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IDLE RPM

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dgraham7(at)TWCNY.RR.COM
Guest

Posted: Sun Jul 25, 2010 4:48 am Post subject: IDLE RPM

Hello. I need to set the idle speed on my Rotax 912  80HP. The maintenence manual states

 that the idle speed should be 1400 RPM. I have a  mechanic friend who feels that would be too low. He is

 concerned about the gearbox. Any comments  ?

 Damien

 Zodiac 601 HDS

    [quote][b]

hgmckay

Joined: 23 May 2006
Posts: 397

Posted: Sun Jul 25, 2010 10:35 am Post subject: IDLE RPM

Damien,

  

 Some have stated that 1400 is OK, but I get way too much  "chatter" in my gear box at that rpm, and I know that is not good for  the gears in the gear box. I set my idle speed at 1800 because that is  where my engine idles best. Some set theirs at 1600. I would recommend an idle  speed in the range of 1700 to 1800 rpm.

  

 Hugh McKay

 Allegro 2000

 Rotax 912 UL

 380 hours

  



  From: Damien (dgraham7(at)TWCNY.RR.COM) 

 Sent: Sunday, July 25, 2010 8:48 AM

 To: rotaxengines-list(at)matronics.com (rotaxengines-list(at)matronics.com)  

 Subject: IDLE RPM

 



 Hello. I need to set the idle speed on my Rotax 912  80HP. The maintenence manual states

 that the idle speed should be 1400 RPM. I have a  mechanic friend who feels that would be too low. He is

 concerned about the gearbox. Any comments  ?

 Damien

 Zodiac 601 HDS

[quote]



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[b]

ami(at)mcfadyean.freeserv
Guest

Posted: Sun Jul 25, 2010 10:50 am Post subject: IDLE RPM

I find it useful to be able to pull back on the  throttle to get the revs to 1400, so that (in a slippery 'plane) a less  flat approach angle can be followed (in practice this does means 1400 rpm  in the glide) there is also less taxiing on the brakes, and to allow shut  down with less kickback. But agree that 1600rpm is a better normal idle  speed.

  

 Duncan McF

 [quote]   ---

rampil

Joined: 04 May 2007
Posts: 870

Posted: Sun Jul 25, 2010 11:21 am Post subject: Re: IDLE RPM

Check a recent manual and or (preferably both) Rotax Bulletins.

Years ago idle was 1400 nominal. Field experience changed

recommended setting to1800-2200 or thereabouts

_________________
Ira N224XS

Roger Lee

Joined: 10 Jan 2006
Posts: 1464
Location: Tucson, Az.

Posted: Sun Jul 25, 2010 11:34 am Post subject: Re: IDLE RPM

Hi Damien,



You can set the 80 hp rpm a little lower than the 100 hp Rotax because the compression is lower on the 80 hp. If you set the rpm too low on the 100 hp you start to bang away at your gearbox and it's the same for the 80 hp. Yes it will fly, but you will be doing a gearbox rebuild later as you build some hours. The 80 hp is better off for the gearbox sake at 1700-1800 rpm at idle. With the lower 80 hp compression you can get away with 1700 rpm for idle. 1400 rpm is just too low. I have a very slippery Flight Design CT and I can set in down in 1000' at 2800 rpm. So if someone says you can't set down a high drag aircraft at 1700-1800 rpm then I would have to say it's time for more training or more precision flying. I know I can do mine at 2800 because the guys in the UK challenged me. I typically leave some throttle in right to touchdown. makes landings smooth all the time. They said they had to be at 1500 rpm to land. It's all technique and not always what we want to blame it on.

So I agree with Hugh and your mechanic.

_________________
Roger Lee
Tucson, Az.
Rotax Instructor & Rotax IRC
Light Sport Repairman
Home 520-574-1080 TRY HOME FIRST
Cell 520-349-7056

dgraham7(at)TWCNY.RR.COM
Guest

Posted: Sun Jul 25, 2010 11:59 am Post subject: IDLE RPM

Thanks for all the helpful responses.

 Regards,

 Damien

 DO NOT ARCHIVE

 [quote]   ---

dirtfly7(at)yahoo.com
Guest

Posted: Sun Jul 25, 2010 12:07 pm Post subject: IDLE RPM

Brake pads are cheaper than gearboxes.

