in mind that a 1500-1600 rpm static idle will give about 1900-2000 rpm with a 55K approach

[John]

Sorry, but an 1,800 RPM idle is too much thrust for a clean plane like the CT to carry in on final. The minimum RPM is, as it has always been published 1,400RPM for the 912ULS in the Rotax manuals, that is not a misprint. Approx 1,500 RPM is not a misprint, it is based on flight testing. 1,800RPM is the MAX idle speed that can be used for this engine, where appropriate for the airframe. If you have a high-drag airframe you can get away with that. Making the CT high drag by flying far behind Vl/d on final is not something I would ever recomend god forbid you should find your self ever having to land without power.

 

As far as legal issues go, we'll continue to follow the manual.

 

Finally, what is taught in the class is 1,400 - 1,800RPM, according to the airframe manufacturer. I have been to these classes as well, even taught a few semairs myself, and am familiar with the syllabus.

 

No, the engine is not going to die because it is set at 1,550 RPM. If it can't run there than there is another issue.

 

I am making this final post on this board in the interest of safety, however, I do not feel it is appropriate that I post here further. Therefore there will be no more comments from me on the subject. We are always happy to answer your tech support questions via email, or on the phone.

 

John

[josjonkers]

1800rpm is just fine and works like a beaut...........

 

 

Sorry, but an 1,800 RPM idle is too much thrust for a clean plane like the CT to carry in on final. The minimum RPM is, as it has always been published 1,400RPM for the 912ULS in the Rotax manuals, that is not a misprint. Approx 1,500 RPM is not a misprint, it is based on flight testing. 1,800RPM is the MAX idle speed that can be used for this engine, where appropriate for the airframe. If you have a high-drag airframe you can get away with that. Making the CT high drag by flying far behind Vl/d on final is not something I would ever recomend god forbid you should find your self ever having to land without power.

 

As far as legal issues go, we'll continue to follow the manual.

 

Finally, what is taught in the class is 1,400 - 1,800RPM, according to the airframe manufacturer. I have been to these classes as well, even taught a few semairs myself, and am familiar with the syllabus.

 

No, the engine is not going to die because it is set at 1,550 RPM. If it can't run there than there is another issue.

 

I am making this final post on this board in the interest of safety, however, I do not feel it is appropriate that I post here further. Therefore there will be no more comments from me on the subject. We are always happy to answer your tech support questions via email, or on the phone.

 

John

[S3flyer]

When the appropriate manual states a specific RPM for idle is that static or when the engine is warm? Mine is a bit high (higher than all the recommendations in this thread ) and I think I'll trundle out to the hangar today and turn it down a bit.

[Tom Baker]

When the appropriate manual states a specific RPM for idle is that static or when the engine is warm? Mine is a bit high (higher than all the recommendations in this thread ) and I think I'll trundle out to the hangar today and turn it down a bit.

 

I don't know if you have had rotax training or not, but setting the idle down is not a simple as just turnung a screw. Remember you have 2 carbs and they need to stay in balance. Tom

[Roger Lee]

Hi John,

 

