The Kellett Autogiro Company was founded during the "Golden Age of Flight" in 1929. Its early work involved licensed production of Cierva autogyro aircraft. Despite their appearance, autogyros were not true helicopters as their main rotor masts were not driven under normal power. Instead an "autorotation" effect was used to develop vertical lift while an engine-driven propeller provided the needed forward pull / push. The concept was developed by Spaniard Juan de la Cierva and an example first-flown in January of 1923.
Kellett produced several of the early-form Cierva products before shifting attention to a more internally-oriented design. This work produced the Kellett "KD-1" which held attributes akin to the Cierva C.30 model autogyro. The aircraft carried a traditionally-arranged airframe which sat the engine at the nose and two open-air cockpits directly aft of this. The engine (Jacobs L-4 radial unit of 225 horsepower) drove a two-bladed propeller. The tail unit incorporated the necessary horizontal and vertical planes while the undercarriage was fixed in flight and wheeled for ground-running actions. The notable physical characteristic of the aircraft was its three-bladed, mast-mounted main rotor which was installed over and ahead of the crew positions and over the engine compartment.
The prototype emerged as the KD-1 and only a single example was built. Upon completing the needed testing and certification, the KD-1 was brought to the commercial sector as the "KD-1A". Three were built to this standard. The "KD-1B" was similar in form and function but offered enclosed cockpits. Two of this mark were built.
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.
KD-1 - Prototype; single example
KD-1A - Fitted with Jacobs L-4 radial engine of 225 horsepower; three examples completed.
KD-1B - KD-1A model with enclosed cockpits; two examples completed.
YG-1 - KD-1A for evaluation by U.S. Army
YG-1A - YG-1 fitted with radio equipment
YG-1B - U.S. Army production model; seven examples completed.
YG-1C - One-off YG-1B fitted with constant-speed propeller for testing; redesignated to XR-2.
XO-60 - Fitted with Jacobs R-755 radial of 225 horsepower; seven examples completed.
YO-60 - XO-60 model with Jacob R-915-3 radial of 300 horsepower; six modified to this standard.
XR-2 - Redesignation of YG-1C; Jacobs R-915-3 radial of 300 horsepower.
XR-3 - One-off YG-1B modified to XR-2 standard
Kayaba Ka-Go - Prototype by Kayaba of Japan; based on KD-1A model.
Kayaba Ka-1 - Fitted with Argus As 10 engine of 240 horsepower.
Kayaba Ka-2 - Fitted with Jacobs L-4MA-7 engine of 245 horsepower.
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