Several owners operating their Carbon Cubs in high ambient temp conditions this past summer have reported CHTs that were uncomfortably close the redline after extended climbs, and even high power cruise, in these conditions. Consequently we undertook a summer long project to study the issue and try to improve cooling margins.
The Carbon Cub's relatively high power engine in its relative low speed airframe indeed presents a challenge. Nevertheless a solution was needed, so we began with understanding the airflow around the cowl... IMG_7850.jpg
After pursuing several concepts we ended up with a set of "ground adjustable cowl flaps" as the solution. This is probably not surprising to many of you since adjustable cowl flaps are common on many aircraft from Cessna 182s to Bonanzas, but the low speed envelope of the Carbon Cub presented a bit of a unique challenge. After working on several solution concepts though we found that a set of supplemental cowl flaps simply worked best.
Do they work? I've attached a copy of the flight test cards from the last test flight which was not on our demo aircraft but rather on a fresh production plane so as to ensure accuracy. Please review it if you're curious, but the bottom line under well controlled conditions was a 20° reduction in CHTs, an 18° reduction in oil temp, and a 4 mph reduction in airspeed caused by the increased cooling drag, a sign that there is indeed more air flowing through the cooling system. Our beta testers have confirmed these results and further report that the cyliners cool off significantly faster when leveling off at the top of a climb.
These cowl flaps are ground adjustable to three positions yielding an exit slot that is 1", 2", or 3" (fully open). Owners should adjust the flaps to suit their ambient conditions, but during summer temps of 90° or above they should be fully opened and then either fully closed or in the middle position for winter operations in most parts of the country.
With this posting I am announcing a Service Instruction (SI0017) that is about to be released for the CC11-160 regarding retrofitting these cowl flaps. This is a service instruction and therefore NOT mandatory, but all CC11-160 owners will be offered a set of these flaps at n/c. Further, for those aircraft on warranty, we will cover installation labor. All owners (other than our beta testers) will receive a mailing on this, but feel free to contact Ethen Hughes at ethen.hughes@cubcrafters.com or 509-248-9491 to make arrangments for your aircraft. We are producing parts at a rate of three sets per week so it make take a few weeks to get yours, we appreciate your patience.
CC340 hot weather operation (high CHTs)
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