I live and fly a lot in So Cal where the summer temps hit 110f, I don’t let the temperature stop me from flying but the Rotax does complain, oil temps will hit 245f on days when the air temp is over 80f in a continuous climb.
Obviously step climbing or lowering RPM will reduce the temps but……………….
I looked for a way that I could maintain 220f or less oil temperature regardless of the outside temperature or demands I put on the engine. My muffler is already spaced 1 ½ inches from the oil cooler via a lower cowl mod.
First I tried different prop pitches, no significant improvement. Next I tried shielding the oil cooler from the radiant heat of the muffler, that seemed to help a few degrees, but not the improvement I was looking for. I placed a baffled in the front plenum to divert some coolant radiator air to the oil cooler, this gave hotter coolant but didn’t improve the oil temps. I opened the rear of the cowl by 20% where the air exits using spacers under the cowl screws, no improvement. I created a ram air intake of approximately 8 square inches using the oil cooler door, no improvement.
My final idea was to add a second oil cooler. After a long search I decided on a Setrab 50-116-7612 oil cooler. It has a frontal area of approximately 4” square and is 2” thick with 16 rows. It has roughly the same total surface area as the OEM cooler. I plumbed it in series with the OEM cooler. Aircraft Specialty Flightlines, was very helpful and built the new hoses for the conversion. I opened a 2.5” hole to the right of the existing oil cooler air passage, directed the air with a 2.5” scat tube to a custom plenum that covers the new oil cooler. The new cooler is mounted 3” from the firewall to allow the air to exit easily.
I currently have 15 hours on the mod and have been able to get the oil to 235f by force, flying totally wrong, 85f day, 1 notch flaps, 61 kts, climbing 700fpm for 4000’. As soon as I dropped the nose and hit 85 kts within 2 minutes the oil was 222f and falling. Typically I see 200 – 205f in cruise 5400 RPM, 116 kts, 3500ft. On normal climbs that I would have hit 245f by the time I climbed 3000’ I now can climb continuous to 8000’ and stay under 220f. I will add more flight temperature info when the daily temps get back in the 100’s. The total added weight of the conversion was 2.8 pounds.
oil cooler – https://www.amazon.com/dp/B0011FPOCM?psc=1&ref=ppx_yo2ov_dt_b_product_details
Oil cooler fittings – https://www.amazon.com/dp/B078CR3V9D?psc=1&ref=ppx_yo2ov_dt_b_product_details
Conductive teflon firesleeved hoses – https://www.aircraftspecialty.com/rv-12.html
I made the oil cooler mount out of .062 aluminum, it supports the oil cooler off the firewall 3”. The plenum was designed using a blue foam core to get the internal shape and alignment for the SCAT tube. Fiberglass and epoxy resin was used for the layup. Another 2 ½ fiberglass tube was made then attached to the lower cowl for the incoming air. The SCAT tube is 2 ½ “ by 12 “.
The following pictures should help explain the process.
