Hoverfly Mk.I was the British name for the Sikorsky R-4B. It equipped Royal Air Force and Fleet Air Arm units set up in 1945 to evaluate helicopters for service use. In total the R.A.F./F.A.A. operated seven YR-4B (pre-production machines) and forty-five R-4B Helicopters. Subsequently the Sikorsky R-6A would also operate in the British Armed Forces as the Hoverfly Mk.11.