Joel



--- On Sun, 7/25/10, Roger Lee <ssadiver1(at)yahoo.com> wrote:

 	  Quote: 	
		  

From: Roger Lee <ssadiver1(at)yahoo.com>

Subject: Re: IDLE RPM

To: rotaxengines-list(at)matronics.com

Date: Sunday, July 25, 2010, 1:34 PM



--> RotaxEngines-List message posted by: "Roger Lee" <[url=/mc/compose?to=ssadiver1(at)yahoo.com]ssadiver1(at)yahoo.com[/url]>



Hi Damien,



You can set the 80 hp rpm a little lower than the 100 hp Rotax because the compression is lower on the 80 hp. If you set the rpm too low on the 100 hp you start to bang away at your gearbox and it's the same for the 80 hp. Yes it will fly,  but you will be doing a gearbox rebuild later as you build some hours. The 80 hp is better off for the gearbox sake at 1700-1800 rpm at idle. With the lower 80 hp compression you can get away with 1700 rpm for idle. 1400 rpm is just too low. I have a very slippery Flight Design CT and I can set in down in 1000' at 2800 rpm. So if someone says you can't set down a high drag aircraft at 1700-1800 rpm then I would have to say it's time for more training or more precision flying. I know I can do mine at 2800 because the guys in the UK challenged me. I typically leave some throttle in right to touchdown. makes landings smooth all the time. They said they had to be at 1500 rpm to land. It's all technique and not always what we want to blame it o!

 n.

So I agree with Hugh and your mechanic.



--------

Roger Lee

Tucson, Az.

Light Sport Repairman - Maintenance Rated

Rotax Repair Center

520-574-1080





Read this  topic online here:



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 [quote][b]

Thom Riddle

Joined: 10 Jan 2006
Posts: 1597
Location: Buffalo, NY, USA (9G0)

Posted: Mon Jul 26, 2010 2:31 am Post subject: Re: IDLE RPM

When I owned an Allegro with 80 hp 912, I found it very difficult to get the airplane to descend as steeply as I liked with the idle set higher than 1600 rpm. 



In aircraft with lower glide ratios this is not a problem. If you keep the carburetors well balanced/synchronized, the low compression 912 should idle quite smoothly at 1500-1600 rpm with no gearbox chatter. The worse the carb balance, the higher the rpm required to avoid unwanted gearbox stress.

_________________
Thom Riddle
Buffalo, NY (9G0)

Don't worry about old age... it doesn't last very long.
- Anonymous

Roger Lee

Joined: 10 Jan 2006
Posts: 1464
Location: Tucson, Az.

Posted: Mon Jul 26, 2010 5:09 am Post subject: Re: IDLE RPM

Hi Guys,



Thom, my CT has a glide ratio of 14:1. many people at first had high approaches and landing speeds too fast. We all found out that flying a high glide ratio quick handling plane is different from most GA aircraft. It is all in the set up , lower, better approaches, slower speeds, early setup, better spot landings. I use 3800-4200 in the late downwind and base. Then I reduce throttle to 2900-3000 rpm on final. I let that stay all the way to the ground. I have found that it is all about setup and speed control and proper positioning in the pattern. Many of the CT people can set it down in 1000' or hit a spot on the runway. Flying a CT or an Allegro or any fast handling, high glide ratio plane makes people a better pilot and helps hone their skills. No plane is really bad it is all in what you get used to and how much you practice to make you better. No matter if he is at 1600 or 3000 rpm at landing the stick will control his speed and ultimately his sink rate. I personally don't think rpm is as big a factor as people think, they just need to change the way they setup their landings in their minds.

_________________
Roger Lee
Tucson, Az.
Rotax Instructor & Rotax IRC
Light Sport Repairman
Home 520-574-1080 TRY HOME FIRST
Cell 520-349-7056

goodings(at)yorku.ca
Guest

Posted: Mon Jul 26, 2010 6:47 am Post subject: IDLE RPM

The following strategy comes from the person who was the chief engine 

engineer for many years for Katana aircraft which then employed only the 

certified Rotax 912 80 HP engine.  It has worked well for us so far for 7 

years with the uncertified 912S 100HP engine in our CH601HD Zodiac.  We 

set the idle at 1400 or so (it goes to 1600 when the engine is hot) to 

have low RPM on shut-down - the only time we pull the throttle right back 

with the engine running.  Of course, one must pull the throttle right back 

on start to engage the choke.  BUT as soon as the engine starts, we set 

the RPM to 2000 or slightly above to save the gear box.  This is easy to 

do because we have twin throttles with a good friction nut out of crashed 

C-172s.  Other throttle set-ups may not work as well for this strategy. 

