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04-26-2011, 10:01 PM
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Join Date: Feb 2007
Location: San Diego, CA
Posts: 1,670
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Also confused
I thought a Vernatherm would do two things: (1) Control the oil Temperature by regulating the amount of oil flowing through the cooler, and (2) Acting as a Pressure Relief Valve in the case of a blocked oil cooler, such as might happen on a very cold day at startup.
Wondering why the anti-vernatherm comments? Reasons? 
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Pete Hunt, [San Diego] VAF #1069
RV-6, RV-6A, T-6G
ATP, CFII, A&P
2020 Donation+, Gladly Sent
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04-26-2011, 10:26 PM
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Join Date: Mar 2007
Location: Sonoma County
Posts: 3,821
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Quote:
Originally Posted by DanH
Well, I think so, but at this point I have my antenna up....wondering if I've missed something else. God is in the details.
For example, consider the pressure relief spring. As mentioned in the previous post, careful measurements says the cone should contact the seat at about 188F....but that doesn't mean it will seal. There is a pressure difference between the two sides of the bypass hole. The cooler pressure drop per the SW chart is 3.5 psi at 235F OT and 46 lbs/min. Add hoses, fittings, and a more realistic oil temp (more viscosity) and the total difference is probably 10 psi or more. The bypass seems to have an area about 0.25 sq in, so the vernatherm will actually need to extend far enough to compress the pressure relief spring to 2.5 lbs. Put another way, it won't actually seal at 188F. Full sealing will require some higher temperature.
I have the vernatherm in my hand right now, and I can compress the spring about 0.125" with just my thumb and forefinger. Don't know the actual spring rate, but I'll find out tonight. It's not real stiff:
Here's the thing. Recall that temperature vs length plot? Unloaded there's only 0.040" between 190F and 212F.
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Rostra site has a range of 20 to 100 lbs. for the bypass.
This spring is only a problem if your oil cooler is not cooling.
Does your oil warm up quick? Or does the oil get too hot? What made you want to test the Vernatherm?
Last edited by gasman : 04-26-2011 at 10:29 PM.
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04-27-2011, 04:49 AM
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Join Date: Feb 2005
Location: Huskerland, USA
Posts: 5,862
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Quote:
Originally Posted by PCHunt
I thought a Vernatherm would do two things: (1) Control the oil Temperature by regulating the amount of oil flowing through the cooler, and (2) Acting as a Pressure Relief Valve in the case of a blocked oil cooler, such as might happen on a very cold day at startup.
Wondering why the anti-vernatherm comments? Reasons? 
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I'm not looking to start a vernatherm war, but it seems to me we could do better. Oil temps are critical for keeping an engine free of sludge and moisture, I would think we can all agree on that. There are dozens of threads about oil cooler doors, air duct flaps, duct tape over the oil cooler, etc., to keep oil temps at proper operating temps. The vernatherm never seems to be enough, or work right, or fails, or needs adjustment, ect, ect, ect. If it worked as advertized there would be no need for oil cooler doors, duct tape, ect.
It just seems to me to be an overlooked issue inwhich the brilliant engineering minds at VAF could overcome. We should be able to design an oil tempurature control system that works. A precision external oil thermostat seems to me to deserves a look anyway.
JMHO.
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RV-7 : In the hangar
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RV-44 : 4 place helicopter on order.
Last edited by Geico266 : 04-27-2011 at 05:29 AM.
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04-27-2011, 10:15 AM
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Join Date: Dec 2008
Location: Big Sandy, WY
Posts: 2,567
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Quote:
I just came from the shop and measuring the spring rate. This pressure relief spring starts compressing at 14.75 lbs. That matches Lycoming statements about the relief pressure being 60-90 psi drop across the cooler circuit; a pressure relief valve with a port area of 0.248 sq in and a 14.75 lb spring would start to open at 59 psi. The spring rate is quite flat; it only rises to 17.75 lbs at 0.110 compression.
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Very cool Dan, worthy. I don't know how you find the time & tenacity to get real numbers like you do.
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Actual repeat offender.
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04-27-2011, 12:17 PM
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Join Date: Oct 2005
Location: 08A
Posts: 9,500
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Quote:
Originally Posted by PCHunt
I thought a Vernatherm would do two things: (1) Control the oil Temperature by regulating the amount of oil flowing through the cooler, and (2) Acting as a Pressure Relief Valve in the case of a blocked oil cooler, such as might happen on a very cold day at startup.
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Correct on both Pete, although the pressure relief wouldn't come into play at start because the vernatherm is cold and thus the bypass port is fully open. You would taxi out, warm to 100F (or whatever you prefer), then launch. Sometime later when the oil reached ~180F and the vernatherm tried to close the bypass, a congealed cooler would result in enough pressure differential to force the cone off the seat by compressing the pressure relief spring.
