Operational History
Versions of the HUP built for the U.S. Army were designated H-25 Army Mule. The tandem overlapping rotor configuration was a development by Piasecki and was used in future helicopter designs by the company and successors including the H-21, HRB-1/CH-46, and CH-47.
The design featured two three-bladed, 35-foot-diameter (11 m) rotors in tandem in which blades could be folded for storage in ships’ hangars. The HUPs were powered by a single Continental R975-46 radial engine, with a take-off rating of 550 hp (410 kW). To provide rescue without crew assistance, an electrically operated door, available after folding the copilot’s seat forward, opened through which a rescue sling could be lowered from an overhead winch.
The HUP was produced for the Navy in four versions: HUP-1, -2, and -3. The HUP-2 was the first production helicopter equipped with an auto-pilot. The US Navy also tested a system called 'Raydist that allowed an unmanned HUP-2 to be directed from a ground station and by radio ordered to hover with in 5 feet of the desired point. Edo tested a HUP-2 with a fiberglass hull and outrigger floats for amphibious operations. In addition to those delivered to the U.S. Navy and Army the HUP/H-25 helicopter was also delivered to the Canadian and French Navies. A total of 339 aircraft were delivered over the 20-year life of the aircraft.
The US Army H-25 designation was adopted by the other services in 1962. The final units were withdrawn from US service in 1964. It also served with the French Navy from 1953 to 1965.
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