We hold some power on on final, and never have less than 2000-2200 RPM 

even on touchdown.  In summary, one can set the idle low, but essentially 

never go below 2000 RPM when the engine is running.



John Goodings, C-FGPJ, CH601HD with R912S, Carp/Ottawa/Toronto.

Thom Riddle

Joined: 10 Jan 2006
Posts: 1597
Location: Buffalo, NY, USA (9G0)

Posted: Tue Jul 27, 2010 3:32 am Post subject: Re: IDLE RPM

Roger,



I am assuming your CT is the SW since that is the more common of the models sold to date in the USA. If my assumption is correct your airplane does not have the very high aspect ratio wing that the Allegro has plus its wing area is considerably smaller than the Allegro's and the typical flying weight is a good bit heavier with the same payload. The net result is your airplane has a much heavier wing loading and span loading, both of which dramatically affect the descent rate at idle power or power off.



				CTSW/		Allegro 2000

Wing Span			27'-11"/	      35'-6"

Wing Area			107 sqft/	      122.5 sqft

Aspect ratio               7.29  /              10.29

Typ.Wt w/ sam load	1200 lb/	     1100 lb

Wing loading		11.2 lb/sqft /    9.0 lb/sqft

Span loading		43.0 lb/ft	/      31.0 lb/ft



Of these factors, the one that most affects descent rate is the span loading. That is why gliders and sailplanes have such high aspect ratio wings.



Due to these major differences in the wing design, an idle speed as high as you recommend in an Allegro 2000 will dramatically reduce the idle power descent rate and extend the idle power glide compared to a CTSW with the same idle speed regardless of piloting technique or approach speed. 



My Kolb Slingshot has 22' wing span and 110 sqft of wing area with a 4.4 to 1 aspect ratio. During a typical flight it has a wing loading of 7.5 lb/sqft and a span loading of 37.3 lb/ft. My approach speed in the Slingshot is 55 mph with full flaps as it was in the Allegro 2000. At idle power I get a descent rate of 1500 fpm in the Slingshot and the Allegro was 700 fpm at same airspeed with full flaps. Both of these were without slipping.



Bottom line is that what airframe the engine is installed in makes a huge difference in acceptable idle speed for short approach capability.

_________________
Thom Riddle
Buffalo, NY (9G0)

Don't worry about old age... it doesn't last very long.
- Anonymous

Thom Riddle

Joined: 10 Jan 2006
Posts: 1597
Location: Buffalo, NY, USA (9G0)

Posted: Tue Jul 27, 2010 3:45 am Post subject: Re: IDLE RPM

Roger,



A bit more on descent rate differences. I just did my BFR in a Pitts S2-B. Its idle power descent rate at normal approach speed is 2500 fpm and this airplane has no flaps.  



This bi-plane has 20' wings and a total wing area of 125 sqft. MTOW is 1625 lb. and flies like a high powered rock with exceptional flight controls.

_________________
Thom Riddle
Buffalo, NY (9G0)

Don't worry about old age... it doesn't last very long.
- Anonymous

Roger Lee

Joined: 10 Jan 2006
Posts: 1464
Location: Tucson, Az.

Posted: Tue Jul 27, 2010 4:37 am Post subject: Re: IDLE RPM

Howdy Thom,



Nice to hear from you again. Hope you are staying out of the heat and humidity. It's been a little too humid here with the heat lately. It's our monsoon time.

We kind of got off the original topic question about gearbox and idle rpm. While we are on it. You can momentarily shut down one mag and then after the rpm drops some shut down the last one. It will help with that final hard stop to the gearbox.

The Allegro does have a longer wing than my CTSW, but I have flown both and they land very similar. To me the key in our light planes is being able to round out at 12" or less off the deck and hold the stick steady without pumping it until touch. Just a tad more back stick as it descends. 



