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.

 

[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.

[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.

[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?

[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?

[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.

[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.

[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.

[sandpiper]

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?

 

If I have a closed throttle approach and landing, my RPM doesn't get lower than 1900 even at 50K. Somewhere in the flare to touchdown, if I don't touch the throttle, it will get into the yellow on rollout. I don't let this happen, or if it does, only for a moment.

 

During the approach, landing and touchdown I don't hear anything unusual from up front.

 

Again, my idle is set such that RPM stays in the green, until roll out, without having to add throttle.

 

During air work such as slow flight, power off slow glides, or power off stalls, the prop does not stop, nor come close to stopping. I would never intentionally set my idle so low that the prop would stop in flight.

 

I have tried idle at 1800 but did not like the landing result.

 

Being an LSRM-A I have the advantage over those who, technically, are not supposed to mess with idle settings. My changes do not cost money, only my time.

 

Of course, saying it doesn't cost me money is technically incorrect as the certification was $4G plus travel and expenses. But worth it to me. :wacko:

[FlyingMonkey]

I think I will ask Tim to set the idle at 1700rpm, and if he wants to go lower I'll make him talk me into it. That 200+rpm difference from where I am now will probably help a lot with my problem of often being too high with a 15 flaps setting. It's fine at 30 flaps, but after the recent unpleasantness I am going back to basics and practicing my 15 flaps landings.

 

BTW, Tim's home airport is Lumpkin County (9A0) in the foothills of the North Georgia mountains. I flew up there this morning out of curiosity, since it's only 40 miles from here. A very picturesque and challenging airport for my skill level. It's 3100x75 feet with a 700ft displaced threshold due to trees at each end, so usable runway is 2400 feet, and it has a significant slope to it, runway 33 is strongly uphill and 15 downhill. Lots of ridges inside the pattern so you are close to terrain quite a bit. I highly recommend checking it out, I had a lot of fun landing there.

[FlyingMonkey]

Agreed Roger, I'm not asking the engine to fix my flying. I just want the option for the engine to be "idle" instead of "low power".

[FlyingMonkey]

Hi Andy,

 

You have been moving down the right path since day one. No since in slowing or stopping now. i'm a big believer in grabbing the bull by the horns. I have actually been know to do that for real. Turned out okay 1-2 times and a few times it was the bulls turn. I'm kind of one of those best 2 out of three types so I'm dumb enough to keep going back for no other reason than it's fun.

 

[FastEddieB]

After 31 posts on the topic, I have a confession...

 

...I don't know what my idle speed is.

 

I think the last time it was futzed with was when Roger did my carb balance last October. So I assume it's in the ballpark.

 

If I think of it, I'll check it next time I fly.

 

I guess my point is there's an acceptable range, and it's easy to overthink this sort of thing

[FastEddieB]

AAAH come on what do we ever over think?

 

I don't know.

 

Let me think on it a bit!

[Jim]

After 31 posts on the topic, I have a confession...

 

...I don't know what my idle speed is.

 

I think the last time it was futzed with was when Roger did my carb balance last October. So I assume it's in the ballpark.

 

If I think of it, I'll check it next time I fly.

 

I guess my point is there's an acceptable range, and it's easy to overthink this sort of thing

 

Idle speed is one of those things that seem to mean a lot when you're a rookie, and less as you accumulate hours and learn to control your airspeed with the stick. I think you've accumulated a few hours (: