Background

In the late 1940s the basic concept of the helicopter had been proven and numerous aircraft were in production. Design engineers then set their sights on how to extend the helicopter’s speed envelope. Conventional helicopters are speed limited to about 180 knots due to the rotor’s inability to generate lift and forward propulsion at higher speeds. Many companies were experimenting with high speed concepts to overcome these limitations. Sikorsky proposed a “stowed rotor”, where the helicopter rotor was stopped and retracted into the top of the fuselage of a fixed wing aircraft. To make the geometry work, a single bladed rotor was envisioned with a large counterweight to replace the opposite blade.

The Igor I. Sikorsky Historical Archives, Inc. has numerous documents on the history of Mr. Sikorsky’s life and career, and the history of the company he founded. One of the most interesting documents is an unpublished book written by “Prof” Sikorsky, Igor’s cousin, prior to his retirement in 1968. It documented the various aircraft produced by Sikorsky up to that time. Much of the following information on the S-57 Convertiplane and “Stowed Rotor” aircraft is taken directly from that unpublished book:

“Aviation development in fixed-wing aircraft was being applied to increased speed with a resulting increase in take-off and landing runs. The helicopters capable of vertical landings and take-off were limited in their design mission because of insufficient cruising speed and range. Numerous configurations, entitled “Convertiplanes”, were proposed by industry. These configurations combined the vertical take-off features of the helicopters and the forward-flight-and-range characteristics of fixed-wing aircraft.”

The concept of a stowed rotor V/STOL aircraft was originated by Sikorsky Aircraft in the late 1940s. It was recognized early that stowing the rotor into the fuselage would be difficult due to the rotor size in relation to the airframe. This led to the unusual idea of using a single bladed rotor with a counter weight. This was flight tested on a Sikorsky R-4 helicopter in August 1948. The blade counterbalance proved acceptable, but the aerodynamic vibratory loads were a concern. The test did give the design team sufficient encouragement to proceed with the concept.

“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. 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. The technical knowledge required to achieve this combination was the subject of intensive study from 1951 through 1958. During this interval, the US Air Force actively supported the development of this concept under two contracts with Sikorsky Aircraft.”

“In early 1950, a joint Air Force-Army design competition was held to select various convertiplane configurations for a design study phase. This was to include preliminary engineering, wind tunnel tests and a mockup. These designs were to be used for observation and reconnaissance missions, and to explore the feasibility of application of convertiplane principles to larger aircraft. Three convertible configurations were selected – the unloaded rotor principle proposed by McDonnell Aircraft [the XV-1 compound helicopter with a reaction drive rotor system], the tilting rotor configuration proposed by Bell Aircraft Corp. [the XV-3], and the single bladed fully retractable rotor proposed by Sikorsky Aircraft Division [the XV-2].

“Numerous rotor and propulsion systems were examined [by Sikorsky] in an attempt to achieve the most favorable aircraft configuration. 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. Gas-driven rotor systems were examined, but were restricted to relatively low rotor disc loadings in the order of four lbs. per sq. ft. 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. Accordingly, these early configuration studies were based on a counterweighted single bladed main rotor with cold cycle propulsion.

“The Sikorsky model S-57 stowable-rotor convertiplane design was developed…during 1951, 1952, and 1953. Several configurations were studied during that period, but the final design was essentially a jet-powered airplane with a one-bladed, counterbalanced, pressure-jet rotor which could be stopped and started in flight; thus providing both helicopter and airplane flight characteristics in the same aircraft. To prevent excessive blade flapping at low rotor speeds, the design provided for the flapping hinge to be locked below 25% of the design rotor speed. In this regime, lift forces on the blade are transmitted directly to the aircraft as moments which could produce pitching and rolling motion of the aircraft. During a normal autorotation stop, the blade drag at zero blade lift provides the torque necessary to stop the rotor. A normal start would be made by using power to start the rotor at zero blade lift. Hence, for normal starts and stops, the blades would contribute essentially no rolling and pitching disturbances even though locked.