I sorry you feel you can no longer post here. There was no personal attack here, but a different opinion from a different perspective only. If you felt this was personal then I apologize to you and the forum.These types of discussions helps everyone to see things from all directions and never look at anything from just one view. You are always welcome and we would encourage your post. Not everyone agrees here and we as a forum agree that it is okay to disagree at times. I would step out on a limb here and say most everyone on this forum are friends and that has been proven by the very close camaraderie shown at our Fly-In's and the invitations we all extend to each other to visit as we fly from state to state or country to country.That's one reason we live in the US and people from all over the world enjoy free speech. I do agree that once in a while it does help to have a little thicker skin with some free speech. This is why we or any forum are out there, to talk about the good, the misunderstood and ways to make things better and improve. Without open talk forums people can't share ideas and that's what makes all these forums good for everyone. A bad forum is a censored forum. This forum is really no different than the FAA's LSA committee where everyone has a voice or an idea that can freely be expressed and explored regardless if it's popularity. Sometimes in a forum where we only see printed text things can be misconstrued and someone may take something on a personal level which is certainly hard to see through sometimes. If one steps back on this forum there has never been a post that has ever been a personal attack, so each person , including myself, at times needs to step back and read the message as was intended and not look at it a personal. Censoring forum post has killed other forums and been destructive in nature and no one benefits from that type of exchange. No one here has to agree with the me or the next person, but they do get to post their disagreement or reasons why. This way we all learn something, that's what a think tank does. The reason I posted the items in the previous post about legal issues is I spent 28 years in the public safety and court system and more hours than I can count on the courtroom stand. I know exactly what will happen when faced with lawyers and my comments above are only meant to shine some light on what may happen in a courtroom venue. Forewarned is forearmed. I know how lawyers will look at the "standard of care practices" of an overall group to judge one. If we always agreed and there was never a discussion or difference to discuss where would we ever gain knowledge. I'm proud to say I belong to the CT family as it has proven to be exceptional in its bonding of people from around the world like no other forum and group of people I have witnessed.

I'm a sponge for knowledge and love to learn and teach. I always have a thirst for more and will always listen to anyone or research until all stones are overturned. I want to learn everything no matter where it comes from or how bad it may sound so I can make the best decision that is right for me. Shielding myself from unwanted knowledge defeats that purpose.

 

 

Please, I would ask that you reconsider and be an active member. It would be a shame to lose the insight we gain from you and for you to lose the insight from all of us.

 

 

Learning, research, exploration and above all, sharing knowledge with all is the key to civilization.

 

 

Thank You,

Roger Lee

[207WF]

My minimum idle is set at just over 2000 rpm warm. Roger Lee has said this is too high, but it works just fine on landing for me, and I like the idea that the gear box is taking it easy. I am in a warm climate in SoCal, never less than 55F temperature. OK, so I reduce power to idle abeam the tower and not abeam the numbers, but that was pretty easy to adapt to. WF

[Roger Lee]

Howdy 207WF,

There is nothing wrong with an idle at 2000 rpm. It is all a matter of preference after 1700 rpm and it can land just as well at 2000. Pilot skill and knowledge determines where and how you land not rpm. I know a number of people in other aircraft that like 2000 idle rpm. 1800 is only a bench mark or target idle to help stay away from unwanted vibrations. I try to set most of the Rotax 912's that I see here at 1750-1800. Warm up when the engine is cold should be around 2200-2400 normally to keep things smooth and allow all the different metals to expand until you reach Rotax's minimum operating temp of 120F. I tend to warm up at 2400 during the winter and 2200 during the summer, but this is just a personal preference.

[GlennM]

I would be curious to hear what an actual Rotax engineer would have to say about their manuals and the idle rpm controversy. I know I hear many misunderstandings and mis-statements of actual limits when I go to training classes at my company. I would think the courts would use the published manual and not hearsay of a training course in their judgements. Many things are not "fact" until they are written down. I would think an omission such as proper engine speeds would be fixed right away by Rotax. Just look at the recent operating instruction service bulletin from Flight Design. For a rounding error! Of course, it was an important number. If the aircraft weight is listed at more than 1320lb. on paper, it is not an LSA unless it has a waiver from the FAA, like the Terrafugia. With LSAs, I get worried doing anything that is not per the aircraft manufacturer's published limits. I think many times you don't need to worry about the courts, but insurance companies. I am sure an insurance company would love to use the idle speed as an excuse not to pay on a claim for a hard or barely off runway landing. That was just my .02.

[FlyRatz]

Hi John, hi Roger

 

I think both of you are a little right and I will bring the friction torque of the gearbox into this discussion.

 

My experience with the Rotax engines is, that the actual friction torque determines the minimum usable idle speed. If the f.t. is in the low numbers (<40 && >30 Nm), the engine will run smoothly with 1500 RPM. If it is in the higher area you will need a little more RPM to run the engine without producing extensive wear at the gearbox.

