I have spent a lot of time and effort working on cooling and cooling drag on my KR. ?At the last gathering, most probably didn't notice the cowl flap in the trailing edge of the lower cowl. ?
I started with the same cowl as Larry except over a C-85, but didn't close up the cowl inlets. The back left cylinder tended to run hot during a hard climb, but otherwise the cooling was more than adequate, although quite uneven. ?At 350 hours, I replaced the C-85 with an O-200. ?In order to accommodate the larger oil tank on the O-200, I had to push the outlet area out a bit larger. ?My cooling was awesome during a hard climb, but the engine ran too cold at cruise. ?I was seeing one CHT drop down to 175?F at full throttle under cruise conditions. ?I started installing covers on the cylinders to move the air around and even out the cooling. ?That worked pretty well, but the engine still was running too cold. ? About a year ago, I topped the engine and changed to 8.5:1 compression pistons and electronic ignition (E-mag). ?I knew the advance on the electronic ignition would generate quite a bit higher CHTs at the altitudes I fly (7000' - 14,000'), but I also wanted to reduce the cooling drag as the lower cowl had quite a bell shape to it that had to be creating a significant amount of drag. ?The problem was how to keep the engine cool during a climb. ? I ended up cutting the bottom of the cowl off and replaced the bell shape with a flat bottom. ?Then to ensure I had adequate cooling during climb, I added a cowl flap to the trailing edge of the lower cowl. ?I had enough motion in the trim servo I used that I decided to build it so I could open it roughly 3" like a normal cowl flap, close it to a streamlined position in line with the rest of the cowl, and a third position where the cowl flap is retracted roughly 2" up inside the back of the cowl to cut down the air flow around the oil tank, originally intended for wintertime use. What I expected was a bit more drag than before with the cowl flap deployed and less drag than before with it in the streamlined position, and no change from the streamlined position to the retracted position. ?But what I found was, the more I could close off the air flow from the cowl, the lower the drag and faster the plane flies. ?Retracting the cowl flap into the cowl lowered the drag and I was able to fly faster than with the cowl flap in the streamlined position. ?I found that my cooling is barely adequate for the left rear cylinder during climb with full advance in the ignition, but under cruise conditions, the engine still runs a bit on the cool side with the CHTs at 300?F across all 4, or 275?F during the winter operations. ?However the cowl flap and ignition advance gave me better control over the oil temp so now during wintertime ops the oil temp is running warm enough to keep the moisture boiled out of the oil. The point is that your KRs are capable of a pretty significant range of speed. ?Engineering your cooling for cruise may create a situation where you can't adequately cool the engine during a hard climb. ?Or engineering your cooling for climb may generate too much drag and over-cool the engine during cruise. ?It's a real challenge to get your cooling tuned to work well under all circumstances. ?Admittedly, I added a level of complexity to my plane by going to the spark advancing electronic ignition, and I fly my KR under more challenging conditions than most others see, so I often run into issues most other builders won't see. -Jeff Scott Los Alamos, NM > ----- Original Message ----- > From: Larry&Sallie Flesner > Sent: 03/27/14 05:02 PM > To: KRnet > Subject: Re: KR> High Altitude > > At 04:58 PM 3/27/2014, you wrote: > >I mainly need to reduce the size of my engine air inlets. > >++++++++++++++++++++++++++++++++++++++++++++ > >What are the benefits you will gain from decreasing the inlets? > >And will it effect cooling? > >_+++++++++++++++++++++++++++++++++++++++++++++++++++ > > The greatest percent of airframe drag comes from engine cooling. Any > additional air rammed through the cowling over what is necessary to > cool the engine is just additional drag. Ideal airflow is a high > pressure on the top side, low pressure on the bottom side and outlet, > and no more air than necessary. A plenum better directs and limits > the amount of air needed for cooling, thus reducing drag and > increases cooling efficiency. > > I started with what was supposed to be an 0-200 cowl. I had to > considerably modify the cowl and one mod was to cut down the inlet > holes at least 1 1/2 inches on each side. I still have very good > cooling, even without an oil cooler. > > Larry Flesner
