English: A U.S. Navy Bell XHSL-1 (Bell Model 61) prototype in flight. This was the first USN helicopter fitted with a dipping sonar, although mostly only the prototypes (BuNos 129133-129136) were used for flight test. Most production models were stored directly. The photo is printed in conjunction with the work of LCdr. Stewart Graham, USCG. He was transferred to the Naval Air Development Squadron (VX-1) in Key West, Florida (USA), in February 1951. Here he taught Navy helicopter pilots and crew members the techniques he had pioneered while testing the dipping sonar until September 1953 when he returned to the USCG.[1]