I agree if we are trying for a short field landing, no power is better. Even in a good gliding aircraft the speed and decent rate can be controlled with the stick. Pull that stick back far enough and the plane will slow and descend, not to mention if we find ourselves too high we can slip. When we are at an idle powered approach we can keep the approach speed around 50-60 which ever you use for your plane. If I carry power like 2800 rpm to the touch I can keep it at the same speed by pulling the stick back. Will I have a slightly longer roundout at the bottom, yes, but I don't care about the extra 75'-100' because all I'm looking to do is have better rudder and elevator authority with the prop wash and slightly reduce the sink rate at roundout to make a nice soft touch. I too had a Kolb Mark III and I used the same rpm to touch which made it much more controllable at really slow speeds and always a nice soft touch.

_________________
Roger Lee
Tucson, Az.
Rotax Instructor & Rotax IRC
Light Sport Repairman
Home 520-574-1080 TRY HOME FIRST
Cell 520-349-7056

lucien

Joined: 03 Jun 2007
Posts: 721
Location: santa fe, NM

Posted: Tue Jul 27, 2010 6:25 am Post subject: Re: IDLE RPM

 	  Roger Lee wrote: 	
		  

You can momentarily shut down one mag and then after the rpm drops some shut down the last one. It will help with that final hard stop to the gearbox.



	




Don't mean to jump in, but I've always been skeptical of this particular shutdown procedure. In fact, I had a nasty fight with a guy about it on another list not too long ago to the point he ended up sending me almost mentally-ill private emails about it and I had to cut the exchange off.



But you and Thom are "da men" on the Rotax so I don't feel bad about commenting on this and I'd be interested in your input. 



My observation on my 912ULS has been that the really hard stop that joggles the gearbox is far below even 1400 rpm - it's nice and smooth when cut off until it gets to that very low rpm. And, to my mind, the staggered mag shutdown method actually introduces the possible hazard of idling the engine, if only momentarily, below 1400 rpm while it's running on only one mag at that point. I should think that that would be worse long-term than the part you _can't_ do anything about no matter what, which is the joggle right as the engine comes to a stop. 



So it seems to me you're just as well off killing the engine straightaway from 1800 or 2000 rpm, thus avoiding the possibility of too low of an idle with the engine still running. Like I said, you can't really do anything about the joggle at the very end, since that occurs at an rpm that's far below where the engine could even run anyway. 



Anyway, like I said, I'm just skeptical of the value of doing the staggered mag shutdown. I can't really see what it buys you and it has the drawback of the possibility of a too-low idle during shutdown. 



But of course, I'm fully prepared to be wrong and learn if there really is a benifit to doing this? 



Thanks,



LS

_________________
LS
Titan II SS

Thom Riddle

Joined: 10 Jan 2006
Posts: 1597
Location: Buffalo, NY, USA (9G0)

Posted: Tue Jul 27, 2010 7:06 am Post subject: Re: IDLE RPM

On all the 80 hp 912 UL engines and the one 80 hp 912F3 I help to maintain, I've done a mag check at my preferred idle rpm of 1600 and got virtually no rpm drop. I've only done this on engines with well balanced carbs and would not even consider doing it on an engine with the carbs out of balance.



I don't see any advantage of doing a sequential mag grounding rather than simultaneous grounding of both mags, at least in the lower compression engines. Can't speak with any authority on the high compression 912 engines but don't see any advantage in theory. 



However, as Lucien pointed out, if it is important to have the idle speed of the hi-comp engines at 1800 minimum idle rpm (hot) and shutting off one of the mags results in an rpm drop, this seems to be contradictory advice.  If it does something strange like kickback upon shutdown at high idle speed, maybe the high idle speed is not such a great idea or something else is amiss. 



I've never seen any problem at normal shutdown of the 912ULS engines I've worked on with the hot idle speed set at 1800 rpm (and well balanced carbs). Most aircraft mag switches, at least those made in the USA and installed on GA aircraft, including the Diamond Katana with 912F3 engine, do not have the capability to ground the two mags independently, so the question is moot on those aircraft.

_________________
Thom Riddle
Buffalo, NY (9G0)

Don't worry about old age... it doesn't last very long.
- Anonymous

Roger Lee

Joined: 10 Jan 2006
Posts: 1464
Location: Tucson, Az.

Posted: Tue Jul 27, 2010 9:11 am Post subject: Re: IDLE RPM

Hi Lucien and Thom,



One of the first things to consider here with the gearbox is what prop people use. Is it a light composite or a heavy 72" nickel edge Warp Drive. I have a 66" Warp, square tip on my plane. Rotax doesn't want anything over 68" when using the heavier props like a like Warp and no nickel edges. 