 

And yes, the more idle RPM the more complains from our pilots. For security reasons, it is not advisable to come in with a lower IAS than 1.3 times Vs0. Therefore a higher idle RPM always results in a lower vertical speed at approaches. This is unwanted if you do not have a field with a 3 km runway.

 

By the way: I always use this device to check the smootheness of the engine at idle.

 

Greetings

 

Markus

[Jeremy MacGregor]

Markus made an excellent point about the gearbox friction torque. You need to keep in mind that Rotax 912 series engine have found there way into 100s of different aircraft over the years, all of which very drastically when it comes to prop configurations ( diameter, #of blades, total weight of prop) all come into play when determining mass moment of Inertia. MMI along with Idle rpm are the two biggest factors that effects longevity of the (3) disk springs in the uls gearbox. Note: since some of you are flying the 80hp engines. There are only (2) disk springs in this configuration but because of the faster spinning prop and the lower compression, disk spring wear is slowed by quite a bit and you guys should be able to get away with 1500rpm with out a problem.

 

Things to consider....

 

Low engine rpm will speed up wear on disk springs

 

Heavy props wear gearbox faster than lighter ones

 

Low friction torque makes idle sync difficult

 

Out of sync carbs at idle will increase wear on friction torque

 

My opinion...... 1600rpm at static warm Idle will give you less float for most pilots but will increase gearbox wear, 1800rpm will increase float but will save you having to pull your gearbox off at 500hrs to replace disk springs which run about $60 but has some labor associated with it. If you run more than30% 100LL then this may not make any difference to you since your pulling the box at 600hrs anyways. If you running 91 and you don't have to pull it till 1000hrs then do yourself a favor and increase your Idle. Although I agree there should be some standardization among the fleet of same type aircrafts, I do believe that there should be other factors to consider...

[Roger Lee]

Right off the R.O.A.N. website:Rotax Blog, 30 March 2011 19:08

Engine Roughness at Low RPM

Written by Rotax-Owner.com

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There are several factors that can cause or influence engine roughness, especially at lower RPM's.

 

A rough running engine can sometimes be a combination of several contributing factors.

 

One cause of Low RPM roughness can be a gearbox with low spring pack tension (or a "loose" gearbox).

 

 

 

Keep in mind that even though the Rotax Operator's Manual for the 912 and 914 series does allow for a minimum idle of 1400RPM, it is recommended to keep the RPM a little higher to reduce roughness and engine wear. 1800 - 2000 RPM is a good place to set your idle. Prolonged idling at low RPM's can cause accelerated wear in gearbox components, which can cause increased engine roughness. Low idling and rough running can also cause wear in other components such as carburetors. Worn carburetors can cause engine roughness - it can become a wicked cycle. Therefore, proper maintenance of the gearbox is important.

 

[Bugs]

I can certainly agree with the idea that the chosen idle speed for the Rotax 912ULS is a compromise just like many engineering judgements. In the end, it amounts to the one with which you are most comfortable after weighing all the information. I am surprised that there has been ( to my knowledge) no mention of the need to constantly apply considerable braking to control aircraft taxi speed when idling at 1600 RPM or higher on hard surface. It may not be of any concern when operating on turf. My CT is idling at about 2050 RPM. On hard surface. I don't know how fast it will taxi because I don't let it reach the max speed. It's just too fast for comfort because of the ever-present need to apply more than a little braking effort. That's not good either. For that reason alone, I'm about to set the warm idle speed to 1700 RPM.

[westcoastoz]

My warm idle speed is set at 1800 RPM which I find makes for less violent engine shutdowns as it drops off another 100 RPM or so by the time you get down to No.1 ignition circuit.

The excessive taxi speed was an issue when I had the old Italian brakes fitted however since installing the Matco brake units, there is sufficient drag to maintain a comfortable taxi speed at 2000 RPM on a hard surface.