HJP-1 / HUP-1 / H-25 Mule
Since the Army Air Forces was supporting the single main rotor configuration, the Navy turned to other types of rotor arrangements, not being sold on any one design in particular. In an effort to obtain the best ship-based helicopter for spotting, rescue, and utility missions on board battleships and cruisers, the Navy, in 1946, contracted with both Sikorsky and Piasecki for two helicopters from each company. The competitive contract resulted in Piasecki developing the PD-14 (XHJP-1), the first overlapping tandem-rotor helicopter, while Sikorsky entered the S-53 (XHJS-1), a design using many components of the R-5 (HO2S-1). According to preliminary characteristics and design performance requirements, both machines were to be configured for a gross weight of less than 5,000 pounds including two passengers.
After comparative evaluation, the Navy selected the Piasecki XHJP-1 over the Sikorsky XHJS-1. One major reason for the selection of the XHJP-1 (later redesignated by the Navy as the HUP-1) was that the Sikorsky model required a ballast change in order to accommodate a change in the loading. Sikorsky designed another helicopter to correct the ballast problem but was too late for entry into the utility evaluation. The HUP-1 was finally developed for the Navy with a 600-horsepower engine and seats for four passengers. The helicopter was restricted to a gross weight of 6,000 pounds and an air speed of 104 knots.
The Piasekci H-25 was an eight-place tandem rotor helicopter developed for the U.S. Navy for naval rescue operations. With minor modification, it met Army needs for a light cargo and utility helicopter. The maximum Army inventory reached 63 in 1955. All inventory aircraft were later turned over to the U.S. Navy. The H-25 had two three-bladed counter rotating rotors. The H-25 was powered by a single Continental Motors nine-cylinder radial R-975-46 475 hp engine and had a cruising speed of 92 mph (80 knots).
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