Interesting Oil Cooler Observations

[MichaelFarley56]

Hello All,

Over the past several months, I've made a few small modifications to my AeroVee including the addition of a top mounted oil cooler, just like the Onex. Once I removed the Sonex style lower oil cooler I added a cover plate on the lower baffling but left the oil cooler cowling opening in place.

After talking to Kerry at the factory, I wanted to see what would happen to my oil temperatures if I closed over the oil cooler hole in my cowling. I figured if nothing changed, I would simply fiberglass that opening closed and give myself a smooth cowling from the spinner on down. Tonight I took some tape, covered up the cooler hole, and took the Waiex up for a 10 minute test flight to see how it flew.

After only a few minutes, I was shocked when my oil temperatures quickly rose to over 200 degrees in by the time I leveled off around 1500' AGL. Normally in this case I would be looking at temps in the 150 degree range. I quickly reduced power and came back to make a normal landing, and by the time I got on the ground my oil temperatures was around 210 degrees. Certainly not over any redline, but on previous flights I would be hard pressed to get my oil over 170 in cruise, and they quickly dropped as I reduced power for landing.

Lesson for the day...

First, a top mounted oil cooler will fit under a stock Sonex or Waiex cowling. It's a tight fit but it does work. If you decide to go this route however, it is still very important to have cooling air going over the bottom of the AeroVee or else the oil will get warm. To be honest I'm not sure how the Onex guys do it. I love my top mounted oil cooler but will absolutely be keeping the baffling over my lower engine block per the Sonex plans in order to keep things nice and cool.

Just thought I'd post this in case anyone else is thinking about going with a top mounted cooler.

Thanks guys!!

[falvarez]

Mike, thank you for the update on the top mounted oil cooler. I'm actually surprised by your findings...I figured by blocking off the lower inlet, that you would have a greater pressure differential allowing more cooling air to flow actually keeping the engine cooler. Were you able to determine what the impact on the CHT was by closing off the lower inlet?

[MichaelFarley56]

Great question Frank...that was actually why I wanted to close off that opening in the first place. Like you, I figured less direct pressure going into the lower cowling the better, which would give me lower CHT readings.

At my home base tonight it was around 76 degrees, and probably 90% humidity as some big rain storms had just passed though. On a full power climb up to around 1500' AGL my hottest cylinder got up to around 355, where it's normally around 365. So there may have been a slight benefit there.

Strange thing though...once I leveled off in cruise the hottest cylinder leveled off around 330, which is actually around 10 degrees warmer than before! I'm more used to seeing 320 on my hottest cylinder, so i think to make a long story short, I really didn't see much, if any, cooling benefit by covering up the oil cooler opening. The main things I noticed was a much higher oil temp, maybe as much as 40-50 degrees, and it also seemed like I lost 2-3 psi of oil pressure as well. I went from around 45 psi to around 42 psi. Again, nothing alarming, but noticeable. I don't know; maybe if the oil stays cooler it helps keep the cylinders a little cooler as well?

This was obviously just one quick test, and I'm sure I could tweak the upper baffling by the oil cooler a little to get better oil temps, but for me personally I'm just going to remove the tape off the oil cooler hole in the cowling and call it a day!

[radfordc]

In your installation did you do anything to force cooling air through the top mounted oil cooler? If not, then I can fully understand why your oil temps went up so fast. In the VW auto installation they use a shroud around the cooler and a fan to force air through it. I think in order for the top mounted oil cooler to work you need to install a separate intake and exhaust duct for it.

Also, the top mounted oil cooler isn't a "full flow" system like the bottom mounted oil cooler. Again, this degrades the ability to cool the oil somewhat. Here is a diagram of how the top mounted cooler works: http://www.thesamba.com/vw/forum/viewtopic.php?t=280293

[MichaelFarley56]

In your installation did you do anything to force cooling air through the top mounted oil cooler? If not, then I can fully understand why your oil temps went up so fast. In the VW auto installation they use a shroud around the cooler and a fan to force air through it. I think in order for the top mounted oil cooler to work you need to install a separate intake and exhaust duct for it.

Great question. I modified my baffling to incorporate an air shroud for exit airflow, just like the factory now does with the Onex. I don't have a fan to force air through the cooler, but instead was expecting the high pressure air in the cowling to force air through the cooler then out the back of the baffling and out the bottom of the cowling. The exact same setup at the Onex.

It's no big deal...I have nothing wrong with leaving the oil cooler shroud on the bottom of the block. Works just fine!

[SvingenB]

In your installation did you do anything to force cooling air through the top mounted oil cooler? If not, then I can fully understand why your oil temps went up so fast. In the VW auto installation they use a shroud around the cooler and a fan to force air through it. I think in order for the top mounted oil cooler to work you need to install a separate intake and exhaust duct for it.

Also, the top mounted oil cooler isn't a "full flow" system like the bottom mounted oil cooler. Again, this degrades the ability to cool the oil somewhat. Here is a diagram of how the top mounted cooler works: http://www.thesamba.com/vw/forum/viewtopic.php?t=280293

That diagram makes no sense to me. Does the top mounted oil cooler has a thermostat? There is no way the oil will flow like that unless there is a thermostat valve mounted on the oil cooler, but there is none in the drawings, and I didn't believe the top mounted cooler has one.

