Bensen B-6 Gyroglider
In the early 1950s, a Russian immigrant, Igor Bensen developed a novel and surprisingly successful means of introducing aviation to people safely and cheaply. His Gyroglider and subsequent Gyrocopter designs opened a new field in sport aviation.
Igor Bensen began his association with autogyros during World War Two, when he managed General Electric's rotary wing program, which developed plastic rotor blades for the Kellett XR-3 autogyro. After the end of the war, he tested one of the more unusual rotorcraft designs to originate during the conflict - the tip-jet powered Doblehoff WNF 342 helicopter, captured by American forces in Austria. While at General Electric in 1946, Bensen designed, built and flew the B-3 Gyroglider - an un-powered gyroplane that relied on a tow from a vehicle or aircraft as its means of propulsion. It was similar in design to the Rotochute, an ultimately unsuccessful British wartime project to enhance the precision of airborne landings. The B-3 was an exciting project for him, but General Electric did not see any practical reasons to produce it in the post-war environment. He also worked on the Rotachute bomb - an alternative to the small parachutes used on anti-personnel fragmentation bombs. Bensen joined Kaman Helicopters in 1951 to develop this idea further as a means to recover spacecraft and deliver atomic weapons. In 1953, he formed his own company to produce autogyros and towed rotary-wing gliders.
Bensen Aircraft Corporation's first project to take form was actually a helicopter that that relied on blade mounted propellers to turn the rotor, like the Nagler-Rolz NR 54 V2 (see NASM collection). The Navy eventually cancelled this research project, designated B-4. Meanwhile, Bensen began construction on a towed "Gyroglider" design, similar to the Focke-Achgelis FA 330 (see NASM collection) submarine-based observation kite, but much simpler in construction.
Like a standard autogyro or even a windmill, the rotor blades on the Gyroglider provides lift as the airflow in forward flight or in vertical descent results in a center-of-pressure that acts forward of their center-of-gravity. This causes the blades to autorotate with sufficient velocity to provide lift. As in a helicopter, the blade rotation allows the aircraft to take off and land at much lower speeds than an airplane. However, without an engine to power the main rotor, autogyros of any type cannot hover, unless there is a strong headwind. A conventional propeller would typically provide the forward motion necessary for sufficient airflow to turn the autogyro rotor. However, the Gyroglider, relied on a vehicle or boat with a towrope to provide adequate forward motion for the rotor blades to achieve autorotation. If the pilot released the towrope, or the towing vehicle stopped, the reduced airflow through the rotor disk caused the blades to slow and the Gyroglider to descend. As long as the aircraft remained pitched for an appropriate forward speed, the pressure of the onrushing air provided the rotor with enough momentum to flare out into a safe landing at the end of the descent. This is the same technique used by helicopters, though their variable pitch rotor blades add an additional complication during autorotation.
The Gyroglider prototype consisted of a two-bladed rotor and single fixed vertical stabilizer mounted on a welded steel tube structure with an old ski serving as a keel. The landing gear consisted of three small wheels, including a steerable nose wheel. The control column was an overhanging yoke fastened directly the rotor head, which the pilot tilted in the desired direction. The small blades of the Gyroglider did not have gyroscopic forces strong enough to warrant cyclic control of the rotor. The rotor blades bolted directly onto the rotor head without any hinges, though a simple teetering mechanism allowed them to flap to prevent asymmetric lift on the rotor disk in forward flight.
By April 1, 1954 Bensen had completed the B-5 Gyroglider prototype and began towed flights with it. The aircraft was stable in pitch, but occasionally oscillated wildly when the pilot made lateral control inputs. Bensen simplified the construction of the prototype to prepare it for production under the B-6 designation. The improved Gyroglider utilized inexpensive galvanized water pipe to create a simpler, but larger, frame with two wooden skids. Dual vertical stabilizers mounted on the aft ends of the skids alleviated the oscillation problem caused by the B-5's small single vertical stabilizer. The B-6 had a single overhead-mounted control stick instead of the prototype's yoke.
Bensen envisioned the Gyroglider as a means for high school-aged boys to experience the thrill of aviation without significant cost. He intended to market the Gyroglider as a kit, for as little as $100. The cost increased by more than $300 dollars if the buyer wished to purchase pre-fabricated versions of the rotor blades and hub to avoid their difficult construction process. The plans offered wide latitude in the style and technique of fabrication for everything but the rotor blades and control assembly. The basic version used flat wooden skids, but the plans had an option for small rubber wheels and steerable nose gear, if the owner intended to operate the Gyroglider off pavement. The simple design of the B-6 allowed for easy disassembly during transport or storage.
Before takeoff, the pilot turned the rotor by hand to start it spinning before autorotation could begin. The Gyroglider became airborne at a relative wind speed of 31 kph (19 mph), and cruised between 40 kph (25 mph) and 97 kph (60 mph). Three hundred feet of towrope allowed the B-6 to reach its maximum altitude of 46 m (150 ft). In case of emergency, a release mechanism jettisoned the rope to initiate an autorotative glide, though the pilot could also make untethered autorotations for fun or practice. As long as the Gyroglider remained attached to the towrope, the Civil Aeronautics Authority (CAA) did not define the B-6 as a glider or any other aircraft. Thus, the agency did not require any form of license to operate it. A safe autorotative glide was possible with an airspeed of only 11 kph (7 mph). If operating the wheeled version, the operating manual advised the pilot to wear steel-soled shoes to aid in braking.
By the time Bensen had drawn up the flyers and blueprints for the B-6 kits, he had already constructed an improved model, the B-7, which then became the first Bensen model to go into production. This design became quite popular and Montgomery Ward even sold the Gyroglider through its catalogs. Amphibious versions with floats or boat hulls flew for more than 20 years in wildly popular stunt shows at the Cypress Gardens attraction in Florida. The B-7 was large enough to mount a motor and pusher propeller to transform the aircraft into a true autogyro, which Bensen designated as the B7-M. Bensen called this innovation the Gyrocopter - a name that has become associated with many modern gyroplanes. The B-7 Gyroglider was soon followed by the improved B-8 its motorized version, the B8-M (see NASM collection). Bensen's Gyrogliders and Gyrocopters quickly became the most popular line of sport aircraft up to that time, and spawned a large number of imitators. In 1966, Bensen Aircraft Corporation donated the first kit-plan B-6 to the Smithsonian Institution.
Rotor Diameter:6.10 m (20 ft)
Length:2.03 m (6 ft 8 in)
Height:1.63 m (5 ft 4 in)
Weight:Empty, 47 kg (103 lb)
Gross, 160 kg (353 lb)
References and Further Reading:
Bensen, Igor B. A Dream of Flight. Indianapolis: The Abbott Company, 1992.
B-6 curatorial file, Aeronautics Division, National Air and Space Museum
R.D. Connor
