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912 rebuilds 2000 hours…. Evidence of 5000/5400 rpm cruise differences. Avgas or Mogas differences.


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Can I ask if we have anyone willing to discuss their experience of dozens of Rotax 912 rebuilds that have known operational conditions of…

 

4900/ 5000 rpm cruise WOT versus 5300-5450 rpm WOT Cross country pilots as one point fo comparison and  school aircraft as another to compare equally.

 

Same goes for Avgas versus Mogas. 

 

Proof must be in the eyes and measurements of the guys actually doing this. I lean on Robert Lee in USA as the guy doing this for decades. It’s 5400rpm approx cruise and Mogas. Anything else as detrimental.

 

However, seeing as there is conjecture I’d really like to have more data based input.

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Without manifold pressure this discussion is meaningless. 5400 wot at sea level is not the same as 5400 wot at 5500 feet. Service letter SL-912-016 provides information on how rotax would like the engine to be operated. 75% cruise power is 5000rpm and 26in MP.

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Started writing the question with map and I changed to WOT.

 

The strong opinions must be coming from somewhere and you’d think it would be the rebuilders who are aware of pilot habits. 
 

Mine at 2000 hours is going to be the 5400rpm WOT guy.

 

The other obviously a 5000rpm with a known throttle habit/ setting that’s prob also less than WOT

 

 

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Okay 5200rpm seems to be the sweet spot for cruise based on what I’ve been reading and below and maybe mentioned by one or two pilots here. Bold at bottom via Roger Lee the most definitive. 5200 combining fuel economy with engine at favourable RPM for longevity.

 

Rotax bulletin.

 

https://legacy.rotaxowner.com/si_tb_info/serviceletter/sl-912-016.pdf

 

Rotax bulletin 3.1.2 No WOT continuous below 5200 RPM and that’s what your ear/ feel is telling you when it labours down at 5000rpm WOT

 

I was trying some 5000 RPM low MAP with consideration to fuel economy yesterday. Had my own personal runway rain cloud for landing so didn’t get a lot of time to play around. Pics yesterday below. Flying to commute so have to cop cross winds and rain within reason.
 

Seems to be fairly dramatic drop in fuel consumption. 130kn 5400 rpm 27 map is some 22lph. Dropping map back to 20 and 110kn around 12 lph… I’ll test this more. Seems too big a drop in lph. 390nm trip and get there in 3 hours/ 66l or 390nm trip in approx 3 1/2 hours for 36l and save some 60-70 dollars or stretch a trip to avoid a refuel and get there at the same time with less dollars spent. Maybe…

 

This provides a reason to run 5000RPM range lower MAP. However, is it easier on the engine based on Rotax 2000 hour tests? Don’t know.

 

This lifted from Robert Lee in CTflier 2012. 
 

The Rotax 912 series engine was not designed to run under 5000 rpm. It was specially designed to run in the mid 5000's. Running in the 4000's will cause excessive vibration (no you can't feel it) to the engine and more heat. The jetting in the carbs is set up to run in the 5000's. Running in the 4000's with 100LL will absolutely cause more leading and faster. If you run a steady diet of 100LL run the engine around 5300-5400 in cruise to help keep the lead blown out. We are not talking about 4000 rpm to land or slow to look at something. That is intermittent. We are talking the most used or sustained throttle position that is important to the engine.

Set your WOT to be around 5500-5700 for a ground adjustable prop (closer to 5600 is better). Use a cruise rpm of 5100-5400 rpm. Your Rotax 912 can really run at 5500 rpm all day 24/7 if you wanted. That said I see no reason to run at 5500 24/7 just because it burns more fuel, you don't go that much faster than 5200 or so and there is a little more wear at the higher rpms if it is a 24/7 run. My 2006 CTSW cruises around 135 mph at 5100-5200 depending on the days conditions. The 6 CT's at my field I have set up are about the same and yes they can go faster with more throttle in cruise. (i.e.5500 rpm) Can a CT fly faster at 5700 rpm WOT compared to 5500 rpm, yes. IF you set the prop to get 5700 rpm at your average altitude then it will run better at the higher altitudes because it will be able to get more rpm up high verses too much pitch causing it to bog down some at the higher altitudes due to loss of HP and torque to turn the prop at that particular pitch setting. The rule of thumb is a 3% loss of HP for every 1K ft.

 

This from another forum. 

 

I guess I will share a few more tidbits about the Rotax that you won't find in print.

In the manual it says to only run 5500-5800 rpm for up to 5 minutes. In truth it could run 5800 all day. It could actually hit 6200 rpm and not cause any damage. The engine probably won't float a valve until around 7000 rpm. Rotax set the 5500-5800 rpm and time limit as a liability safety margin. So now you can see that it was designed for higher rpms and running way down in the 4000's isn't great. That's almost like running a 3 speed manual shift car in 3rd. gear at 15 mph. Not quite that bad, but it makes my point.http://ctflier.com/public/style_emoticons/default/wink.gif

 

From Roger Lee some 6 years back….

