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After engine cooling, the second most troubling cooling area seems to be oil cooling.
I have seen many different ways of accomplishing this task for UPDRAFT engines, but it seems the most successful is simply mounting the cool directly on the cowl (bottom or top) and putting a ramp in front of it to cause a low pressure area just aft of the dam. The best position for the dam is about 1-2 inches in front of the opening. Improvements can be had by properly radius the exit openings of the cooler. Check out Vance Atkinson's oil cooler arrangement.
For a tightly cowled downdraft engine, oil cooler placement is much more difficult. In my installation I wanted to totally separate the oil cooling duct work from the engine cooling ductwork. My rational is that separation of the two cooling systems would greatly simply trouble shooting (and it certainly has). I could independently solve either the engine cooling or oil cooling issues if necessary. In my initial installation I ended up with too much engine cooling and not enough oil cooling.
Ok, lets get into oil cooler specifics.
First with downdraft cooling the placement of the oil cooler is more difficult. Where does the air come from? You CAN NOT take air from general cowl area (a traditionally done with an updraft engine) because it is too hot to be successfully used after cooling the engine to cool the oil (tried that) and there is not enough pressure for the small fins of the cooler. You need a dedicated air source either from either the engine inlet plenum or a separate oil cooler plenum.
As you can see in the pictures, I first built a dedicated duct drawing air from the bottom of the strake via a P-51 style scoop, into the cooler and back out below the cowl.
You can see a side view of my proposed ducting with a Positech cooler.
I believe oil cooling is a direct reflection of INDICATED airspeed. At lower altitudes above 140 kts true and 135 kts indicated (about 95 mph across the cooler) the oil temp was acceptable. At 15,000 ft at 140 kts true, the INDICATED airspeed drops to about 110 kts and the airspeed across the cooler also dropped to about 75 mph, and my oil temp started to climb. Somehow I must get more air though the cooler. So you really need to size the oil cooling capacity for high altitudes which will be more than ample for lower flight levels.
This is the way the system was initially designed initially. You can see I replaced my Positech cooler for a more efficient Stewart Warner cooler. Changing the manufacturer of the cooler decreased my temp about 8 def F, but it was still a bit too high for me.
I had the inlet on the bottom of the plane as well as the outlet. My oil cooling was less than successful. I had good oil flow lines into the scoop, but apparently there was some sort constraint on the outlet.
At 120 kts (my standard measuring speed), I could only get 86 mph across the cooler. This resulted in oil temps around 208 F in normal operation or 220 f in a hard climb. I tried to put VG just in front of the outlet, before the inlet, a ramp before the outlet and finally gill type louvers on the bottom of the outlet. The installation of louvers seemed to help the most.
After reading about though the wing oil cooling in a NACA study , I found that the aft curved surface of the upper cowl might be an optimal place for the outlet. According to the study, this is a very nice low pressure area (on the aft portion of a curved wing surface). I installed a small static port into a suitable area and measuring the pressure in this area, I confirmed the information in the report.
During a down time, I proceeded to modify cooling system. First was removing the outlet duct, tilting the cooler (toward the upper cowl) and building an outlet duct to discharge the air toward the upper cowl.
I cut an opening in upper cowl, and built a flange with camlocks on the oil cooler duct to facilitate removal of the cowl.
A second NACA report discussed cowl flap design as it applied to engine cooling. I used this report to constructed louvers over the outlet to maximize airflow and ensure outlet air is aligned to the air stream and a cowl flap on the bottom of the plane to aid in cooling during climb out.
I used some foam to hand carved the gills, and used the Cozy Girrrls LoVac method of glassing to ensure an accurate impression of the glass onto the foam.
Surprisingly I took my air nozzle and blew air across the top of the finished louvers and was easily able to get a piece of paper to be sucked up tight against the bottom of the louvers. The surface airflow was causing a strong enough vacuum to support a piece of paper! Amazing!
Could it be possible that by putting in louvers in the PROPER area of the cowl it may be possible to significantly improve cooling of the engine? I completed the mod in December 2005 and so far my oil temp has not been above 195 deg f on climb, and typically about 186 F in cruise. The real test will be in summer.
(Note: after flying with it for 3 years, I find the oil will slowly rise to 205f in a steady climb to 15,000 ft and quickly level off in cruise to 185f). It is consistently 185f all the time now.
This shows a great shot of the cowl louvers and the tapering of the cowl to remove the rounded area where the cowl closed out (separation) and make it more streamlined.
Overall, the closeout streamlining mod added 4 knots to the plane. Gary Hertzler you were right!
The underside and my cowl flap door.
Since I had a straight entrance into the cooler, I thought I would try using some expanders to try and increase the pressure across the cooler. When I started this mod at my reference 120 kts indicated, I measured 118 mph across the cooler using an airspeed indicator (the ports are on the inlet and outlet duct of the oil cooler plenum). I used some scrap foam, carved ramps and using some aluminum tape, flew the plane to check the results.
This is the shape of the diffuser ramps I installed. The result was an amazing!! I now have 128 mph across the cooler at a cruise speed of 120 kts which is an increase of 10 MPH. Sweet!
A large ramp is on the top, and a smaller one on the bottom. Overall I decreased the inlet size almost 3 sq in with a significant improvement of pressure.
Finally, some pictures of the finished product.