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  #1  
Old 09-28-2021, 03:22 PM
thiggins thiggins is offline
 
Join Date: Mar 2020
Location: Saluda,NC
Posts: 46
Default Exceeded recommended RPM with cold oil

In the POH for the legacy RV-12 the POH mentions a minimum oil temp of 120 degrees along with a max RPM.

Example : “Taxi RPM – 1800–2500 RPM until oil temp reaches
120° F (50° C)”

Why is this? What specific damage can occur?

Why? : A buddy of mine was taxiing with oil temp around 100 degrees and got bogged down in grass…He knew better, but advanced the throttle well beyond the RPM recommendation. He says the engine now makes unpleasant noises he’s never heard before, and is running rough. It has around 570 hours on it.

What could have been compromised? (We fear big $$$)

Who could help him diagnose it? (We need a Rotax expert. )

Thanks
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  #2  
Old 09-28-2021, 03:35 PM
Bob Y Bob Y is offline
 
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Location: Piedmont, SC
Posts: 301
Default

If nobody replies here, you might post the question on the Rotax Owners Forum.

https://www.rotax-owner.com/en/

You do need to subscribe to post questions.
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  #3  
Old 09-28-2021, 04:22 PM
NinerBikes NinerBikes is offline
 
Join Date: Dec 2018
Location: Granada Hills
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Default

The Rotax uses a dry sump tank and oiling system. There is rubber hose between the sump tank and the oil pump, which creates suction.

If the oil viscosity is too high from the oil being not up to operating temperature, and the engine rpms and oil pump demand is too great, the rubber hose, due to low vacuum pressure from the oil pump can collapse inwards and starve the oil pump, and drop the oil pressure to 0, thereby damaging the bearings and bearing surfaces of the engine, causing seizures, galling etc.

That was a big no no that your friend performed. It's possible he just destroyed the engine.
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  #4  
Old 09-28-2021, 05:36 PM
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Piper J3 Piper J3 is offline
 
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Location: Hinckley, Ohio
Posts: 2,564
Default

Just one slight correction for above post. Oil hoses are teflon lined and are very rigid. I doubt that lines would collapse with vacuum. Also, oil tank is elevated above inlet port of oil pump so positive pressure is assured.
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80 hrs Flying Aeronca Chief 11AC N86203
1130 hrs Flying 46 Piper J-3 Cub N6841H
Bought Flying RV-12 #120058 Oct 2015 with 48TT - Hobbs now 750

LSRM-A Certificate 2016 for RV-12 N633CM
Special Thanks... EJ Trucks - USN Crew Chief A-4 Skyhawk
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  #5  
Old 09-28-2021, 05:40 PM
AndrewR AndrewR is offline
 
Join Date: Jun 2017
Location: Ballarat, VIC
Posts: 60
Default

Quote:
Originally Posted by NinerBikes View Post
If the oil viscosity is too high from the oil being not up to operating temperature, and the engine rpms and oil pump demand is too great, the rubber hose, due to low vacuum pressure from the oil pump can collapse inwards and starve the oil pump
The oil suction hoses should be rated for suction, i.e. they should not collapse if the demand is too high.

I think this scenario is unlikely at 100F. While the manual says normal temperature is >120F, in reality I expect there are Rotax engines going to full power at <100F every day, and you would be hearing about more failures if it was a common issue.

It probably depends on the design of the oil system though - i.e. length of the lines, resistance through the oil cooler etc.

Is there oil pressure logging or alarm? That would be the best indication.
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  #6  
Old 09-28-2021, 06:00 PM
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emsvitil emsvitil is offline
 
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Location: SoCal
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It's possible the noises were there before...........

It's just that he really started listening because he ran the rpm up and is being ultra sensitive to any noise he didn't think was there before.
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  #7  
Old 09-28-2021, 06:53 PM
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rvbuilder2002 rvbuilder2002 is offline
 
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Location: Hubbard Oregon
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Everyone has their own opinion based on logic. But the reason already given is the engineering reason given by Rotax for why the limitation exists. Whether it’s collapse of a hose or in ability to suck the oil at a high enough rate from the tank or pick your own idea… But that is the official reason. If there hadn’t been instances of problems in the past I wouldn’t imagine that they would’ve set the limitation.
Now the limitation could just be because Rotax doesn’t know what any given airframe has for oil hoses. It was common in the past that some were just plain rubber hose with clamps on the hose barb city Chand. So maybe it is a limitation that is not realistic for some of the current engine installations but Rotax has never removed a limitation.

EDIT
The second part of the reason for the limit (that I forgot when I initial wrote the post) is that because the crank case pressure is used exclusively to push oil back to the tank, if the rpm gets too high before the viscosity is low enough, the pump can suck oil from the tank faster than crank case pressure can return it to the tank. If this goes on for too long, the oil pick-up can suck air which would result in oil pressure loss.
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Last edited by rvbuilder2002 : 09-28-2021 at 11:17 PM.
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  #8  
Old 09-28-2021, 07:03 PM
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Piper J3 Piper J3 is offline
 
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Location: Hinckley, Ohio
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I routinely run my engine at 2500-2800 RPM at startup. Engine runs smooth at this speed while warming up from a cold start, and at this speed, the engine is not making much power. Smooth engine run is important for gearbox longevity as has been discussed in many threads. The 912 uses a pressed-together roller bearing crankshaft so positive oil lubrication is not super important at engine start.
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Jim Stricker - EAA #499867
PPL/ASEL 1970 - Sport Pilot since 2007
80 hrs Flying Aeronca Chief 11AC N86203
1130 hrs Flying 46 Piper J-3 Cub N6841H
Bought Flying RV-12 #120058 Oct 2015 with 48TT - Hobbs now 750

LSRM-A Certificate 2016 for RV-12 N633CM
Special Thanks... EJ Trucks - USN Crew Chief A-4 Skyhawk
MJ Stricker (Father - CFI) - USAAF 1st Lt. Captain B-17H
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  #9  
Old 09-29-2021, 06:12 AM
thiggins thiggins is offline
 
Join Date: Mar 2020
Location: Saluda,NC
Posts: 46
Default Logs

Does Anyone know of a good Rotax person near Western NC/Asheville , NC?

Is getting a BUDS connector and software worth doing?
Would it show oil pressure in the logs ?
Where do we get a BUDS connector anyway?

Thanks for all the input/information, very helpful.
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  #10  
Old 09-29-2021, 09:16 AM
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jrtens jrtens is online now
 
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Location: Utah
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Quote:
Originally Posted by thiggins View Post
Does Anyone know of a good Rotax person near Western NC/Asheville , NC?

Is getting a BUDS connector and software worth doing?
Would it show oil pressure in the logs ?
Where do we get a BUDS connector anyway?

Thanks for all the input/information, very helpful.
You will be able to get engine and flight data logs from your Dynon or Garmin system. (oil pressure and much more)

BUDS might be necessary to find problems with the ECU, ignition and fuel injection systems, but not always. And, it usually requires an experienced Rotax AMT to read the data.
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