 

Hi Claus,

I see you have a 1680mm (66 inch prop) ground adjustable prop. WOT means Wide Open Throttle.

Since you do have a ground adjustable prop you usually want a prop pitch set to get a good overall balance performance. If anyone is over pitched there are ZERO redeeming qualities with over pitching. You lose out on everything. Typically a good rpm be to be setup for for a balanced performance envelope is 5600-5650 rpm at WOT (wide open throttle) at your average altitude in level flight. 
A prop getting 5200 on take seems a little under pitched and would get upwards of 5800 at WOT in level flight. Nothing really wrong with that. That gives you a good climb prop, but you would lose some speed and fuel economy. In other words you'll have to give it more throttle and rpm to do a decent cruise speed which in turn would eat up more fuel. Most guys are around 4950-5000 rpm at takeoff and that usually puts them in the 5600 range, Adding a little pitch back into the prop will also increase temps a small amount. Probably not much in your case.
Your engine really isn't designed to run its life in cruise at 4500-4800 rpm. It is better to be over 5000 for cruise. Most of the guys I know here have there 912iS prop set up to get that 5600-5650 rpm and cruise around 5100-5400 with the average being 5200. Low cruise rpms is also causing you low engine temps.



 

 

 

 

 

 

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Still leaves me curious what Rotax rebuilders are finding on engine pull downs that makes some favor 100ll over mogas. My reading says without decalin additive 100ll is going to cause valve seat problems, sludge in oil and possibly detonation. Detonation obviously being a major problem. Decalin  converts the lead, why one wonders do some LAME recommend 100LL in a rotax 912 and why isn’t decalin recommended as an additive if so.
 

What experiences are they having that informs this recommendation?
 

Roger Lee again responding to a question on decalin and Rotax response. I think I’ll just make Roger my Rotax guru and leave it at that seeing as he always has his hands on these engines insides.

 

It isn't approved in writing. I classes and verbally they say okay. It's the Austrian's that would have to take it to task and run hundreds if not a thousand hours of test before they would put it in writing. It isn't a priority for them. In the US and other parts of the world real life experience has shown and proved Decalin isn't an issue and works to help scavenge lead deposits.

 

Decalin like TCP is absolutely not a cure all for lead deposits. It only reduces the lead build up. Without these scavengers lead would build up worse than with their use. Anyone that has torn into a Rotax engine using leaded fuel can see the deposits instantly. If it cured all the lead problems then many GA aircraft using these products wouldn't have lead issues and owners like us wouldn't have to have 25 hr. oil changes because the scavengers would have completely removed the lead, but we all know that isn't the case.

Decalin and TCP only reduce lead build up and by changing the lead additive TEL from an oxide to a phosphate.

 

 

Cut and paste:

"What is TCP Fuel Treatment and what does it do? TCP stands for tricresyl phosphate which is an effective tetraethyl lead (TEL) scavenger. During the combustion process, TCP chemically converts the TEL to lead phosphate, which is less conductive, thereby reducing spark plug fouling. TCP use does not impact the power provided by leaded fuel."

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This is from Rotax in Austria

 

Can I use Avgas?
Yes, Avgas is approved. But; special consideration must be given to the lead
contained in Avgas. This lead will be found in the oil and combustion area as it is
not completely burned by the engine.
Please note the following for Avgas use:

Changing the oil every 25 hours
Use a semi-synthetic oil as a fully synthetic oil does not capture the lead.
Use Good operating practices such as high manifold pressure high RPM
to minimize the build up of lead. An engine that is “lugged” will accumulate
more lead as it is not burned off in the combustion chamber.

Watch for sticking wastegate on the 914. Lead will gum this up as well.
 

 

Good operating practices
Successful operation of the 9 series engines depends on understanding its
differences and treating it accordingly. The basic difference is that this is a
geared engine that is designed to run at 5500rpm. The other conventional aircraft
engines are direct drive and run at about 2500 RPM.
Gearbox: anytime you have a propeller and pistons connected you can have
problems, huge forces are at work and the gearbox is able to handle them but
the operator must treat the engine properly in order to maximize the longevity
and reliability.

This is not a snowmobile or chainsaw, do not use rapid throttle
movements as this causes undue wear on the gearbox. Smooth and
steady is the mark of an expert.