That said I just turn both of my mags off all at once and don't worry about it. The technique I stated is right out of a Rotax class with Eric and this very question was answered. You don't let one mag set for a long while it is just for a few moments to help reduce some rpm. The rpm issue and vibration is usually directed at people who tend to let them idle like that for long term. The gearbox pounding should not be an issue for a few moments, but will be for those who allow this to be the norm at idle during warm up and taxi. And like we have talked about here, it is not as big an issue with the lower compression 80 hp as the 100 hp Rotax.



p.s.

I don't think this is a real issue for us three, but will hopefully answer a few questions or put some newer minds at ease about shut down and the sudden stop.

You can do a lot of things to a Rotax that aren't in it's best interest and it will still run for a while, but eventually you will be doing a fairly expensive repair before the other guy for not following what is recommended. After I started to see how much labor and material was a number of years ago I changed my ways. I want to make it as long as possible and keep my money.

_________________
Roger Lee
Tucson, Az.
Rotax Instructor & Rotax IRC
Light Sport Repairman
Home 520-574-1080 TRY HOME FIRST
Cell 520-349-7056

ami(at)mcfadyean.freeserv
Guest

Posted: Tue Jul 27, 2010 1:34 pm Post subject: IDLE RPM

That is not correct!

Try landing in a marginal 350 yard strip with hedges at both ends and trees

close to approach (meaning rotor and the need to keep speed up meaning a

lower rate of descent at approach speeds). This is the strip I use 

routinely.

With a 2 knt tailwind you'll miss it. With a 2knt headwind - easy.

Crosswinds are the worst as no headwind component, additional turbulence and

need for greater margin above stall.

Being able to pull back the idle makes a significant difference to sink

rate and the ease of getting in (=safety); of course, in flight that doesn't 

mean 1200rpm, more like 2000, but it would be 1200 if the 'plane were not 

moving through the air.



Of course you'll maybe say 'find a better strip', but  life is full of 

compromises!



Duncan McF.

---

Roger Lee

Joined: 10 Jan 2006
Posts: 1464
Location: Tucson, Az.

Posted: Tue Jul 27, 2010 5:58 pm Post subject: Re: IDLE RPM

Hi Duncan,



I was challenged to do this on a 300m strip which is about 1000' by some friends in the UK that said they needed to have that low an rpm to land. So I took it to task and measured off the runway up at an airport. No trees, but it was easy to do at idle or 2800 rpm with 30-40 flaps with an approach speed of 50 knots. You just had to hit the beginning of the runway and leave nothing behind. I do it on a dirt strip up from me and it is 1700' and I use 30-40 flaps on it over the trees and make it down. If you miss your spot or land too fast then good brakes can be a must.

The other person didn't say he had lots of obstacles. If the runway is open it's very doable. Add high obstacles and we have an all together different scenario.

_________________
Roger Lee
Tucson, Az.
Rotax Instructor & Rotax IRC
Light Sport Repairman
Home 520-574-1080 TRY HOME FIRST
Cell 520-349-7056

lucien

Joined: 03 Jun 2007
Posts: 721
Location: santa fe, NM

Posted: Wed Jul 28, 2010 6:06 am Post subject: Re: IDLE RPM

Ok, thanks gents for the info.



I kind of figured the shutdown procedure was down in the noise of other factors like the prop. For what it's worth, my 912ULS originally had a 3 blade IVO medium on it which was a significant concern for me from day one. Fortunately, the pitch change mechanism quit working right one day and that finally gave me an excuse to get that monster off there. 



It weighed so much I literally had to redo my W&B once I fitted my powerfin, which was just under 10 lbs lighter! It was like dropping an anvil off the back of the plane....



I did try a Warp Drive 68" on my plane first, but had to abandon it due to a harmonic vibration problem that had nothing to do with prop itself. But even the Warp Drive was like a feather compared to the IVO and the engine was much happier below 2200 rpm with the WD than the IVO. And Daryl has told me that the WD meets the MOI limits of the 912 gearboxes in probably all diameters. For sure, my 912 ran/idled normally (that I could tell) with my WD fitted to it even with a 3" extension.



When I did my GB preload check a few months ago it came out right in the middle of the acceptable range, so I think I got the IVO off there in time . But there's just no comparison in how nicely the engine idles and runs in general with the Powerfin vs the IVO. 



So the monster prop thing is definitely a contributing factor, IMO....



LS

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LS
Titan II SS

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