“Simultaneously with the design studies, a twentieth scale wind tunnel model of a dart wing aircraft configuration was tested in 1951 to provide information on stability and control about all axes. Modifications to wing and control surface designs were made as a result of these tests. At that time it was decided that design work should be limited to that required to establish the general size and weight of the aircraft. The most important technical aspect of this concept centered about the rotor system during conversion. In particular, the rotor behavior from an aeroelastic and control standpoint during conversion was identified as the critical aspect of this concept. It was, therefore, agreed by [the Air Force-Army] and Sikorsky Aircraft that all future effort should be focused on this area. To this end, a one-fifth scale gas-driven, single blade rotor was designed in 1952, after determination that a full size rotor … was impractical due to lack of a suitable engine-load compressor combination.

“In 1953, fabrication of this scale rotor was completed. It was then tested on the outdoor rig to verify nozzle, ducting, and aerodynamic performance and to determine basic structural integrity. Following this, the rotor was installed in the Massie Memorial Wind Tunnel at Wright Field. An aerodynamically similar model of the airframe was also constructed which allowed evaluation of the mutual effects of wing and rotor. Both dart and conventional tapered trapezoidal wing planforms were tested in the Massie tunnel for approximately three months. During this investigation, the autorotational characteristics of a flapping and of a locked rigid blade were determined for a rotor advance ratio from 0.3 to infinity. Acceleration and deceleration characteristics were determined as a function of shaft angle of attack, blade collective and blade cyclic control inputs. Information was obtained with regard to aircraft rolling and pitching moments produced during rotor starting and stopping. The sensitivity of these moments to rotor control inputs was also determined. Rotor dynamic behavior in passing through all blade resonant frequencies during acceleration and deceleration was also studied in detail. This first Air Force contract was completed in July 1954. Considerable knowledge was gained regarding the Stowed Rotor concept, particularly with respect to rotor behavior during conversion. Information was obtained as to sensitivity to rotor control and time history of moments produced during the phase when rotor flapping is locked out.”

Although not clearly explained in the Prof Sikorsky book, these test results were not fully successful, and the design team moved on to a two-bladed rotor as described in a latter section of this write-up.

Also note that the aircraft configuration was evolving. The isometric cutaway drawing shown above and the three-view drawing shown below are of the initial configuration with rotor stopped, the blade rotated 90º about its pitch axis, and stowed on top of a large pylon on the top of the aircraft. The painting shown earlier and the display model shown below have a cleaner design where the stopped rotor is retracted into the fuselage.

Configuration Features

The unique configuration of a single counterweighted blade was driven by the low disc loading technology of the time. The S-57 disc loading (gross weight/rotor disc area) was only 2.9 lbs/sq. ft. Many modern helicopters have disc loadings of around 10 lbs/sq. ft. or even higher. This led to a very large rotor in relation to the size of the aircraft itself. And thus the decision to try a single blade so that it could be stopped and stowed as the aircraft was converted to a high speed fixed wing.

Three vertical fins were initially included. On the early configuration the center fin included the rotor pylon extending from the rotor to the tail of the aircraft to provide the structure to support the stopped rotor blade. The outer fins were mounted just inboard of the elevons on the underside of the wing and included rudders for fixed wing yaw control.

The airplane configuration was also a technological stretch. It used a “delta” wing with an aspect ratio of 1.37 to 1, and a taper ratio of 3.6 to one.

The rotor blade construction was envisioned to be an aluminum extruded spar with aluminum trailing edge ribs, skin and tip. The compressed air for the tip drive passed through the spar.

The engine was to be installed in the rear portion of the fuselage. A compressor, driven by the engine, was installed forward of the engine to provide the compressed air to drive the rotor or a tail nozzle for fixed wing propulsion.

Conventional helicopter flight controls were to be used. The cyclic stick was to control both the rotor and the elevons on the trailing edge of the wing. The rudder pedals would also control the rudders. Quoting a Sikorsky report at the time: “Directional control in the helicopter hovering condition will be accomplished through the use of a new and novel directional control device operated off the main rotor, since no anti-torque rotor will be used.” No further details were provided.

A two person cockpit was included for a pilot and a copilot/observer.

The rotor stopping and stowing concept for the second configuration is illustrated by the following photographs of a 1/16.7th scale demonstration model.

General Arrangement Drawing – Initial Configuration