Avoid low idle speeds; at idle the piston pulses are more pronounced and
the gearbox has to deal with a lot of pulsing. This is worse when
compounded with a heavy prop. (Rotax has a limit for the propeller
“moment of inertia”)

Do not take off or cruise at low engine speeds. The engine was designed
to take off at 5800rpm and run its whole life at 5500 rpm; the ignition,
carburetion and valve timing are all designed to be at their best at this
rpm.

o Low piston speeds actually contribute to piston wear as the
“rocking motion” duration is increased.

o High prop loading at low rpm increases stress throughout the
drivetrain.

o More combustion byproducts (carbon and lead) build up in the
cylinder head with low engine speeds.

Avoid excessive carb heat; this is not a C-150, this engine is not prone to
carb ice so the teachings of the average C-150 pilot are not relevant. If
safe and sensible a quick check for a normal rpm drop when carb heat is
applied is all that is needed.

 

 


 

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Two of my beliefs regarding Rotax 912 UL and ULS engine operation are:-

 

1.  For any engine / prop set up that achieves 5,200rpm WOT in climb or S&L is to reduce rpm by at least 100rpm within 5 minutes as the engine is fully loaded / stressed.

2. Best to set prop pitch to achieve rpm for WOT at 5,800; and don't run more than  5 minutes at that rpm.  Maximum continuous rpm to be 5,500.

 

Then considering that most installs have springs set to pull throttle to max rpm if throttle linkages brake; so should the throttle linkage brake and carbs go to 5,800 then you will need to stop engine before exceeding 5 minutes at 5,800 otherwise serious dame may occur to the engine.

 

Therefore a perfect setup in my opinion would be to set throttle / prop itch for 5,800 and have adjustable high throttle stops that prevent exceeding 5,500 in the event of a throttle linkage failure.

 

Also when my WOT setting is achieved I will never run at that rpm continuously.  At present WOT is 5,450 and if I was to run at a continuous rpm it would not be higher that 5,350rpm.

 

 

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I might add - 5800 is a good goal, but  that prop (over) speed can be strongly dependent on airspeed.... 

 

ideally- a need rev limiter /governor

 

 

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5800 rpm is mainly for CSU type operation. Ground adjustable should be set for 5500 WOT.

 

If you set a ground adjustable one at 5800 when you throttle back to cruise rpm you are now on the back side of the thrust curve from the prop..less pitch is on at say 5000 rpm   if you set max rpm WOT at 5500 then of course you have more pitch on at 5000 rpm so the aircraft will cruise faster. Obviously for takeoff the 5500 setting for a gound adjustable in theory will produce slightly less takeoff performance that the one set at 5800 rpm. No different to an outboard engine prop. Ski boats are usually a finer pitch to get the skier out of the water faster but high speed skiers have more pitch on the boat prop.

 

The other thing to think about is those throttle springs on the rotax. Set your engine at 5800 WOT then if the spring breaks...and that is not uncommon then you have 5 mins or so before the engine goes bang or you turn it off and find somewhere to land . If you set the prop for 5500 WOT in the air then if the spings break well then you have a good level of margin to get where you need to go and just turn the engine off when you get close to the field

 

This isnt specific to a particular type or brand of prop it is just a "makes sense" for the engine operation.

I had my Bolly set for 5500 rpm at WOT for the 10 years I was flying the girlfriend. I was told to do this by a Rotax engine guy who also is a racecar engine builder for his profession. 

He is also a flying instructor and has been for a long time. I took his advice more than 10 years ago about this. 

It also happens that this is how Eprop have designed their props to operate. The new Glorious CSU inflight adjustable prop is set at the 5800 WOT because it is a CSU type and you can set whatever pitch you want which of course is the ideal way but there is a big difference between 3k and 12k 

 

Mark

 

 

 

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Caution Skippy?...my opinion and proven in practice on my own equipment. Also design and operation from the company in the world that makes more than 24,000 propellors a year. What are your qualifications?

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FWIW, the RV-12 has a ground adjustable prop and Vans suggest a WOT of around 5650 is a good number.

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Posted (edited)

There are a few different prop combos...

 

fixed.

ground adjustable

In flight adjustable - a few steps

in flight adjustable  - continuous.

True constant speed (and governor) 

 

I feel the answer to the above discussion must always be prefaced with exactly what you have bolted to the engine.

 

Certainly though, if the geatbox has to couple LESS power per revolution, that is -  operation at higher RPM rather than lower RPM ,  then I think that rules the roost. 

 

 I dont know what the relationship is with gearbox wear and RPM. Turbs would know.  (IE whether the extra wear at purely due to higher mesh rate  is inconsequential compared to  operating the prop too slow (high gearbox load per revolution) leading to higher wear loads.

-glen

Edited by RFguy
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There’s a zillion of these donks in operation world wide, if half the old wives tales here mattered there’d be busted donks littering the countryside😂

They sure are one tuff donk as many are in the abusive training environment👍

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Its an aviation motor; its designed and approved to be run flat out continuously at (x) manifold pressure and (x) max rpm right up to zero tbo without failure... if it fails before zero tbo something went wrong.

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