CTSW idle RPM

[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.

[Roger Lee]

1700 sounds like a good place to be. Fix the sight picture way out on final and remember rpm doesn't cause the issues. There is also more than one way to loose altitude if your high. More flaps and or a slip are only two ways.

[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".

[Roger Lee]

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.

[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

[Roger Lee]

AAAH come on what do we ever over think?

[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 (:

[Doug G.]

The place it makes the most difference to me is taxiing. I set mine at 1800 per Rotax recommendations and feel it is too fast on taxi. I have to use the brakes a lot.

[FlyingMonkey]

The place it makes the most difference to me is taxiing. I set mine at 1800 per Rotax recommendations and feel it is too fast on taxi. I have to use the brakes a lot.

 

Same here Doug...at 1900+ it wants to taxi at 25 knots. WAY too fast IMO.

[sandpiper]

Same here Doug...at 1900+ it wants to taxi at 25 knots. WAY too fast IMO.

 

Then taxi at the bottom of the green and use brakes to keep it slow.

 

I would rather change some brake pads (doubtful) than damage the gear box.

[Jim Meade]

The place it makes the most difference to me is taxiing. I set mine at 1800 per Rotax recommendations and feel it is too fast on taxi. I have to use the brakes a lot.

 

Doug,.

Where do you find the Rotax recommendation for 1800 rpm? I've been looking and haven't found anything other than the 1400.

[Tom Baker]

My first CT when delivered idled at 2000+. I really had to use brakes while taxiing.

[FlyingMonkey]

 

 

Then taxi at the bottom of the green and use brakes to keep it slow.

 

I would rather change some brake pads (doubtful) than damage the gear box.

 

Hmm...people have talked about gearbox "chatter"... This is the first time somebody has explicitly mentioned "damage". Is damage possible? At what RPM? How long does it take to cause damage at that RPM? Does anybody even know the answers?

 

I'd rather have an idle set around 1700 than the 1900+ I currently have, it's just uncomfortable for me to be on the brakes all the time, especially since I recently had air in the brake lines and they quit working. I'd hate to to that develop at a fast taxi. I can always throttle up to whatever the recommended taxi RPM is, if it's over 1700.

 

I also don't like that when I'm at idle in the air at approach speeds I'm still making 2100rpm or more. It just seems like more power than I want an idled engine making. YMMV of course.

[FlyingMonkey]

My first CT when delivered idled at 2000+. I really had to use brakes while taxiing.

 

Mine idles there until it warms up, then drops to 1900-1950. It's a lot of work!

[Ed Cesnalis]

I can use a bit more power to taxi up here, 1,700 - 1,800 is fine for me. It seems it can be hard to nail a target number so in that range works for me.

[Roger Lee]

Depending on which brake set you have makes a big difference too. the Marc brakes with standard round ball bearings roll very easy and the Matco's with the tapered roller bearings have a little more friction on the roll. Like I said the idle rpm number is a range so set it for what you like. just know too high or too low causes hard starting and too low causes excessive vibration. You're doing things right just keep going. Ed's 1700-1800 is a good place to be for most people. Remember there is no exact number and no exact number is good for everyone.

[FastEddieB]

Data point -

 

My 5.00x5 Marc Ingegno wheels use tapered roller bearings.

[Doug G.]

Jim, I thought I had found it in one of the manuals, but it is not there. I also took the yellow indication on my tach to be a negative.

The source for my setup ends up being the videos on Rotax- Owner on mechanical and pneumatic carb balancing, and a previous conversation on this forum where I was told 1800 was where it should be set.

On the other hand I have heard the gearbox chatter when it is set low and there is no way I would go anywhere near 1400. (I do plan to go a bit below 1800 though.)

Thanks for correcting me. I had followed the manual on my first annual and assumed that the idle was there.

[Doug G.]

CTLSi - Do you know this from experience?

[Ed Cesnalis]

I found 1,400 in writing.

 

http://www.rotax-air...okus/d04670.pdf

 

 

1,400 here too

 

http://documents.flightdesignusa.com/SW-POH.pdf

[FastEddieB]

Well, that settles that!

[Roger Lee]

"Well, that settles that! "

 

Nope, not so fast Chumlee. :unsure:

 

 

It is another not so good item in print. CTLSi is right because the IS engine has that high compression and it's idle should be up between 1700-1800. This unfortunately is just like the poor interpretation for the 912ULS. Rotax didn't take the time to make a correction. It just doesn't run at 1400 because the compression is identical to the 912ULS at 10.8:1. It has the same gearbox and it will take a beating not to mention transfer all that vibration into the crank and other parts.

 

You have to read between the lines and interpret.

 

Not my doing, but it has to be done.