Sikorsky YR-4B 43-28225 in the NACA full scale wind tunnel, Langley Field, Virginia, 1944. A technician is preparing strobes to take stop-motion photographs of the helicopter’s rotor blades while they turn at normal operating r.p.m. (NASA)
Sikorsky YR-4B 43-28225 in the NACA wind tunnel, 1944