Transferred from the United States Navy, Bureau of Weapons.
Fore-and-aft twin-rotor helicopter; wood and fabric; dark blue; 1945.
Piasecki XHRP-X Dogship
While Sikorsky's first helicopters proved successful in their limited military service during World War Two, they were restricted in their ability to carry passengers and cargo because of their small payload, and cramped passenger compartments. The situation was further complicated with these single-rotor designs because their center-of-gravity had to align closely with the axis of the rotor mast to ensure effective control. A twin-rotor helicopter could solve these problems, but the Platt-Lepage XR-1 (see NASM collection), the Army Air Force's (AAF) contracted design, suffered from control problems inherent to its lateral-rotor configuration. When Congress criticized the Navy for largely ignoring the helicopter's enormous military potential during a 1943 Senate investigation chaired by Harry S. Truman, the service was forced to actively search for a new helicopter design. As the only two manufacturers of military helicopters had their hands full with the AAF's contracts, the Navy had to select from a handful of designers - none of whom had yet completed a helicopter suitable for production. However, the design that emerged from this effort became the basis for a family of aircraft that would serve in many nations throughout the remainder of the twentieth century.
A young, enterprising engineer named Frank Piasecki was awarded the contract on the basis of a demonstration held at Washington's National Airport with his experimental single-seat PV-2 helicopter (see NASM collection). The smooth handling qualities of the PV-2 along with a new proposal for a new helicopter design with unprecedented load-carrying capacity provided the Navy with all of the incentive needed to select Piasecki. On January 1, 1944, Piasecki's company, the P-V Engineering Forum, received the contract for construction of the new helicopter, known inside the company as the PV-3. At that time of its completion, the PV-3 was indisputably the world's largest helicopter, with enough room for eight passengers, in addition to the two-member crew. Piasecki insisted that the first prototype remain as company property after completion of the contract because it was intended only as a technology demonstrator and not as a prototype. Once the "dog-ship" had demonstrated the validity of Piasecki's approach, the company would construct test models for the Navy. The craft was dubbed the "dog ship" because of its role as a testbed (dogs were frequently used as the "guinea pigs" of the time). Piasecki was greatly concerned about the final disposition of the "dogship" because he felt the company's wartime contract was doomed as soon as the war ended, and knew that the company had to have at least one model worthy of production with which it could enter the peacetime civil market.
Several of the first helicopter pioneers constructed tandem-rotor designs, but the results had been ambiguous, because the power and control limitations did not make their load-carrying advantage apparent. Piasecki realized the potential of the configuration after his work as an engineer at Platt-LePage on the XR-1 project gave him insight into some of the pitfalls of helicopter design. He noticed that the lateral rotor arrangement on the XR-1 seemed to perform better when flying sideways. Additionally, he realized that if he eliminated the large weight of the design's supporting outriggers, then the helicopter could carry a bigger payload. Most of Piasecki's peers did not share his enthusiasm for the tandem configuration, because they thought that in forward flight, the downwash from the forward rotor would destabilize that of the rear rotor, leading to a rapid loss of control. Fortunately, experiments quickly demonstrated that if the rear rotor would avoid interference from the forward rotor if it turned in a slightly higher plane.
The Navy's designation for the PV-3 was the XHRP-X, which stood for Experimental Helicopter, Transport ("T" was already taken for Trainer), Piasecki. The "-X" indicated that this was not a Navy aircraft, but a company prototype. The official Navy prototypes fell under the XHRP-1 designation. However, the helicopter's unusual bent fuselage earned it the nickname "flying banana," a moniker which stuck to the HRPs, and the later H-21 series.
The XHRP-X's airframe consisted of a lightweight steel-tube truss frame, equipped with a fixed tricycle landing gear. The pilot occupied the foremost of the two tandem seats for the crew. Initially, the helicopter flew without a fabric skin, though a Plexiglas windscreen protected the crew from the onrushing airflow. A single Continental R-975 engine, located in an aft compartment, powered the two three-bladed rotors of the helicopter.
