The Bell 533 was an experimental helicopter used by Bell Helicopters to test new technologies and product developments.
The Bell 533 was built in 1959 from the first YH-40 (one of six pre-production aircraft that led to the UH-1) in response to a request from the United States Army's Transportation Research Command for a vehicle to test emerging rotorcraft technologies.
During the first phase of rebuilding, a general clean-up of the airframe was performed to reduce drag. New rotorhead fairings were developed, and a cambered vertical stabilizer was developed which, in cruise flight, aerodynamically unloaded the tail rotor. A variable-tilt rotor mast replaced the rotor's stabilizer bar. During this phase, the helicopters original Lycoming T53-L-13 turboshaft engine was retained.
The reconfigured helicopter was first tested in a NASA Ames Research Center wind tunnel, which confirmed the modifications had indeed reduced the aircraft's drag. The helicopter made its maiden flight on August 10, 1962 at Bell's Fort Worth, Texas headquarters, and program flight testing began shortly thereafter. During this part of the program, a speed of 163 knots (302 km/h) was achieved in a shallow dive.
Additional modifications were then tried. A two-bladed rotor system similar to the UH-1B's was installed, followed by a three-bladed rigid rotor system, with swept blade tips. Two suplementary 1700-pound-thrust Continental J69-T-9 turbojet engines were mounted on the fuselage, and on January 17, 1964, a level flight speed of 182 knots (338 km/h) was obtained.
The next phase of modifications converted the aircraft to a compound helicopter with the addition of two wings, which produce enough lift at high speed to partially unload the main rotor. The turbojet engines were upgraded to J69-T-29 models, and the Bell 533 became the first helicopter to break the 200 knot barrier with a top speed of 205 knots (380 km/h) on October 15, 1964.
In 1968, the wings were replaced by ones that were slightly shorter, with two large Pratt & Whitney JT12A-3 engines, each producing 3,300 pounds of thrust on the ends. Ever faster flight speeds were recorded until the record 274 knots (over 300 mph) was obtained.
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