Bell 533

The Bell 533 was a research helicopter built by Bell Helicopter under contract with the United States Army during the 1960s, to explore the limits and conditions experienced by helicopter rotors at high airspeeds. The helicopter was a YH-40—a preproduction version of the UH-1 Iroquois—modified and tested in several helicopter and compound helicopter configurations. The Bell 533 was referred to as the High Performance Helicopter (HPH) by the Army, and reached a top speed of 274.6 knots in 1969, before being retired.