April 9, 2005

Rosie ended up coming down with a little oil temp issue. Apparently, his oil temp was running much hotter than usual on climbout, and the strange part was that once he levelled off, the temp kept climbing. He had just installed a second oil cooler a day or two before, since his oil temps had gone up a bit after installing his new constant-speed prop. The second oil cooler did the trick, according to a test flight yesterday. And literally yesterday everything was running nice and cool -- but today was a totally different story.

Gary's theory was that the vernitherm (vernatherm? I've seen it spelled both ways) probably wasn't working properly. The vernitherm is a valve that is essentially like the thermostat in your car's engine. When the engine is cold, the vernitherm contracts, allowing oil to bypass the oil cooler. This helps the engine warm up. As the oil temperature rises, the vernitherm expands and closes off the bypass, forcing most if not all oil to pass through the oil cooler. It basically enables the engine to self-regulate its oil temperature. If the vernitherm fails, one of three things is likely to happen:

1. If it fails in the expanded position, oil will always pass through the oil cooler. The oil temp might be overly cool. (Of all problems to have, this one is probably the least harmful.)
2. If it fails in the contracted position, oil will always bypass the oil cooler. The oil temp is likely to skyrocket depending on ambient conditions.
3. If it fails somewhere in the middle, some oil will pass through the oil cooler, and some will bypass it. It is unlikely that you'd even notice this in all but extreme hot or extreme cold conditions. Oil temp would tend to be proportionate to ambient temperature and inversely proportionate to airflow, if I'm not mistaken. In other words, on climbout, it might seem hotter than usual, and on descent it might seem colder than usual.

Rosie was gonna fly home to diagnose the problem in his own hangar, but after a little coercing from Gary and me he decided to work on it here. The likelihood was that he would have the oil temp problem all the way home, and he had to climb high over mountains to get back. Best to get it done here...lots of people to help, with tools and resources nearby.

Rosie pulled his cowl and removed the vernitherm. We set up a test contraption, which was basically a little stove with a flask full of old oil. We brought the oil up to well over 200 degrees and dipped the vernitherm into it.

This activity attracted a crowd from the local RV yokels.

Basically, from the freezer to the hot oil and back, Rosie's vernitherm only moved about .040" or so. That sure didn't seem right. We expected it to expand and contract at least about 1/4". But without a "known good" vernitherm around, we had no way to tell. None of us had done this experiment before to know exactly what to expect.

So Rosie and I took some tools and rags over to Mike Holland's hangar, just down the way from me. Mike is building an RV-9A and has an O-320. Gary has an O-320 in his RV-6 and knew that the O-320 and O-360 (Rosie's engine) use the exact same part. So Rosie asked nicely and Mike obliged...we pirated his vernitherm. Thanks, Mike! Rosie ordered him a replacment.

Anyway, we tested Mike's brand new vernitherm in the oil bath and immediately it expanded about 3/8" or more. Yep, there you go. That was the problem.

Before Rosie installed the pirated vernitherm I shot this photo to show where this puppy installs. Some people might not know about this. The vernitherm installs from the bottom up into this hole, right above the fuel pump and near the oil cooler send fitting.

More food for thought. Vernitherms are stamped with the temperature that they are designed to maintain. In the case of O-320, O-360, etc. engines, we usually use 85C, or 185F.

Rosie got everything buttoned back up, and we went out and did some formation practice. Rosie's oil temp was around 180 the whole time!

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Dan Checkoway ()