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Diesels in Pattern
I started out with a CAP 21. It was equipped with a Webra .61 side exhaust, a J.R. Apollo 6-channel radio, and standard servos. All up, the ship weighed 7.50 pounds and performance was pretty sharp. Then I began the diesel experiment.
I received a diesel head conversion kit for the Webra from Davis Diesel Development. The whole conversion consisted of merely removing the standard cylinder head and replacing it with the diesel head. The fuel system tubing had to be changed to neoprene tubing to allow use of model diesel fuel. The whole conversion took half an hour.
After reading the instructions carefully, I was mildly surprised to have an engine that started rather easily without the glow plug. The engine performed well, with a 13x6 prop. Normally I use a 12x6 prop and get 12,000 rpm.
I finally arrived at the optimum prop: a cut down 14x8 prop that measured 13x7.5. With this prop I recorded a top rpm of 10,500. Idle speed was 1,100 rpm and fuel consumption was 1/2 oz. per minute.
So, what does all this mean? In a nutshell, I have found the Davis Diesel Conversion head to be feasible for pattern performance in the CAP 21, which has a pretty honest airframe. The conversion allowed this ship to perform better vertical maneuvers, slowed down the horizontal flight speed, received better mileage, and it was quieter! The diesel conversion produced a sound that was a cross between a 4-stroke and a well-muffled 2-stroke.