more prototypes (designated XHRP-1), the PV-3 would go into production as the HRP-1 in 1947. The HRP-1 was commonly nicknamed the "flying banana" because of the upward angle of the aft fuselage which ensured the large rotors did not hit each other in flight. The name would later be applied to other Piasecki tandem-rotor helicopters of similar design. An evolutionary follow-on design to the HRP-1, designated HRP-2, used an all-metal skin and switched crew seating to side-by-side instead of tandem; however, the limited power meant only five were built, all for the Coast Guard. In 1949, Piasecki provided the H-21 Workhorse to the United States Air Force, an improved version of the HRP-2 with a more powerful Wright R-1820 Cyclone radial engine. Piasecki's tandem-rotor helicopters flew higher than competing single rotor designs, and offered a smoother ride. At approximately the same time the HRP-1 and HRP-2 were being developed, the Navy commissioned Piasecki to design a smaller tandem-rotor utility helicopter; the resulting prototype, which Piasecki called the PV-14, was designated XHJP-1. These went into production as the HUP-1 (PV-18), with the first variants delivered to both the Navy and the United States Army (as the H-25) in 1949; in total, 339 were delivered to the militaries of the United States, Canada, and France by 1954. The HUP was designed with overlapping main rotor blades, which reduced the size so they could be carried on aircraft carrier elevators. Piasecki is forced out Don R. Berlin was brought in as president and director of Piasecki Helicopters in 1953, while Frank Piasecki was chairman of the board. Under Piasecki, the company began the PV-15 large transport tandem helicopter project (designated H-16). The prototype PV-15 was first flown in 1953, but a fatal crash in January 1956 led to the cancellation of the project. The majority owners eventually lost faith in Frank Piasecki's leadership and by May 1956 he was forced out of the company. He had formed a new company, Piasecki Aircraft Corporation to pursue the development of compound helicopters and other rotorcraft. In two successive special stockholders' meetings the board then changed the name of Piasecki Helicopter to Vertol (for vertical take-off and landing) Aircraft Corporation and amended the bylaws to bar Piasecki's re-election as a director, on the grounds that he was running a rival company. Acquisition by Boeing In 1956, Vertol began developing a successor to the HUP with improved