Oil Thermostat for the Rotax 912uls

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Oil Thermostat for the Rotax 912uls

Postby scubarider2 » Mon Apr 06, 2009 7:50 am

Well, I now have 126hours on my bird with one annual under my belt.  If it were not for that stupid thing called a "job" I would have triple the hours!!!  :x    The weather has not cooperated much lately either.  Spring break is here for me this week and yesterday was BEAUTIFUL with 75 degree weather.  Tomorrow they are calling for SNOW.... :?
Anyway, I have been questioning my oil thermostat.  All winter it allowed it to run pretty cool.  Now the weather is getting warmer it is running sometimes in the yellow.  Does not sound like it is doing its job.  What do you think?  Is there a way to test it?  Should I just buy a new one?
Dennis
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Postby Sir Real » Mon Apr 06, 2009 9:14 am

I'm baffled  :lol:
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Postby scubarider2 » Tue Apr 07, 2009 10:08 am

Me too, that is why I am asking you EXPERTS  :shock:
I am in trouble now...................  8)
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have you checked the hose routing?

Postby KevinC » Tue Apr 07, 2009 1:37 pm

Dave from SD gave me some good pix of his hose routing- which is not real intuitive.  Have you checked that?  I'd suppose it would run pretty cool during the winter if not properly plumbed.

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Postby scubarider2 » Tue Apr 07, 2009 6:13 pm

I wondered about that.  I remember I had a hard time getting oil pressure up when I first cranked it.  After lots of trials etc., found that I had the thermostat reversed.  After replumbing it, oil pressure up.  Wonder if I could have it bass@asswards?  Hmmm....back to the instructions... I hate that  :roll:
Thanks,
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Postby gme9261236 » Thu Apr 09, 2009 5:54 pm

Dennis, Just wondering what temps you've been seeing ,we put our thermostat in two flights ago,  flew today at 40 degrees oat . After runup to 140 degrees we flew at 4800 rpms for about 15 minutes and it read 158 degrees. The manual says 190 to 230 is normal.At what oat are they saying is normal.158 sounds low, but if the thermostat is working corectly the oil cooler should be out of the circuit. Just wondering what everyone is seeing in cold weather,is my enigma off maybee thanks,Glenn
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Postby scubarider2 » Thu Apr 09, 2009 8:48 pm

Good questions.  On the coldest days that I flew the temp hung out at about the 140degree range.  When the weather is warmer she goes to around the 190 range.  When I push her with the nose up for awhile in hot weather she pushes into the yellow range.  I would love for that engine to be kept in the 190-210 range.  I guess that is asking too much...   :?
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Postby Lynng » Fri Apr 10, 2009 10:24 pm

Guys,

According to Uwe, the oil warm up is much faster the high end of the temp range is not effected. I haven't bought one yet but plan to after seeing Uwe's installation.

Lynn :wink:
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Postby fuji0010 » Wed Apr 29, 2009 5:33 am

An oil thermostat ensures that the oil is never too cool: under ... No need for unsightly duct tape over your oil cooler! Approved by Rotax .

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Common problem with 912 oil thermostat

Postby migbro » Thu May 14, 2009 9:08 pm

Dennis:
The root cause of the scenario you are seeing is in the design of the oil thermostat itself.  The oil thermostat works by bypassing the oil cooler when the oil temperature is low and by routing most of the oil through the oil cooler when the oil is hot.  The type of oil thermostat that is typically installed on the Rotax 912 has two design features that affect its performance on the 912.  First, the thermal motor activates at a nominal 170F, closing the bypass and sending most of the oil through the oil cooler.  170F is a lower operating temperature than is ideal for the 912 and results in the low oil temperatures you have been seeing in the winter.  Second, because of the design of this type of oil thermostat, even when the oil thermostat is “closed,” a significant percentage of the total oil flow – perhaps as much as 10% - is able to bypass the oil cooler, reducing the effective cooling capacity of your oil cooler and resulting in higher oil temperatures in hot weather.  The fix to both of these problems is to install an oil thermostat with a higher operating temperature and a lower bypass flow when closed, together with ensuring that your oil cooler has sufficient cooling capacity for all operating conditions, including high power settings with high ambient temperature.  If you’re interested, take a look at P/N P6-H-190 in the attached data sheet or here.
Mike Brown
www.thermostasis.com
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Postby scubarider2 » Fri May 15, 2009 7:09 am

THANKS Mike!
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Oil Thermostat for the Rotax 912uls