Quote:
Originally Posted by Geico266
There are dozens of threads about oil cooler doors, air duct flaps, duct tape over the oil cooler, etc., to keep oil temps at proper operating temps. The vernatherm never seems to be enough, or work right, or fails, or needs adjustment, ect, ect, ect.
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Larry, the Thermostasis unit for the Rotax works on the same principles as the Lycoming system. When cold they allow oil flow to both the cooler and a cooler bypass. When hot they both block the bypass. They both use a vernatherm to do the job. Neither can force warming as neither can block the cooler.
That said, it does not appear to have a pressure relief valve, and if it doesn't a congealed cooler would result in oil starvation when the vernatherm closed the bypass. The same is true of a 912 without the Thermostasis unit, but it would happen at startup. With a stock 912 oil system, all oil must pass through the cooler to get to the pump.
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Dan Horton
RV-8 SS
Barrett IO-390
Last edited by DanH : 04-27-2011 at 01:21 PM.
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04-27-2011, 01:51 PM
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Join Date: Oct 2005
Location: 08A
Posts: 9,500
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Just had a nice conversation with Ken Lionello, chief engineer at Rostra.
Ken says there is really only one significant failure mode, and eventually all vernatherms die from it.
Imagine a steel thimble filled with a hydrocarbon wax, then capped with a rubber diaphragm. When you warm the closed thimble, the wax expands and the diaphragm pushes outward against a little rubber plug. The plug pushes against the rod described in a previous post. That's the basic operating principle.
Ken says given enough heat cycles and calendar time, the rubber components get hard. Eventually the verntherm loses its ability to extend. He asked what Lycoming was specifying for a replacement interval; I told him "at overhaul".
He was kind enough to send a proprietary drawing because it lists some specifications. From the drawing and the conversation:
(1) Valve travel to be a minimum of 0.160" between 150F and 185F.
(2) Valve should close against seat between 183F and 187F.
(3) Pressure relief spring force is 15 to 20 lbs when the seat is pushed away from the nut a distance of 0.050". The result is a cracking pressure of 60-90 psi.
(4) Seat to sealing surface dimension (for us the bore depth in the oil filter adapter) is 3.280". The seal is assumed to be an aluminum crush washer of 0.0625" thickness. Reworking the seat may increase the 3.280 dimension and raise regulated oil temperature. A thinner crush washer will reduce regulated temperature but also reduce pressure relief clearance.
So, there we have it. Apparently there is nothing wrong with my vernatherm, but I've learned a lot about them. Hope you have too.
__________________
Dan Horton
RV-8 SS
Barrett IO-390
Last edited by DanH : 04-27-2011 at 01:53 PM.
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04-27-2011, 02:18 PM
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Join Date: Aug 2010
Location: Wichita, KS
Posts: 1,958
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Dan, mind telling us what, if anything, made you suspect your Vernatherm might be in need of replacement? Just reached what you thought would be a good replacement interval? Lower than expected oil temps indicating a possibly failed (open) Vernatherm? Looking for a little education myself...
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04-27-2011, 03:21 PM
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Join Date: Oct 2005
Location: 08A
Posts: 9,500
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Quote:
Originally Posted by krw5927
Dan, mind telling us what, if anything, made you suspect your Vernatherm might be in need of replacement?
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I've been seeing high oil temps lately (205-210) usually on the second leg after some hot time on the ground. I don't recall ever breaking 200 back in October when I started flying this crate. Perhaps OAT was generally lower and I was not paying attention, and I was flying with the largest of my three exit area panels for the first 6 hours. Whatever...it seemed sensible to check the vernatherm installation before proceeding with cooling experiments. As usual, one thing led to another.....
Full picture here:
http://www.vansairforce.com/communit...ad.php?t=68241
__________________
Dan Horton
RV-8 SS
Barrett IO-390
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04-27-2011, 03:29 PM
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Join Date: Nov 2005
Location: Davis, CA
Posts: 1,156
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Thanks for the information, including the "how they work" information. I was most curious. Similar operating principle to an automotive coolant thermostat, as I expected. Roughly the same failure mode, too. In any case, nice to know a little more about the inner workings of these engines.
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Lars Pedersen
Davis, CA
RV-7 Flying as of June 24, 2012
960+ hours as of June 30, 2020. Where did the time go?
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04-27-2011, 04:57 PM
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Join Date: Feb 2005
Location: Huskerland, USA
Posts: 5,862
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Quote:
Originally Posted by DanH
So, there we have it. Apparently there is nothing wrong with my vernatherm, but I've learned a lot about them. Hope you have too.
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Good info Dan, as always.
So what is next if the vernatherm is working properly? Fly it and see or are you going to pursue other cooling measures?
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RV-7 : In the hangar
RV-10 : In the hangar
RV-12 : Built and sold
RV-44 : 4 place helicopter on order.
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