Bell 533

The Bell 533 was a research helicopter that Bell Helicopter built under contract with the United States Army in the 1960s to explore the limits and conditions that helicopter rotors meet at high airspeeds. It was a YH-40, a preproduction version of the UH-1 Iroquois, modified and tested in several helicopter and compound helicopter configurations, and the Army called it the High Performance Helicopter. Bell built it from one of six preproduction UH-1s after an Army contract awarded on 7 August 1961, and it made its maiden flight on 10 August 1962 at Fort Worth, Texas. The second configuration added a pair of Continental turbojet engines for extra thrust, tested from 21 October 1963. It reached a top speed of 274.6 knots in 1969 before being retired, and it is on display at Fort Eustis, Virginia.