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Another thread on oil coolers, mass flow, etc.

bjdecker

Well Known Member
Ambassador
I'm sure the answer is somewhere on this forum, I'm just not smart/patient enough to find it; Posting a question for the hive mind --

Dataset:

Two RV-7's, Airplane 1 & 2, both with IO-360-A1B6 Angle Valve, SilverHawk RSA-5 fuel injection/Avstar fuel injection, Dual Plasma III Ignitions @ 20°BTDC.

Airplane 1 had a SW 8406R "7-Row" oil cooler, fed by a 3" SCEET duct from the #4 Cylinder back baffle, aluminum box duct transition to oil cooler.

Airplane 2 has a SW 10599R "9-Row" oil cooler, fed by a 3" SCEET duct from the #4 Cylinder back baffle, custom fiberglass transition/plenum from duct to oil cooler face.

The oil temps for equivalent power settings, DA and TAS***, and CHT's are essentially the same between each aircraft ... ~185°F

Why is this the case?

In my pea-brain, I would have expected the 10599R to reject about 30% more heat than the 8406R, the OT should run about 10% or so cooler.

Unfortunately, Airplane 1 was lost, and no more data from that aircraft is available -- so I am left chasing variables for Airplane 2. Going to a larger duct and/or SW 10611R is interesting and would probably solve the problem - but overkill IMHO and hiding the real root cause of the loss of efficiency.

***Here's one additional data point -- Airplane 1 was painted, and would yield a 180KTAS+ cruise at test power settings, Airplane 2 is bare metal, rough fiberglass and cruises at 172KTAS with test power settings. Could the difference in TAS be driving this temperature delta?

Cheers!
 
The oil temps for equivalent power settings, DA and TAS***, and CHT's are essentially the same between each aircraft ... ~185°F

Why is this the case?

Two likely reasons. The vernatherm is doing its job, regulating to 185F, and/or the 3"D duct is, in effect, the system throttle.
 
If the oil cooler isnt being maxed out, the vernatherm should be regulating oil flow to the cooler to keep a constant temperature. If you both had 185 degree valves, then this makes sense. If you both did a slow, max power climb that put enough heat into the oil to raise the temp, I imaging the aircraft with the 9 row cooler would see a slower increase and/or a lower peak temp.
 
My bet is that the 3" scat tube is throttling the cooler flow. I have a 3.5" scat tube and have determined it is too small. Pressure drop between upper cowl and cooler face is about half of delta-P between upper and lower cowls. Too much loss. So I am getting ready to replace with 4" tube and a better diffuser transition to the cooler face.

3" is really small for the big cooler and the cooling capacity needed for angle valve engines with piston squirters.
 
Thank you Paul, Steve & Dan!

Both aircraft have an 86°C vernatherm, so this all makes sense now -- OT hits 86°C (187°F) and the Vernatherm expands, and the hot oil runs through the Oil cooler. Air blasting through the Oil Cooler takes the excess heat away, and the temp stays around 187°F.

Next step, measure Delta P in Plenum/Cowl Bottom and Plenum/Face of Oil Cooler. Depending upon the differences I'll change to a 4" duct first...then maybe a larger cooler. I'd like to have a bit more margin for those hot day/long climbs.

Attached pic is a transition that I whipped up last night.
 

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There are so many variables in this system and very small changes can have significant impact.

One of my concerns has always been the use of SCAT tubing on these systems as ducts. The inside ridges can affect flow mostly due to the set up of standing waves. Might not effect some systems but if you are marginal to start it might be a factor. Using a smooth wall duct if possible is a much better practice

My cooler takes air from behind #4 that is routed Via an aluminum plenum to a smallish cooler mounted in the engine mount. A short neoprene boot allows for motion between engine and plenum/cooler. I installed a cockpit controllable swinging door to block flow and raise temps in winter

I recently added a very small “splitter” vane In front of the outlet hole behind 4. This small vane splits airflow between going to the cooler and the cylinder. This small vane dropped 4 CHT by 15-20 degrees and raised oil temps by about 7 degrees -

my point is that small changes can make big differences. Fine tuning and attention to detail matters. Things like that nicely shaped plenum with good seals all add up.
 
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Smooth wall duct

There are so many variables in this system and very small changes can have significant impact.

