<p>Although Igor Sikorsky could not claim to have invented the helicopter, he most certainly developed the basic concept into a practical flying machine which was to be the foundation of an important sector of the aviation industry. His successful introduction of the anti-torque tail rotor overcame the last major control problem, following months of trials with various auxiliary rotors fitted to his VS-300 prototype, which had made its first tethered flight on 14 September 1939.
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<p>By the spring of 1941 the VS-300 was achieving free flight at forward speeds of up to 70 mph (113 km/h), and the Vought-Sikorsky Division of United Aircraft received a contract for the development of a two-seat version which was designated XR-4. It had a fabric-covered fuselage with an enclosed cockpit and was powered by a 165-hp (123-kW) Warner R-500 engine which drove both the main and tail rotors through gearboxes and driveshafts. The first flight was made at Stratford, Connecticut on 14 January 1942, and by April Sikorsky had become sufficiently confident to demonstrate the helicopter to governmental representatives and senior military officers.
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<p>Over the period from 13 to 18 May, the XR-4 was flown to Wright Field, Ohio for evaluation, accomplishing the world's first cross-country helicopter delivery flight. During the 761-mile (1,225-km) journey, which was completed in 16 hours 10 minutes flying time, the XR-4 made 16 landings.
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<p>Thirty production R-4s were ordered by the USAAF, the first three being designated YR-4A and the rest YR-4B; all were powered by the 180-hp (134-kW) Warner R-550-1 engine and were equipped with a 38-ft (11.58-m) diameter main rotor in place of the 36-ft (10.97-m) diameter rotor fitted to the prototype. Production was completed by 100 R-4Bs with 200-hp (149-kW) R-550-3 engines.
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<p>Among the R-4's achievements was the first helicopter landing aboard ship, accomplished on 6 May 1943 when a USAAF YR-4B, flown by Colonel Frank Gregory, landed on the USS Bunker Hill, using a deck area only 14 ft (4.27 m) greater than the rotor diameter. The type also recorded the first helicopter rescue, probably that made by a machine from Colonel P. D. Cochran's 1st Air Commando which, in April 1944, rescued the four occupants of a light aircraft which had crashed behind Japanese lines in Burma.
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<p>An earlier example of the helicopter's value in a humanitarian role was provided on 3 January 1944 when, in strong winds with sleet and snow, Commander Frank Erickson of the US Coast Guard flew his YR-4B from Floyd Bennett Field, New York to Sandy Hook, New Jersey with plasma for badly-burned survivors of an explosion aboard the destroyer USS Turner.
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<p>The US Navy had taken a decision to acquire helicopters for evaluation in July 1942 and its first HNS-1 (a YR-4B on loan from the USAAF) was delivered later that year, to be supplemented by two more in March 1943. A total of 25 HNS-1s was eventually to be operated by the US Navy and US Coast Guard.
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<p>British observers had been present at Sikorsky's demonstration flight in April 1941 and had been active in the later deck operation trials. Seven YR-4Bs and 45 R-4Bs were flown by Royal Air Force and Fleet Air Arm units under the designation Hoverfly I, entering service with the Helicopter Training Flight at RAF Andover in 1945 and with No. 771 Fleet Requirements Unit at Portland in September of that year.
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<p>The R-4 helicopter was subsequently developed into the R-6 which retained the earlier machine's rotor and transmission system, but installed in a streamlined fuselage with a semi-monocoque all-metal tail boom. Powered by a Lycoming O-435 engine of 225 hp (168 kW), the prototype XR-6 was flown on 15 October 1943 and was followed by five XR-6As with a 240-hp O-405-9 engine. The 26 YR-6As were similar. Production of 193 R-6As followed, of which 36 went to the US Navy as the HOS-1 and 150 to the UK as the Hoverfly Mk II. The R-6B was a projected version with a 225-hp O-435-7 engine but this was not proceeded with.
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<hi style=hdr1>Specification</hi>
<hi style=hdr2>Sikorsky—YR-4B</hi>
<list style=aflst>
<item><hi format=bold>Type:</hi> two-seat training and rescue helicopter
<item><hi format=bold>Powerplant:</hi> one 180-hp (134-kW) Warner R-550-1 radial piston engine
<item><hi format=bold>Performance:</hi> maximum speed 75 mph (121 km/h); service ceiling 8,000 ft (2,440 m); range 130 miles (209 km)
<item><hi format=bold>Weights:</hi> empty 2,020 lb (916 kg); maximum take-off 2,535 lb (1,150 kg)
<item><hi format=bold>Dimensions:</hi> main rotor diameter 38 ft 0 in (11.58 m); length 48 ft 2 in (14.68 m); height 12 ft 5 in (3.78 m); disc area 1,134 sq ft (105.35 m<su>2</su>)
