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Oil Temp in a Climb


FlyingMonkey

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My CTSW has pretty minimal engine instruments, all in small analog gauges: Tach, CHT, Oil Pressure, Oil Temperature, Volts. Normally all these indicators stay in the green, but in a WOT climb on a hot day (hot here is 90-95° OAT), the oil temp starts getting high. CHT stays good, usually does not break 220°. But the oil temp will creep up into the 230-240° range. It comes down a few minutes after I level off, reduce climb angle, or throttle back, so I think this is normal.

 

My question is what is an acceptable oil temp to maintain for a long climb, versus "too hot to let it stay there"? The yellow on my gauge starts at 230° so I usually start thinking about taking action if it stays above that level for more that a few minutes. Is that too conservative? Too aggressive?

 

I read Roger's suggestion in another thread on this topic to ensure the rubber "skirt" is forward and not folded back under the cooler, I'll check that before next flight. Any other suggestions to make sure my bird's blood doesn't boil?

 

Thanks!

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Some days I have to step climb or cruise climb at about 95K. Yesterday was an example, 87F on ground (175'), full power climb to 3500 msl at 95K. OAT at 3500 was 81F. At time of level off I had 245 oil temp and it would only cool to 230F at 5200 RPM. That's bottom of yellow. Finally when I reduced to a 5000 RPM cruise the oil temp came down to 220. CHT is never a problem. The temp was still climbing slowly as I leveled 3500. What would have happened if I needed 7000 msl. Guess I would have needed to pray for a more rapid OAT decrease.

 

To me, this is a major PITA and is a design flaw. In the winter, you get to tape up the radiator if you ever want to see 190F. Even here on this part of Oregon where this is needed with OAT 40F to about 50F. So, lets see, is this a 1, 2 or three piece of tape day? OOPS! Guessed wrong, only needed 2, now it's running too damn hot. Cow flaps would cure this, maybe even the summer problem.

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Last summer (2012), I had an oil thermostat installed in my CTSW. The work was done at a FD service center and FD approved the alteration for my aircraft. I don't think that there is a blanket LOA. My apology for not providing more specifics. But, the paperwork is with the airplane undergoing annual condition inspection.

 

But, an oil thermostat is an alternative to consider...

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Talk to the oil engineers at the oil companies. Get past the salesman that first answers. That is where I get my info. For a full or semi synthetic the 240F is fine, but it shouldn't be a full time temp. The shear forces, high mechanical action, temps, ect... is why you are not supposed to use a straight mineral based oil.

 

 

John,

 

You have an issue some where. It is very possible that one of the oil hoses gets hot with the oil and then the radius reduces due to a partial fold in the hose. This causes high oil temps. the usual offender is the oil return line off the bottom of the engine. FD's routing and hose lengths weren't that good. If the hose is a tad to long then it gets a reduced radius when hot and being encased in fire sleeve you would never see it. The next offended is the line that comes off the oil radiator on the right side and does a 180 degree turn back into the oil pump housing. You might also try and see if you can swing the coolant radiator on the bottom edge more forward to help flow air better through both radiators.

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Roger - at hose change I re-routed and shortened the one from the tank to the bottom of engine. The other one you mention looked OK when I replaced it. It didn't show any signs of collapse. I'll look at the orientation of the radiators.

 

Anway, this is nothing new. It is a "from day one" problem.

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I've seen numerous references to oil temps being fine at 240 degrees. This Rotax spec sheet shows "normal" temps as "lower than 250", and refers to "occasionally high oil temperature" as being over 248 degrees.

http://www.rotax-air...okus/d04899.pdf

Tim

 

Thanks for that Tim. It looks like Rotax is unconcerned with temps below 250° and even above that for short periods, when using a synthetic or semi-synthetic oil. I'd still like to run mine cooler rather than hotter, but I won't start to sweat when I see 240° in a hard climb.

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Seeing Red. Less than one hour flight at 5000 RPM, below 2500 MSL, OAT 87 F, landed at Honolulu International. One runway closed, so much taxi time, about 17 minutes, mostly holding on taxiway. At idle 1900 RPM, CHT went to 253, 251. red light on for less that one minute. Turned into wind, set to 3000 RPM, cooled within seconds. Did not kill engine before take off on return flight. Coolent check OK before the flight and after the flight from HNL back to HJR all well no loss of fluid. Never saw that before, have you?

 

Farmer

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Hi Larry,

 

Yes I have seen this. It happens because there is no air flowing through the radiator to help with the cooling so when you gave it more fuel the extra fuel helped with the cooling because down at 1700-1900 at idle there is very little fuel flow. Also you turned into the wind so that probably gave you a little more help. The additional rpm also increased the coolant flow. The prop alone does not generate enough air flow through the radiator to really help on long waits at idle on hot days.