The XHRP-X first took to the air on March 7, 1945. Like all of the early helicopter designs, many control adjustments had to be made, using trial-and-error, before the aircraft could safely fly for any length of time. Early in testing, excessive control pressures mandated the use of two pilots, but P-V engineers soon corrected the problem. The addition of a fabric covering greatly increased the forward speed. The aircraft was almost lost during one early test because of a reliance on poor-quality transmission components manufactured to automotive standards, which were not sufficient for the high demands of the XHRP-X's large gearbox.
The Navy selected the McDonnell XHJD-1 (see NASM collection) as a backup design, in case the XHRP-X proved unsuccessful. However, that design matched the deeply troubled development cycle of its XR-1 predecessor, and the Navy eventually cancelled its funding. In 1946, Piasecki completed two refined versions of the Dogship, which satisfactorily met Navy requirements stipulated in the original contract. The first of these was the XHRP-1 Static-Dynamic Test Article (SDTA). Piasecki originally intended this aircraft to serve as a ground test airframe, but the Navy allowed him to operate it as a flying prototype, because ground testing at the time could not accurately duplicate the flight loads on helicopter systems. The next aircraft in the XHRP's development cycle was a pre-production sample of the HRP-1 "Rescuer". The Navy was satisfied with the results, and by mid-1947 production had begun on twenty HRP-1s. Initially intended for the Coast Guard, which only received three, the balance went to the Navy and Marine Corps, because of their increasing need for the unique capabilities of the helicopter. Ultimately, the HRP-1s played their greatest role with Marine helicopter squadron HMX-1, which used the type extensively to develop vertical assault tactics. Marine Corps tacticians realized that their amphibious invasion forces were vulnerable to atomic weapons, which prompted an intense search for alternative methods of delivering combat troops to the battle zone. The helicopter quickly emerged as the best option and the HRP-1 was the first model with sufficient troop-carrying capacity to make vertical assault operations practical. However, the limited number of HRPs did not see combat service.
Igor Sikorsky correctly viewed the HRP as direct threat to his company's hold on the military helicopter market and set about devising a new troop-carrying helicopter. His new S-55 design, known as the HRS-1 in Marine Corps service, successfully outclassed Piasecki's tandem-rotor models until the advent of the Piasecki H-21 in 1953. However, HRP-1's versatile load-carrying capability was essential to the development of anti-submarine "dipping" sonar for the Navy.
The next variant in Piasecki's "flying banana" series was the HRP-2. This refinement of the HRP-1 featured a greatly improved fuselage layout, but this cosmetic and aerodynamic enhancement reduced the useful load so much that Piasecki only produced five, all of which went to the Coast Guard. Beginning in 1953, the HRP-2's new fuselage was adapted to the highly successful H-21 airframe. This helicopter provided much of the U.S. Army's vertical assault capability during the early years of the Vietnam War until its replacement by the Bell UH-1 Huey (see NASM collection). The HRP also inspired the compact HUP series of fleet utility helicopters used throughout the 1950s and the massive YH-16 flying crane. Frank Piasecki's tandem twin-rotor helicopter designs still demonstrate their usefulness in the form of the XHRP-X's descendents - the Boeing CH-46 Sea Knight and the CH-47 Chinook.
Rotor Diameter: 12.5 m (41 ft) each
Length:14.38 m (47 ft 2 in)
Height:4.24 m (13 ft 11 in)
Weight:Empty, 1941 kg (4,279 lb)
Gross, 2,912 kg (6,420 lb)
Engine: Continental R-975, 525 hp (takeoff power)
500 hp (maximum continuous power)
References and Further Reading:
Adams, Frank, ed. The Golden Years. Philadelphia: Frank M. Adams, 2000.
Labermont, Paul. Helicopters and Autogyros of the World. Cranbury, N.J.: A.S. Barnes and Company, Inc., 1970.
Prewitt Aircraft Co. History of the Helicopter vol. 3. Clifton Heights, PA.: Prewitt
Aircraft Co., 1950.
Spencer, Jay P. Whirlybirds: A History of the U.S. Helicopter Pioneers. Seattle:
University of Washington Press, 1998.
R. D. Connor