S-49/R-6A/hos-1/Hoverfly Mk II helicopter sikorsky s-49/R-6a army Serial No. 43-45379

The main gear box with a magnesium housing stood vertically between the engine and main rotor. It conveyed power to the main rotor at reduced speed ratio of 12.96 to 1 and to the tail rotor at a reduced speed ratio of 1.309 to 1. It also included a freewheeling unit to allow autorotation in case of engine failure. A hydraulic rotor brake was located at the rear of the main gear box. The S-49 main gear box contained a planetary system gear reduction system. This was the first use of this method for speed reduction in Sikorsky helicopters and various planetary configurations were used in most subsequent Sikorsky helicopters.