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FYI, When George Braly did the extensive engine heating tests, one of the aspects he tested was the advice to park into the wind on runup. His data showed conclusively that the airplane engine ran cooler when facing downwind than when upwind. His tests were on big Lycomings and TCM. Not sure how a Rotax would respond but the point is don't take the "runup into the wind" as gospel without at least thinking about it.

 

If you get to looking for the reason you park into the wind, it is likely because some CFI told you to and because it intuitively seemed like a good idea. That doesn't mean it was ever tested. Braly tested it.

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Upwind also allows you to look for traffic. Although I suppose you could turn upwind before you cross onto the runway. It depends a bit on where you are I suppose. It is rare that I have anyone in front of me or behind me when I do my runups, I do have to occasionally wait for others landing or departing. I also have never had over temp at those times.

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Upwind also allows you to look for traffic. Although I suppose you could turn upwind before you cross onto the runway. It depends a bit on where you are I suppose. It is rare that I have anyone in front of me or behind me when I do my runups, I do have to occasionally wait for others landing or departing. I also have never had over temp at those times.

You do runups on the runway? Not a practice that is acceptable at most airports I use. If there is any issue with the runup, you could be sitting on the runway long enough for incoming traffic to be affected. One should have time and space to focus on just doing a good runup and watching the engine gauges, not worrying about blocking a runway.

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  • 1 month later...

Jeremy installed a 2nd rubber baffle to cut off any air bypassing my coolers on the top. The seal is impressive but the result is no improvement in oil temps when OAT is 70degrees or above. :(

 

The new fuel pump didn't help either.

 

Climbing from 200' I hit above 250degrees at 4,000'

 

Interesting, I leveled off at 4,000' and at 92% throttle my egt went to 1400 and when i later advanced the throttle it goes back to 1200. Rich mixture does cool but just EGT.

 

Here's my new theory, I want to try a new radiator taping technique. In the winter I use 2 strips of 3" tape across the bottom. For summer I now want to try taping the radiator where there is no oil cooler behind so the air flow will be concentrated at the oil cooler location.

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Ho Ed,

 

That tape adjustment won't help and may cause even higher temps.

 

FYI,

Rotax temps are higher in the mid 4000 rpm's and that's the way the system is set up. The 5000's should run cooler than the mid 4000's especially around 4700-4800.

 

I really believe you may have a hose that has a reduced radius bend. Not kinked. My bet is the oil line off the right side of the oil pump and the right side of the oil cooler or the the return line off the bottom of the engine back to the oil tank. These collapse sometimes when the oil heats them up. I would re-route the one on the bottom to come straight out the left side by cy. #2-#4 and then back to the oil tank. This is a big culprit. You may need a 90 degree oil tank fitting because the one on top of the tank is straight. The hose on the pump to the oil cooler may need to have the fire sleeve removed, re-install the hose and fire the engine up and let it get hot to see if that hose collapses.

 

If you have any reduced radius hose it will slow the oil flow and cause it to get hotter.

Are you using standard 50/50 mix anti-freeze?

Make sure you flush out both cooler fins through and through to make sure there is not dirt and oil to reduce air flow over the radiator fins.

 

Over time the rich mixture should help reduce all temps.

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Roger,

 

My coolant is always cool only my oil reads hot. I got new oil hoses and cleaned the cooler and flushed it till it was like new. No joy there but I will look again for a reduced radius. I'm on my 3 set of hoses now.

 

70 degrees F OAT is the magic number, below that I stay cool, at 70 or above my oil gets hot in climbs or even a little in level cruise. I think this magic number points more towards lack of cooling capacity than at flow, wouldn't a restricted flow be an issue even below 70?

 

Coolers clean, been there done that on most suggestions.

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If the CHT's stay down that means air is moving over the radiator and coolant is flowing normally. If the oil temp is high then something is slowing the oil flow. My money is still on a reduced radius hose. The only way to know for sure is pull off both fire sleeves and watch the hoses as the oil gets hot on the ground with the hoses in their normal state. If you don't just do it like this now you may work at this for ever looking for a cause. this at least needs to be ruled in or out.

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Hi Ed,

 

Remember that the temp for the oil reads after it has gone through the cooler and the oil in the tank is hotter and not been cooled.

One thing that can be checked is the CHT probe is the same as the oil temp probe. They can be switched to compare, but these are usually very reliable. A loose connector on the probe could cause a high reading and make it jump around. I bet there is more going on than a probe issue.

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