“The Sikorsky model S-57 stowable-rotor convertiplane design was developed…during 1951, 1952, and 1953. Accordingly, these early configuration studies were based on a counterweighted single bladed main rotor with cold cycle propulsion. The resulting geometrical difficulties of stowing a large diameter rotor were simplified to a large extent by the use of a single blade rotor system. Gas-driven rotor systems were examined, but were restricted to relatively low rotor disc loadings in the order of four lbs. This early work was severely hampered by limited gas turbine technology since both the characteristics and selection of turbines in the early 1950s were unfavorable. “Numerous rotor and propulsion systems were examined in an attempt to achieve the most favorable aircraft configuration. , and the single bladed fully retractable rotor proposed by Sikorsky Aircraft Division. Three convertible configurations were selected – the unloaded rotor principle proposed by McDonnell Aircraft, the tilting rotor configuration proposed by Bell Aircraft Corp. These designs were to be used for observation and reconnaissance missions, and to explore the feasibility of application of convertiplane principles to larger aircraft. This was to include preliminary engineering, wind tunnel tests and a mockup. “In early 1950, a joint Air Force-Army design competition was held to select various convertiplane configurations for a design study phase. During this interval, the US Air Force actively supported the development of this concept under two contracts with Sikorsky Aircraft.” The technical knowledge required to achieve this combination was the subject of intensive study from 1951 through 1958. At that time, an aircraft was foreseen which could incorporate into a single vehicle both the desirable hover characteristics of a helicopter and the high speed performance of a fixed wing aircraft. “In 1949, an aircraft was envisioned which resembled a helicopter to the extent of having a lifting rotor, but which also incorporated the unique feature of being able to stop the rotor in flight, stow it, and then proceed as a conventional aircraft. Flight test of single bladed rotor on an R-4 The test did give the design team sufficient encouragement to proceed with the concept. The blade counterbalance proved acceptable, but the aerodynamic vibratory loads were a concern. This was flight tested on a Sikorsky R-4 helicopter in August 1948. This led to the unusual idea of using a single bladed rotor with a counter weight. It was recognized early that stowing the rotor into the fuselage would be difficult due to the rotor size in relation to the airframe. The concept of a stowed rotor V/STOL aircraft was originated by Sikorsky Aircraft in the late 1940s. These configurations combined the vertical take-off features of the helicopters and the forward-flight-and-range characteristics of fixed-wing aircraft.” Numerous configurations, entitled “Convertiplanes”, were proposed by industry. The helicopters capable of vertical landings and take-off were limited in their design mission because of insufficient cruising speed and range. “Aviation development in fixed-wing aircraft was being applied to increased speed with a resulting increase in take-off and landing runs. Much of the following information on the S-57 Convertiplane and “Stowed Rotor” aircraft is taken directly from that unpublished book: It documented the various aircraft produced by Sikorsky up to that time. One of the most interesting documents is an unpublished book written by “Prof” Sikorsky, Igor’s cousin, prior to his retirement in 1968. Sikorsky’s life and career, and the history of the company he founded. has numerous documents on the history of Mr. To make the geometry work, a single bladed rotor was envisioned with a large counterweight to replace the opposite blade. Sikorsky proposed a “stowed rotor”, where the helicopter rotor was stopped and retracted into the top of the fuselage of a fixed wing aircraft. Many companies were experimenting with high speed concepts to overcome these limitations. Conventional helicopters are speed limited to about 180 knots due to the rotor’s inability to generate lift and forward propulsion at higher speeds. Design engineers then set their sights on how to extend the helicopter’s speed envelope. In the late 1940s the basic concept of the helicopter had been proven and numerous aircraft were in production.
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