Pentecost HX-1 (Model 100) Hoppi-Copter

Pentecost HX-1 Hoppi-Copter

In the 1940s and early 1950s, several novel designs for strap-on helicopters appeared. If soldiers could take to the air with only the equipment on their back, then they could operate independently of the cumbersome infrastructure necessary to support conventional aircraft and would have unprecedented freedom of movement over short distances. Several engineers attempted to apply newly developed helicopter technology to the idea, with the hope that such products would also have commercial or sport applications. Horace Pentecost was one of the more successful pioneers in this field, but ultimately these fantastic designs did not have enough utility to warrant the safety risks involved in flying such rudimentary aircraft.

The first attempts to develop practical ultra-light helicopters occurred in Austria during World War Two. The Baumgartl Heliofly III and the Nagler-Rolz NR 54 V2 (see NASM collection) both failed as practical models, but the concept did not end there. While working as a Boeing rotary-wing engineer in 1944, Horace Pentecost began to envision one-man helicopters as an alternative to the costly airborne assaults undertaken during World War Two. In his spare time, he began to design a backpack-mounted engine that powered two counter-rotating coaxial rotors. Pentecost felt the design provided an infantryman with the same mobility as a paratrooper but with the ability to select a precise landing zone. He designated this design as the HX-1 Hoppi-Copter because the pilot would simply hop into the air during takeoff. By late 1945, Pentecost completed construction of the design and left Boeing to form Hoppi-Copters, Inc. to work fulltime on testing the HX-1.

The 41 kg (90 lb) "aircraft" consisted of a 20-hp motor that turned the two 3.66 m (12 ft) diameter wooden rotors at 440 rpm. The entire apparatus attached to the pilot with two metal arches that rested on the shoulders. Canvas straps held the assembly in place. The pilot used an over-hanging cyclic stick for directional control and maintained yaw control through a twist grip on the stick, which differentially varied collective pitch between the two rotors. The stick moved vertically to control collective pitch for both rotors. The left shoulder strap held the throttle control.

During initial testing, Pentecost and other "test-pilots" discovered that the backpack arrangement was simply too awkward to handle with the gyroscopic forces of the spinning rotor blades. Combined with the heavy weight of the contraption, the pilot could easily stumble, causing the blades to strike the ground and send potentially lethal splinters in all directions. Pentecost abandoned testing of the HX-1 before it could successfully leave the ground and immediately began redesigning the aircraft with a larger engine and a tricycle landing gear to support the entire weight of the aircraft. On this new model, designated as the 101, the pilot adopted a conventional seated position. By 1949, this Hoppi-Copter design evolved through the 102, 103, and 104 variations. This last design flew well, but like the earlier models, pilots found the controls extremely sensitive. Also, like other ultra-light helicopter projects, disorientation was a significant problem at substantial heights above the ground. Hoppi-Copter pilots did not have any aircraft structure to align with the horizon to provide an adequate visual reference for controlling the aircraft, and the vibration and constant motion of the aircraft rendered their kinesthetic senses useless.

At the time of the Hoppi-Copter tests, the armed services did not share Pentecost's vision of light helicopters as a replacement for traditional paratroops and did not issue any production or research contracts. Hoppi-Copters them began to market their design as a sport aircraft. Just as the company was ready to place the Hoppi-Copter into production, Pentecost's ex-wife forced him out of the company when she managed to become a majority shareholder. The company lay dormant until 1956 when a group of investors took over, with the intention of producing the final evolution of Pentecost's earlier designs, but they could not raise enough money, and the attempt failed. Ironically, by this time the military had finally become interested in ultra-light helicopter projects, such as the Hiller YROE-1 Rotorcycle (see NASM collection). Pentecost started another helicopter company to market a pulsejet-powered design, but it also failed. Hoppi-Copters, Inc. donated the HX-1 to the Smithsonian Institution in 1952.

Rotor Diameter: 3.66 m (12 ft)

Height: 1.52 m (5 ft)

Weight: Empty, 41 kg (90 lb)

Engine: 20 hp

References and Further Reading

Hoppi-Copter curatorial file, Aeronautics Division, National Air and Space Museum

Revised 10/30/01: R.D. Connor