Oil temp dropping in cold weather

[hans]

Hi All,

last Friday I took the first opportunitiy in 2012 (weather-wise) and put some 54 minutes on my S-II.
A friend had invited me to do a sortie into the Alps region the coming summer, so I wanted to check what kind of climb rates I would end up with at 3500' and above. Ground level temps were a few degrees above freezing level.
20 minutes into the flight I noticed that the oil temperature was dropping (and pressure was rising slightly). The oil pressure remained in the green arc all the time, whereas the oil temperature dropped below it. That was the first time I saw a VW with an oil temp that was too low. I went lower into the warmer air, but this didn't resolve the issue, so I returned back to the airport, to be able to land in short order. As flying as low as 1000' didn't warm up the oil again, I entered the traffic pattern and landed.

My oil cooling is the standard top-mounted job, it does not have a separate air feed.

My questions:

• Has anyone noticed similar behaviour before?
  
• If so, how did you solve it?
  
• Should I blank off a section of the top-mounted oil cooler?
  

Thanks!
Hans

[o2koold]

Hi Hans,
I have seen this before. I am the mechanic for a my friends Cygnet. We took it from a 1835 to a 2180, and had to get the oil cooling system working better. We did this around Sept. Come the following Dec., we couldn't get the oil temp into the green in flight. My friend is kind of weak hearted when it comes to cold, so we don't fly if it is colder than about 30°F. Part of the too warm fix was putting in another cowling intake that flowed air under the case, similar to how air would flow if it was in a car(going backwards!). We made a removable plate to cover this intake, and life was good.

I would start by trying to limit air under the engine if you are running any direct flow there. Those fins under the case really work to help cool the oil. If you are not flowing air under there, maybe block a little of the oil cooler, but I would be hesitant to do so. The car doghouse cooler had a thermostat to regulate the temp. I would be afraid of getting too hot with no way to cool it back down.

Scott R

[hans]

Hi Scott,

thanks for relaying your experiences. I'm a bit hesitant to reduce the flow under the engine as I'd expect this to negatively influence the carb (an AeroInjector which is also located under the engine).

cheers
Hans

[o2koold]

I would agree. The Cygnet carb has it's own filtered air supply, so blocking the other flow only stops the over cooling.
Scott

[schrome]

Hans, I had exactly the same experience when the weather turned cool this fall. My cooler is mounted under the case and plumbed for full flow so I simply reduced the size of the smile in the cowling. Now oil temp. is back to 190 F and the Aerocarb hasn't noticed any difference. I should add that I also reworked the cowling exit so that oncoming air can't see the firewall (not because of oil temperature but to reduce drag).

An interesting side effect is that the CHT dropped about 30 degrees too, not that it was high to begin with. Either the gauge has suddenly gone bad or less air entering the bottom of the cowling has improved the flow through the top.

Ed

[hans]

my carb is below the case, the oil cooler atop of it - and with no separate cooling air ducting, just using spill-over from the cylinder head cooling. I guess that leaves me with no option but to partially cover the cooler itself and try to find an equilibrium that works for me.

Thanks for your input!

cheers
Hans

[maddog]

We have to block the oil cooler to ~80percent to be able to fly the 601 in the winter around here. Still get some high oil temps on climb out during the summer if we are not careful on the back stick during climb outs. Just using a piece of cardboard held by the cowling, also helps with the run up times.

Warmer is better, hotter is NOT.

Mike

[hans]

today I put appr. 2.5 hours on my S-II with the oil cooler blocked by appr. 30% (using thin particle board on the front of the cooler).

I went up to 4500' MSL with ground temps a couple of degrees above freezing. The oil temps were still on the low side, still almost too low, but much more stable. I'm not as nervous about it anymore. Perhaps I should block off an even larger percentage...

cheers
Hans