The Sikorsky R-4 (aka “Hoverfly”) is the world’s first large scale, mass produced helicopter. It’s also the first helicopter put into service by the United States and United Kingdom Air Forces.
The R-4 was intended as a training and rescue helicopter, although it was suitable for more general purposes.
Following the successful VS-300 prototype, Igor Sikorsky began work on what would become the R-4. The first prototype was designated VS-316. Clearly showing its relationship to the VS-300. However, the US army had a different designation system for rotorcraft, so they renamed it the XR-4.
The XR-4 first took flight in 1942 and soon smashed all records for helicopters that had come before it. The original XR-4 completed a 1225 kilometer overland flight, set a service ceiling of 3700 meters and had an airspeed of nearly 140 kilometers per hour.
The US Army Air Force ordered many prototypes for evaluation, steadily improving them in conjunction with Sikorsky. The engine power and rotor diameter were increased with each iteration. The tailwheel was also moved further back during development and the exhaust venting changed from a downward direction to a side-venting design. Fuel capacity also grew by 19 liters during this time.
It’s first combat flights took place in 1944 in the China-Burna-India theatre of war. A combat rescue flight by Lieutenant Carter Hartman. Because of fuel and capacity limitations, these early rescue flights included multiple trips and many refuelling legs. However, they would have been entirely impossible using fixed wing craft.
The R-4B is the proper production version of the aircraft and sports a 200 horsepower R-550-1 engine and larger rotor from an earlier variant, the YR-4A. The rotor itself was of a three-bladed design.
In total there were 131 R-4s produced during the two years of its production. The R-4 was succeeded by the Sikorsky R-6.