In 1959, the United States Army’s Transportation Research Command (TRECOM) initiated a program to research methods for reducing helicopter drag and increasing lift. As part of this ambitious program, the Bell company upgraded the fourth pre-series P-40 helicopter, an aircraft later redesignated UH-1B, into the experimental Bell Model 533. This initial modification laid the groundwork for groundbreaking high-speed flight research.
Early in 1960, the Model 533 underwent its first significant modernization, receiving two additional Continental J69 turbojet engines. During this phase, various types of main rotors, including both three- and four-bladed designs, were extensively tested on the helicopter to optimize its performance characteristics. These trials were crucial for understanding rotor dynamics at higher speeds.
Evolution to Compound Helicopter
By 1964, the Model 533 was further refined into a true compound helicopter through the installation of fixed wings. Concurrently, the original J69-T-9 engines were replaced with more powerful J69-T-29 turbojets, providing enhanced thrust. On October 15, 1964, the Model 533 achieved a historic milestone, becoming the first helicopter ever to exceed 370 km/h (230 mph).
The experimental aircraft continued its impressive speed trials. On April 6, 1965, it reached a remarkable speed of 402 km/h (250 mph) and even accelerated to 409 km/h (254 mph) in a shallow dive. These achievements demonstrated the immense potential for combining the vertical takeoff and landing capabilities of a helicopter with the speed of a fixed-wing aircraft.
Pioneering High-Speed Flight
In 1968, the Model 533 received its final major upgrade, incorporating two powerful P&W JT12 turbojet engines, each generating 32.21 kN of thrust. This latest configuration pushed its performance even further. By May 1968, the Bell company proudly announced that the Model 533 had surpassed the 500 km/h (310 mph) barrier, reaching an astonishing top speed of 509 km/h (316 mph).
Technical Specifications
| Modification | Bell 533 |
| Main rotor diameter, m | 13.40 |
| Length, m | 12.98 |
| Maximum takeoff weight | 4173 |
| Engine type | 1 Gas Turbine Engine Lycoming T53-L-9A + 2 Turbojet engines Pratt & Whitney JT12A-3 |
| Power, hp + Thrust, kN | 1 x 1100 + 2 x 32.21 |
| Maximum speed, km/h | 509 |
| Service ceiling, m | 3200 |
| Crew, crew | 1 |
Image and diagram gallery of the Bell 533
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