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By ‘spinners’ I assume you mean what they called butterfly bombs.
They were still being found in the 1950s so I guess that the failure rate was quite high.
Re Diesel engine planes - Diesel engines in aviation were used originally on airships, then on flying boats. The Junkers Jumo was, I think, the engine used in some Junkers aircraft, but not universal.
I was told that the sinister drone was produced by having the engines (petrol or diesel) run at different speeds with prop pitch used to match the power. This produced a ‘beat’ in the tone, the ‘drone’.
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The Mad Landsman
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