One of my concerns has always been the use of SCAT tubing on these systems as ducts. The inside ridges can affect flow mostly due to the set up of standing waves. Might not effect some systems but if you are marginal to start it might be a factor. Using a smooth wall duct if possible is a much better practice

My cooler takes air from behind #4 that is routed Via an aluminum plenum to a smallish cooler mounted in the engine mount. A short neoprene boot allows for motion between engine and plenum/cooler. I installed a cockpit controllable swinging door to block flow and raise temps in winter

I recently added a very small “splitter” vane In front of the outlet hole behind 4. This small vane splits airflow between going to the cooler and the cylinder. This small vane dropped 4 CHT by 15-20 degrees and raised oil temps by about 7 degrees -

my point is that small changes can make big differences. Fine tuning and attention to detail matters. Things like that nicely shaped plenum with good seals all add up.

Where does one procure "smooth wall duct"?
 
In addition to the sources mentioned above I suggest that the wide variety of silicon tubing available from places like Summit Racing might also be an alternative. Not as flexible as Skeet but probably works in a lot of scenarios. It is designed to connect turbos and Inter coolers - similar service to what we are looking for.
 
In addition to the sources mentioned above I suggest that the wide variety of silicon tubing available from places like Summit Racing might also be an alternative.

Generally too stiff for this application.

I recall a recent post where someone mentioned cutting a section from an appropriate diameter inner tube. Seems like a sharp idea to me.
 
Smooth custom ducts

Where does one procure "smooth wall duct"?

First try at making these. Wrapped a 3” pool noodle with packing tape and waxed it up. Used 1 layer of 9 0z cloth with 1” of overlap. Using a hotel room key card spread the red silicone on the outside of the cloth thinking I could wet it out completely, but didn’t work out that way. On the second try used black Permatex 82180 (same as Loctite 598) and put down a layer of silicone first then added the cloth and another pass of silicone on top. Before the silicone could set up both test samples got gently rubbed down with a little sprayed on alcohol to smooth out the exterior surface. When cured they are super flexible and seem strong, I can’t pull the seam apart.

Don Broussard
RV9 Rebuild in Progress
57 Pacer
 

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Generally too stiff for this application.

I recall a recent post where someone mentioned cutting a section from an appropriate diameter inner tube. Seems like a sharp idea to me.

This idea goes back to Scott McDaniels. I forget the size, but an RV-10 main wheel tire has the right sized tube. Word of caution. The "same" size is available as a cheap wheel barrow tube made in China. I tried one. Slightly different diameter tube, didn't fit. So, I stopped in at the local FBO and asked if they had changed any tires recently and there might be some tubes in their trash. Sure enough there were. So I got an old tube for free. Perfect fit.
 
This idea goes back to Scott McDaniels. I forget the size, but an RV-10 main wheel tire has the right sized tube. Word of caution. The "same" size is available as a cheap wheel barrow tube made in China. I tried one. Slightly different diameter tube, didn't fit. So, I stopped in at the local FBO and asked if they had changed any tires recently and there might be some tubes in their trash. Sure enough there were. So I got an old tube for free. Perfect fit.

Steve, is the velocity/pressure situation such that the inner tube will stay expanded?
 
This is my setup. I did it in 2012/13.

I/O360 w/ECI cold air. The reason for my modification, was to pull the oil cooler away from the hot cylinder. I used a 10 row oil cooler, because it fit nicely against the firewall. (I fly out of Phoenix).

I used a small coffee can to build a plug to fit the 10 row oil cooler, and the Coffee can was the right size to use 4” SCEET.
 

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Steve, is the velocity/pressure situation such that the inner tube will stay expanded?

The real aircraft inner tube is actually fairly stiff. The Chinese wheel barrow tube was much lighter. The aircraft tube does want to hold its shape, but also there is some pressure differential between the oil cooler intake flow and the pressure surrounding the tube (if there weren't, no air would flow thru the cooler ;) )
That internal pressure, even though small, will keep the inner tube 'inflated'.
 
The real aircraft inner tube is actually fairly stiff. The Chinese wheel barrow tube was much lighter. The aircraft tube does want to hold its shape, but also there is some pressure differential between the oil cooler intake flow and the pressure surrounding the tube (if there weren't, no air would flow thru the cooler ;) )
That internal pressure, even though small, will keep the inner tube 'inflated'.

Steve, thanks. I was wondering that since some of the pressure is traded off for velocity there could be a problem. I suppose if the main pressure drop is through the cooler itself, the pressure drop due to the velocity in the duct is a small factor.
 
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