Postby migbro » Fri May 15, 2009 10:37 am

Dennis:
You're welcome.  I've spent a lot of time evaluating all of the commonly available oil thermostats and I'm very happy to share what I've learned.  There's nothing wrong with the oil thermostat that you currently have installed (I'm assuming that it's a unit manufactured by Perma-Cool), it's just that its design is not ideal for the requirements of the typical 912 installation.  You'll see these same oil thermostat issues described on other aviation forums - 912 oil temps too low in the winter and too high in the summer.  The high temps in the summer actually result from two issues - (1) some bypass flow even when the oil thermostat is "closed" and (2) an oil cooler sized for an oil circuit originally designed without an oil thermostat, so that the oil cooler cooling capacity is marginal at high ambient temps/high power settings.  With barely adequate oil cooler cooling capacity at high ambient temps/high power settings, when only 90% of the oil passes through the oil cooler then oil temps creep up.  One more related fact.  The oil temperature sensor on the 912 is located at the point in the oil circuit where the oil is coolest - at the oil pump.  The oil leaving the engine to return to the oil tank will be hotter than the measured oil temperature at the oil pump.  This is the point (at the oil tank) at which it is desirable that the oil temp exceed 212F for some time on each flight, so as to boil off any condensation/water contamination in the oil through the oil tank vent.  So....ideally, the oil temp should be controlled so that you see about 190F on the gauge at all times.  Any cooler and you will not be boiling off water contamination.  If the gauge shows significantly hotter than 190F then the oil temp at the oil tank will be very hot.
Mike Brown
www.thermostasis.com
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Postby scubarider2 » Fri May 15, 2009 12:42 pm

Mike that is great research.  Do you have a personal recommendation for one to purchase using a Rotax 912uls?  How about the location of the unit?  I placed mine right on top of the engine with the line going into the oil cooler.
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Oil Thermostat for the Rotax 912uls

Postby migbro » Fri May 15, 2009 6:55 pm

Dennis:
I’d be happy to make a specific recommendation.  Please contact me off list at [email protected].  As far as your other question on placement, here’s what I know about oil thermostat installation on the 912.  First, the specific placement is not that important, providing you observe some key guidelines: (1) the oil hoses should be routed so that they are not in contact with or close to the exhaust system or where they can rub against other components.  If the oil hoses are in contact with any other components, it's a good idea to sleeve them at the point of contact;  (2) the hose manufacturer’s rule on minimum bend radius should not be violated.  For example, Aeroquip AQP Socketless Hose (FBV0800 or FBN0800) has a minimum bend radius of 5.0”.  This sounds like a tight bend, but this bend radius can easily be exceeded if you are not careful.  If the minimum bend radius is exceeded, than the cross-sectional area of the hose is reduced (oval cross-section vs. round) and the hose is more susceptible to being collapsed at the bend by the negative pressure (partial vacuum) exerted by the oil pump.  I once heard Phil Lockwood say that the weakest point on many 912 installations is the oil hose routing to the oil pump inlet.  The 912 is supplied with a straight fitting at the oil pump inlet which can result in a very tight oil hose bend radius at that fitting which in turn can eventually lead to oil starvation as the hose ages, gets softer and progressively collapses at the tight bend.  A 90 degree fitting for the oil pump inlet (Rotax P/N 956 585) can be installed to address that potential problem.  Another important point to consider is the oil fittings used at the oil thermostat.  The most restrictive part of the oil circuit on any 912 is the internal diameter of the fittings used in the circuit.  Rotax recommends a minimum ID of 9mm (.354”) for all fittings (SI-912-011) and also recommends that 90 degree fittings not be used unless they are the large radius Rotax-type fittings (Rotax P/N 956 580).  Many 912 oil thermostats have been installed with hardware store-type hose barb fittings with unknown IDs.  It’s a good idea to use only aircraft quality fittings with known IDs, such as Aeroquip P/N FBM2007 which has an ID of .391” all the way through the fitting or AN840-8 and AN844-8 fittings which have IDs of .400” all the way through the fitting.  Hose and hose clamp selection are also important but I think I’ve written enough on this for now.  The overall idea is not to introduce potential points of failure into the oil circuit with your choices on thermostat type, fitting type, hose and clamp type and hose routing.
Mike Brown
www.thermostasis.com
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Postby scubarider2 » Fri Jul 10, 2009 9:18 am

Still working on the oil thermostat to see what is going on with overheating.  I will be getting some pictures done and working with Mike on the problem.  It just seems like the thermostat isn't working at all.
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