FA: WWII Target Drone Motor (O-15-3 McCulloch)

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McCulloch also built a four-cylinder version of this engine, designed to produce 72 hp at around 6000 RPM. I had one in a small homebuilt airplane I bought for restoration; got rid of it because of its fearsome vibation and fuel consumption. It had been modified to a lower compression and swung a longer prop, and produced around 50 hp. The firing order was the origin of the vibration: the front two cylinders fired at the same time, and then the back two. It sounded like King Kong's chainsaw. Many of them were used on Bensen gyrocopters. These engines were designed for a 15-minute service life, intended only for target drones to be shot down by anti-aircraft gunners. Their magnetos were famous for failing without warning, and they had only the single ignition. Vibration would wear out the carb throttle shaft bearings quickly. In spite of all the drawbacks, many of them flew for years as long as they were looked after and a different carb was used. Magnetos needed good cooling.
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Dan Thomas
The version I have uses a Bing downdraft rather than a Tillotson updraft which was typical for the O-15-3 motors I can find online. Also, the O-15-3 pictures show them with counter-rotating gear boxes that were bolted to an intergral flange cast into the front of the engine case. Mine only has the provision for the prop hub off the crankshaft. It is clearly marked O-15-3 however.
It is an amazingly neat & compact motor weighing no more than ~15 lbs. maybe less. I was able to find info on it and it said it produced 12HP and was used on a 95 lb. WWII USN target drone. I guess it must have been built under license by McCulloch. Unfortunately accurate info on a 60-year-old motor is not easy to come by.
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