The Carbon Cub Stroker 340 runs hot. Those with the Executive Panel know all about the problem with the excellent engine monitoring on the Dynon. Those who do not have engine monitoring only have oil temperature as a guide to engine temperatures. Oil temperature in the Carbon Cub runs low even while cylinder head temperatures are running high. So relying on oil temperature is very misleading in the Carbon Cub.
I have had some experience both in homebuilts and store bought airplanes getting temperatures down. So along with California Cubs and help from Randy Levold at CubCrafters we are beginning the process to evalutate the engine temperatures in the Carbon Cub. Since we are in Southern California we experience some of the greatest extremes of +ISA temperatures, so this is a good place to do hot weather testing.
To begin the test program we are doing some base leveling testing and documentation then plan to move on to look at some quick fix mods then ultimately looking at possible redesign of the cowl, if that is necessary. Right now we are going to focus on quick fix modifications. Randy and the folks at CubCrafters will have to wrestle with redesign of the cowl if the quick fixes don't work.
A program like this needs some goals. My goal would be to see the Carbon Cub be able to climb at maximum continuous power for 5 minutes while keeping all CHT's less than 380 dF up to atmospheric conditions of +15 dC ISA. I have no doubt if we can attain this goal the airplane will cruise in the same range of OAT's at most power settings without any cylinder exceeding 380 dF.
Why 380 dF goal? Most folks (except the engine manufactures) consider 380 dF CHT's or less to afford the most protection to the cylinders and associated engine components to allow the engine to reach TBO. Here is a slide from the Advanced Pilot Seminar that speaks to this issue.
So why does the Carbon Cub run hot? We hope to learn much more about this with testing but here is my guess. I think there is a significant cowl out flow issue. Anyone who has seen a Cub in cruise will note it moves along with cowl looking like it is at a negative angle of attack to the air flow. In my opinion the "smiley" air intake is blanking out the cowl outlet further aft. The cowl outlet has a very minor lip that is not seeing impact air. How do I know this? My airplane just completed a round trip from San Luis Obispo to Yakima for some warranty work. On it's return the airplane was plastered with bugs, except on the lip of the cowl outlet. Except for two small bug hits the cowl outflow lip was clean. So the cowl outflow lip is in "dead air".
Anyone who has flown an airplane with cowl flaps knows what a bug magnet cowl flaps can be. Cowl flaps are placed in a position so that when deployed they create a large negative pressure at the cowl outflow. The goal is to create enough negative pressure to draw cooling air through the baffle system. So some of our early testing will involve installing a large fixed cowl flap on the Carbon Cub to look for cooling changes.
Engine manufactures specify a pressure differential across the cylinders of between 4" and 6" of water to get adequate cooling of the cylinders. So our first step in this test program is to determine the pressure differential in the Carbon Cub. This can easily be done with the Executive Panel since we have two airspeed sources. So I have converted my backup airspeed indicator to measure cowl pressure differential. The pitot line goes to the top side of the baffle system and the static line measures the pressure in the cowl near the exit point. With that you can simply covert airspeed in MPH to inches of water pressure differential. Here is the conversion:
Today I made the first baseline test run. OAT was near ISA today (still pretty cool for California in June). I flew at 35% power and 50% power. Both upper air cowl air inlets were taped shut. The left hand airspeed indicator is measuring pressure differential.
As you can see at 35% power the pressure differential across the cylinders is around 2" of water, while at 50% power the differential is around 2.8" of water. While not pictured here in a climb at full power at 95 MPH the differential was about 3.5" of water. The pressure differential will change with power settings but it is very clear from these first set of tests that the pressure differential across the cylinders is about 50% of what it should be.
So we have a baseline now to begin testing some quick fix modifications. While my presumption is that the cowl outflow issue is the biggest problem I do think there are some aerodynamic issues with the cowl inlet as well that we need to understand.