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In early 1950, the following companies submitted these designs to a Navy specification calling for an HP class (Patrol) helicopter with ASW (anti-submarine warfare) capability, a brand new mission for the helicopter at the time. None was selected in the end (source: Spangenberg Index, April 1950).
- Curtiss-Wright P-592
- Gyrodyne 9 Helidyne
- Hughes 212
- Kellett KH-9
- McDonnell 73/A/B/C/D
- Pennsylvania Aircraft Syndicate (designation unknown)
- Piasecki PD-22B
- Piasecki PH-26/A Convertaplane
- Sikorsky XHPS/-(A)/-( B )/-(C)