I'm posting this for a friend who is having difficulty keeping his RV4 oil cool. Last year the cowling was modified to the Sam James cowl with plenum and 4inch inlets. Engine is an IO360-180hp parallel valve. Oile temp was OK during the winter but is now moving over 200F and it's not summer. Once there engine is hot the temps don't decrease even when you reduce power or dive. Pressure difference in flight between plenum and outside is 10 to 1 (10in.hg to 1 in.hg). Also three different oil coolers have been tested including a new van,s unit and Stewart Warner. Oil thermister valve was also changed. Has anyone got any ideas on where to go from here? Changing the cowl back would be a major undertaking. Apparently there are over 500 RV,s flying with this set up. Any help would be appreciated.
Ok, if the plenum pressure is 10:1 relative to outside pressure, what is the pressure in the lower cowl area? Almost for sure it is not 10:1 with the upper cowl plenum or there would be massive air flow down and out.
My guess is the exit area it too restrictive and there is low air flow down and across the cylinders and the oil cooler. If air is trapped in the lower cowl area due a restricted or small exit area, it is all but impossible to have adequate cooling. The little 7 X 3.25 standard inlets will let a lot of air in but only if it has somewhere to go, so will the 4 inch Sam James jobs.
Here is why I say that.
My cowl is highly modified (and unmodified) over 4 years of screwing around trying to get adequate cooling across radiators with an auto engine. Going back to Lycoming presented some challenges with this cowl but here is the deal as of today. I have approximately 108 square inches of exit area vrs the standard Vans cowl exit of 56 inches (minus the area occupied by exhaust pipes).
This morning I took off from KSUS here in St. Louis, with a heat soaked engine after 2 previous flights at 92F OAT, and climbed post haste to 12,500'. Max oil temp to level off was 178 and CHT 404 (which occurred out of 10,000 at 95 KIAS, increasing speed to 105 dropped it back to 398 quick).
Now I am no expert when it comes to figuring out how to solve problems and literally fell into this cool running Lycoming by accident. The net result of previous attempts to increase air flow through radiators and the cowl obviously works well with an air cooled engine.
Ok, what about drag? So far it seems moot. With the previous engine I never saw Vans performance speed numbers no matter what I did. Supposedly I had the HP, why wasn't it performing? I blamed a less than optimal prop, cooling drag, and myself for not building a clean airplane. Well, guess what - this IO360 does pull the airplane through the air at factory published numbers and not only that, the engine is running very cool on a very warm day. I did another run at 8500' today and it cranked up to 175 KTAS with a fuel flow of 11.8 gph and 2820 rpm. That's 202 mph, slightly above Vans numbers for a 180 HP engine.
My advice is get more air across the cooler. Measure the exit area of the cowl - it must be at least 120% of inlet area to get things moving. You can easily increase exit area by simply cutting out some of the bottom cowl forward of the exhaust pipes. Anything opening on the bottom will increase exit area. I have 2 Bonanza style louvered exits on the side of the cowl that open up about 28 inches. Beech has been using these simple contraptions for about 50 years for oil cooling and they work and they are not draggy. Next time you see Bonanza, look at them. I made a drawing of the openings and copied the shape of inside louvers and made them in a day.
This is getting too long....good luck with your oil cooling.
