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Old 04-14-2022, 10:11 PM
claycookiemonster's Avatar
claycookiemonster claycookiemonster is offline
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Location: Midway, UT
Posts: 440
Default Oil Cooler outlet duct?

I see many mounts and placements for oil coolers, but they all end at the cooler. Air is blown on it, and then...? The interior of a cowling, particularly below the engine has got to be a very turbulent place, and we're just dumping more turbulent warm air into it at the cooler.

Has anyone installed any kind of ductwork downstream of the oil cooler? It occurs to me that some sort of plenum full of warm air could extend to near the cowl exit where we could depend on a low pressure area to pull the air out of the cooler chamber. (P-51 afficionadoes might even achieve the purported thrust that aircraft was said to achieve by it's radiator)

It might only take 12 - 18" of ducting to lead the oil cooler exit flow near the cowl exit to assist the flow through the cooler as well as keeping that part of the heat somewhat contained?

Or, am I (ah-hem) full of hot air?
Clay "Cookiemonster" Cook
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Old 04-14-2022, 11:08 PM
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I have seen a few examples of this here on VAF.

I believe Dan Horton has one.

There was a gentleman named Bob Axsom with a RV6 that did a lot of racing that did a duct.

Alan Judy also, if my memory is correct.
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Last edited by Mike S : 04-14-2022 at 11:22 PM.
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Old 04-15-2022, 05:11 AM
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DanH DanH is offline
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A cooler exit duct is useless if it does not extend all the way to a low pressure area. In fact, it's probably worse, as it adds skin friction.

As our own Steve Smith explained to me, with a standard cowl the pressure drop (inside the lower cowl vs freestream) is right at the plane of the exit. Thus an oil cooler duct must reach all the way to freestream.

Mine is neither fish nor fowl. The long converging exit bell drops pressure below lower cowl pressure at a point prior to the exit plane, but not quite as low as freestream. I wanted to run rather high lower cowl pressures.
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Old 04-15-2022, 06:47 AM
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Dan 57 Dan 57 is offline
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assist the flow through the cooler
Not sure I wanna do that.. like many others O-360 users I struggle to reach ideal oil temps, even with no air to the cooler, as in shutter fully closed.

On parallel thinking, one thing I wonder about is how to replace that Vernatherm automatic valve with a manual control...
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Old 04-15-2022, 07:51 AM
rmarshall234 rmarshall234 is offline
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This is a very interesting question and one that I plan to explore with my biplane build. Through the modify/fly/test route. My oil cooler is FW mounted up high and will receive dedicated ram air pressure for cooling but the outlet will initially be the large area where the cylinder cooing air exits the cowl. All mixed together. Plans are to add an exit plenum that dumps into the low pressure area of the exhaust tunnel and also a shutter valve at the cooler. All done incrementally with test flights in between. One of the smartest engine guys I know - Tom Wilson - suggests that this is the way to go. We'll see.
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Old 04-15-2022, 11:22 AM
Freemasm Freemasm is online now
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To the OP, I'm going to respectfully suggest holding off on this decision and save yourself some possible unneeded work.

There are a lot of variables that will determine how effective your oil cooling circuit will be: cooler size, things that will affect airflow conditions (engine plenum sealing, your flying environment, position of the baffling cooling air take-off), real oil flow rate, etc. There's a pretty decent mix of boundary and process conditions and their respective influencers.

Based on these and the oil cooler exit air temps reported here, increased airflow through it will have lees effect than one would think unless there is a pretty gross error elsewhere. I'd stick to the plans unless you encounter an issue. My dos centavos.
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Old 04-16-2022, 07:46 AM
BillL BillL is offline
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Default What is your goal?

These guys have addressed the basics and DanH quite well as usual. As an observation and pitfall, poor oil cooling has typically resulted from either, (1) speed pursuits where mass flow (and delta P) across the engine was reduced, or (2) excess bypass around the engine (again low delta P) resulting in restricted cowling inlet or exit of the standard cowl.

Either of these will reduce mass flow across the cooler, then the attention of the oil temps begin.

Which direction are you attempting to go, reduced exit, (speed) or something else?

Alan Judy made a beautiful separate circuit for his oil cooler. And a clever variable inlet opening for low speeds. It was needed because he restricted the inlet and exit air areas gain speed. His speed gain was quite successful in the small inlet, small exit category.
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Old 04-16-2022, 09:00 AM
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claycookiemonster claycookiemonster is offline
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Good question. "My Goal" isn't clearly defined yet, as I'm still building.
I've just noticed the great care that goes into many of the "upstream" areas (plenums, oil cooler ducting, Scat tubing, etc) and then the relatively unscripted and chaotic "downstream" areas.
I'm looking intensely at all the contingencies my build may face in all areas, and trying to prepare for them now. I recall the posts trying to address whether the supposed low pressure area at the cowl exit really is low pressure, and how to address that. The Vetterman site has an interesting mod to the cowl exit to address this. So, I began to wonder about whether adding a few ounces of ducting after the oil cooler might make cooling more efficient? Maybe with better downstream management, I'd need less upstream input?

Just curious. I've been left alone in the house. It's either this, or I begin to chew my own sneakers.
Clay "Cookiemonster" Cook
USAF: T-38, F-111F
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Old 04-16-2022, 11:44 AM
gasman gasman is offline
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You might find your answer by researching what Dave did with his oil cooler...

Try to find the entire series.
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Old 04-16-2022, 01:14 PM
jrs14855 jrs14855 is offline
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Default Oil Cooler

Dan 57 mentioned manual control of oil flow to the cooler. Hardware store ball type valve modified for a manual push pull control cable. The vernatherm is removed. I believe the Lycoming parts manual shows the parts that replace the vernatherm.
Alan Judy had a inlet door for the cooler air in that could be completely closed. I don't remember what he had for cooler exit air.
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