United States Army interest in the design was enough to warrant formal evaluation of the product. This led to the developmental designation of "YG-1" being issued and covered a single KD-1A example. The follow-up "YG-1A" was given a radio suite and the first U.S. Army production model became "YG-1B" and seven were acquired. One was modified with a constant-speed propeller for testing purposes and became the "YG-1C" (later the XR-2).
An additional seven examples followed in the "XO-60" guise which carried the Jacobs R-755 radial piston engine of 225 horsepower. The "YO-60" differed in its use of a Jacobs R0915-3 radial engine of 300 horsepower. Six were produced to this standard.
The "XR-2" was the YG-1C redesignated but also carried the R-915-3 engine of 300hp. The "XR-3" was a one-off YG-1B model modified to the XR-2 standard but only ever reached the evaluation stage and was never furthered.
In practical service, the KD-1 proved a successful autogyro attempt. It was responsible for the first-ever scheduled delivery of mail by air by a rotary-wing aircraft when it completed such a flight on July 6th, 1939. For the U.S. Army, it also served as the service's first viable rotary-wing platform before more advanced types were eventually acquired. A few variants also emerged from Kabaya in Japan: "Ka-Go" was a prototype based on the KD-1A and Ka-1 was given an Argus As 10 engine of 240 horsepower. Ka-2 carried a Jacobs L-4MA-7 engine of 245 horsepower.
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