[Sonerai13]

On the prototype Waiex we have the top-mounted cooler and NO lower baffle or opening. Thus, no air is directed over the oil sump. On the prototype Sport Trainer we have the top-mounted cooler but retain the lower cowl opening and baffle directing air across the sump. BOTH airplanes have good cooling and good oil temps. No cooling issues at all. The Sport Trainer oil temps run about 5 to 10 degrees cooler (Fahrenheit) than the Waiex.

Make sure the baffles are properly fitted in the cowl and make sure the cooling outlet at the bottom of the cowl is the proper size (at least 125% of the inlet opening area) and has a "lip" on it to get the airflow right. If the air is flowing properly through the cowl, the cooler will work.


Joe
"Fortes Fortuna Adiuvat"

[radfordc]

In your installation did you do anything to force cooling air through the top mounted oil cooler? If not, then I can fully understand why your oil temps went up so fast. In the VW auto installation they use a shroud around the cooler and a fan to force air through it. I think in order for the top mounted oil cooler to work you need to install a separate intake and exhaust duct for it.

Also, the top mounted oil cooler isn't a "full flow" system like the bottom mounted oil cooler. Again, this degrades the ability to cool the oil somewhat. Here is a diagram of how the top mounted cooler works: http://www.thesamba.com/vw/forum/viewtopic.php?t=280293

That diagram makes no sense to me. Does the top mounted oil cooler has a thermostat? There is no way the oil will flow like that unless there is a thermostat valve mounted on the oil cooler, but there is none in the drawings, and I didn't believe the top mounted cooler has one.

The oil relief valve at the front of the engine controls how much oil goes through the oil cooler. When the oil is cold and thick none of it flows through the oil cooler (it's like trying to pour molasses through a sieve); all of the excess oil not going to the bearings is dumped into the crankcase through the oil relief valve. When the oil is hot it's viscosity is lower and its now able to flow through the oil cooler. As oil begins to flow through the oil cooler the oil relief valve begins to close and less oil is dumped into the crankcase. Note that the top mounted oil cooler is not "full flow" like the bottom mounted cooler. Eventually, the oil relief valve closes completely and all the hot oil is flowing through the oil cooler.

[SvingenB]

In your installation did you do anything to force cooling air through the top mounted oil cooler? If not, then I can fully understand why your oil temps went up so fast. In the VW auto installation they use a shroud around the cooler and a fan to force air through it. I think in order for the top mounted oil cooler to work you need to install a separate intake and exhaust duct for it.

Also, the top mounted oil cooler isn't a "full flow" system like the bottom mounted oil cooler. Again, this degrades the ability to cool the oil somewhat. Here is a diagram of how the top mounted cooler works: http://www.thesamba.com/vw/forum/viewtopic.php?t=280293

That diagram makes no sense to me. Does the top mounted oil cooler has a thermostat? There is no way the oil will flow like that unless there is a thermostat valve mounted on the oil cooler, but there is none in the drawings, and I didn't believe the top mounted cooler has one.

The oil relief valve at the front of the engine controls how much oil goes through the oil cooler. When the oil is cold and thick none of it flows through the oil cooler (it's like trying to pour molasses through a sieve); all of the excess oil not going to the bearings is dumped into the crankcase through the oil relief valve. When the oil is hot it's viscosity is lower and its now able to flow through the oil cooler. As oil begins to flow through the oil cooler the oil relief valve begins to close and less oil is dumped into the crankcase. Note that the top mounted oil cooler is not "full flow" like the bottom mounted cooler. Eventually, the oil relief valve closes completely and all the hot oil is flowing through the oil cooler.

When the oil is cold (with high viscosity) it is cold everywhere. Thus the increased resistance in the oil cooler is balanced by increased resistance in the other parts.

dP = k*q^2 Cold oil has higher k, but the higher k is everywhere. There is no way those diagram can be correct unless the oil IS flowing also in the cooler or the cooler has a thermostatic valve. Remember, the oil flow is not measured there, they only assume there is no flow of oil because there is no heat. But when the oil is cold, there is no heat anyway.

[MichaelFarley56]

Make sure the baffles are properly fitted in the cowl and make sure the cooling outlet at the bottom of the cowl is the proper size (at least 125% of the inlet opening area) and has a "lip" on it to get the airflow right. If the air is flowing properly through the cowl, the cooler will work.


Joe
"Fortes Fortuna Adiuvat"

Quick question for you Joe if you have a second. Has the factory experimented with different cowling exit opening sizes against what's listed in the plans? I don't have my plans right in front of me, but seem to recall that the published exit area size is around 50 sq/in, which I believe is right around the same area as the two cowling openings. I agree and like your "at least 125% of the inlet opening" approach and back early in Phase 1 opened up my exit area up around 25% more, which gave good results. I'm just curious if there's a new "factory approved" exit size. Thank you sir!