In 1944, at the age of 19, Stanley Hiller, Jr. designed, built, and test flew the first helicopter with coaxial rotors to fly successfully in the United States. The XH-44 was also the first helicopter to fly successfully with all-metal blades and a rigid rotor. Hiller used the counter-rotating coaxial configuration to distinguish his designs from Sikorsky's single main rotor designs that dominated the helicopter industry in the mid-1940s.
The first tie-down tests of the XH-44 took place on his parents' driveway and the initial flight tests occurred at the University of California at Berkeley's football stadium, where Hiller was a student. He initially tested the XH-44 with amphibious floats in his family's swimming pool. Up-scaled coaxial Hiller designs failed to sell, but his company prospered with the introduction of the popular UH-12 single rotor model.
Hiller XH-44 "Hiller-Copter"
The fourth American company to begin mass production of helicopters was started by a brilliant and astute teenager, Stanley Hiller, Jr. His remarkable talent for business and inventiveness soon resulted in the XH-44. This coaxially configured twin-rotor rotorcraft was unique among American designs of the time, and although it was never mass-produced, it served its purpose by establishing Hiller Helicopters as a capable competitor in the nascent helicopter market.
Hiller's interest in helicopters began at the age of sixteen in 1941. He was already running a successful enterprise that manufactured gas-powered model racecars. When the United States entered World War Two, Hiller converted his production line to C-47 window frames. By 1942, he had progressed far enough on his helicopter designs to form a new company, Hiller Aircraft, to begin construction on what was to become the XH-44. Hiller and his small group of experienced engineers and craftsmen had completed the first components for the small steel-tube and fabric-covered helicopter by December 1942. As had happened in the construction of Frank Piasecki's PV-2 (see NASM collection), limited financial resources and wartime shortages and restrictions meant that the staff had to scrounge or manufacture almost all their own components. Hiller also selected the same Franklin 90 hp engine (de-rated to 65 hp) to power the X-44 that Piasecki used in the PV-2. Unfortunately, the engine was not commercially available, and Hiller had to plead his case before several government agencies before one was made available to him. The engine was installed in late 1943, and testing began immediately.
The XH-44 differed dramatically in appearance from other helicopters developed during this period, because it employed two rotors, one stacked above the other on a single mast, and turning in opposite directions. Hiller had selected this unusual coaxial configuration for several reasons. The design was already proven in what can be regarded as the first practical helicopter, the Bréguet-Dorand Laboratory Gyroplane, which first flew in 1935. While the mechanism that allowed the rotors to turn in opposite directions was somewhat complex, it allows other weight-consuming components to be eliminated. Direction control was accomplished through cyclic pitch control as on most other helicopters.
In a single rotor helicopter, hinges must be incorporated to allow the blades to flap cyclically to assist in maintaining the same lift on the advancing side of the rotor disc as on the retreating side. On a coaxial helicopter each rotor counteracts the differential lift created on the other, and eliminates the potential loss of control because of retreating-blade stall. Also, as torque is cancelled, out there is no need for a tail rotor that robs a considerable percentage of engine horsepower. The absence of the tail boom, as well as the tail rotor and its transmission, results in considerable weight savings and reduces drag. The greatest penalty for coaxial helicopters comes from the requirement to have very strong "rigid" rotor blades to eliminate flapping which could bring the rotors into contact with each other. As a result, Hiller produced the first practical all-metal blades. These blades held up far better than those of Hiller's competitors, which were made largely of wood and covered with fabric. This resulted in lengthy manufacturing times, difficulty in balancing, and severe quality control problems. The coaxial configuration did have some defects, including poor directional control in forward flight and yaw control reversal in autorotations.
Hiller's decision to develop coaxial helicopters over single rotor types was also grounded in sound business considerations. Hiller realized that it would be very difficult to compete with Sikorsky and his imitators who possessed far greater resources, so he used the novelty of the coaxial design to attract attention and investors. One of the XH-44's great selling points as a home-based commuter aircraft was the absence of a highly dangerous tail rotor.
The first engine run-up inside of Hiller's workshop was so successful that it sucked the skylights from the ceiling. Test flying was a slow and tedious process, because Hiller did not have any prior flight experience, and had to teach himself to fly the XH-44 while test flying it. During this process, the XH-44 was securely tethered, though on the first flight test, made in the driveway of Hiller's family residence, the tether was not adjusted properly and the helicopter tipped over resulting in minor damage. Subsequent testing was completed in the University of California at Berkeley's football stadium. On July 4 1944, Hiller flew the bright yellow craft freely for the first time in the stadium. A public demonstration followed on August 30, 1944 in San Francisco. This event and others like it successfully attracted outside investment, primarily in the form of Henry Kaiser, a wealthy Seattle ship-builder. The infusion of Kaiser's capital allowed Hiller to further refine the XH-44.
The biggest improvement on the Hiller-copter was a redesign of the rotor mast that allowed each two-rotor set to teeter, resulting in smoother handling and improved aerodynamic efficiency. Another change was the incorporation of a new, more powerful engine. By late 1945, Hiller felt the XH-44 testbed had been refined to the level that warranted construction of a new model, the X-2-235, in the hope that this design would lead to an order. No contracts were forthcoming but a refined version, the UH-4, was prepared for production as a commuter aircraft. Unfortunately, the personal aircraft boom that had been forecast for the postwar period did not materialize, and Hiller began to work on a more conventional utilitarian design.
While Hiller was clearly attached to the coaxial configuration, the development of a highly effective stabilization device known as the rotormatic system led to his abandonment of the type. The rotormatic components revolutionized helicopter stability, but the mechanism was adaptable to a coaxial rotor system. Hiller made the decision to adopt the single main rotor with anti-torque tail rotor configuration for his next design the UH-12. This highly successful helicopter was to have the longest production run in history. However, the Hiller Aircraft Company continued to pursue advanced helicopter technology, focusing largely on small lightweight designs such as the foldable YROE rotorcycle (see NASM collection), and the X-18 tilt-wing. The coaxial design has since been abandoned by American manufacturers, but the Russian Kamov design group manufactured a number of the type, beginning at the height of the cold war and continuing through to the present day. These designs proved invaluable as their lack of a tail boom and rotor allowed the relatively large helicopters, used largely for anti-submarine warfare, to be easily stowed on a wide variety of naval vessels.
Rotor Diameter:7.62 m (25 ft)
Fuselage Length:4.06 m (13 ft 4 in)
Height:2.74 m (9 ft)
Weight:Empty, 564 kg (1,244 lb)
Gross, 658 kg (1,450 lb)
Engine:Lycoming, 125 hp
References and Further Reading:
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. Vertical Challenge: The Hiller Aircraft Story. Seattle:
University of Washington Press, 1992.
Spencer, Jay P. Whirlybirds: A History of the U.S. Helicopter Pioneers. Seattle:
University of Washington Press, 1998.
R. D. Connor