It seems a shame that these documents are released for public consumption. The price on these is quite high. It looks like the earliest tests on the theory of the Rotabuggy occurred in August of 1941, which is earlier than I would have thought.
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“1941 to 1944 – Original RAF confidential and restricted file of the most bizarre World War II invention, a “flying jeep”, which became known as “Hafner Rotabbugy”, or “Hafner Gyroplane” in honour of his chief inventor Raoul Hafner (1905-1980), Austrian born British helicopter pioneer and engineer, known for his major contributions to the aerospace industry.”