Staff Writer (Updated: 10/6/2014):
Kaman began a concerted development effort throughout 1946 to bring about a helicopter design that utilized an "intermeshed" rotor arrangement system with power supplied through a piston engine installation. Necessary stability and control would be provided by way of servo-flaps fitted to the edge of the blades. These blades then, in turn, would spin in a contra-rotating fashion, countering the inherent spin caused by engine torque. As such, there would be no need for a conventional tail rotor - and its complicated and costly shaft assembly running off of the power from the main engine. The prototype design was finalized as the "Kaman K-125A" and first flew on January 15th, 1947.
In 1948, a revised and improved form of the original prototype - the "K-190" - went aloft. The K-190 was then followed in 1950 by the "K-225" prototype which introduced seating for three crew. It was this model that took the interest of the United States Navy which ordered a pair for testing in 1950 as the "XHTK-1". Having been satisfied with the ensuing trials, the US Navy placed a quantitative production order to procure 29 examples of the new helicopter. In the USN inventory, the XHTK-1 prototype designation was graduated to become the "HTK-1" (T for "Training"). Kaman went on to develop the "Kaman Model 600" as a dedicated service version to which the US Navy, USMC and United States Air Force all eventually took ownership of.
The USMC knew the Kaman Model 600 as the HOK-1 (O for "Observation") and this form was powered by a single Pratt & Whitney R-1340-48 Wasp reciprocating, 9-cylinder, air-cooled radial piston engine. The HUK-1 (U for "Utility") became another USN version fitting the Pratt & Whitney R-1340-52 radial piston engine series of which 24 were delivered as such. The USAF designated the new helicopter as the more recognized H-43A "Huskie" (this version based on the USMC HOK-1) and eighteen examples of the type were delivered (after 1962, in which the designation system of American aircraft was restructured, the H-43A became the "HH-43A").
A single K-225 prototype was modified to accept a Boeing YT50 Model 502-2 gasoline turbine engine - in effect becoming the first helicopter in history to successfully mount this engine type, first flight being recorded on December 10th, 1951. The USCG also evaluated the Kaman helicopter design under the designation of "HTK-1G" but did not accept the design. A static test drone for the US military was known as the "HTK-1K".
The Kaman's turbine arrangement was generally sound technology for its time but there was natural thought given to the fitting of a new turboshaft technology engine for added performance. The potential was realized when an Avco Lycoming-brand XT53 engine was installed into a HOK-1. The resulting application proved exceptional and led to the introduction of a new production mark - the "H-43B". First flight of the B-model was recorded on December 13th, 1958. An accident involving the rotors and tail section in 1961 forced the Huskie to undergo several modifications to her empennage, which proved an ongoing process throughout the operational life of the helicopter.
The H-43B was dimensionally larger than the preceding H-43A series and held internal seating for up to eight passengers including applicable gear (rescue and otherwise). The United States military ordered 193 examples of this new version and sent examples to allies overseas. The Kaman B-model version helicopter also went on to set several helicopter-related world records of her time in altitude, range and rate-of-climb. As the H-43 before it, the H-43B later became the "HH-43B" after the 1962 designation restructuring. Similarly, the HUK-1 became the "UH-43C" while the HOK-1 became the "OH-43D". A single OH-43D was converted to an unmanned drone as the "QH-43G". The HTK-1 was now recognized as the "TH-43E".
The final Kaman offering of the Model 600 became the HH-43F which was an HH-43B fitted with a Lycoming T-53-L-11A engine of 825 shaft horsepower and utilizing smaller diameter rotor blades. Forty-two of this model were either produced or converted (the latter from existing HH-43B systems).