Bell 533 HPH compound/hybrid helicopter achieves 316 mph (275 kts) in level flight
Tuesday, April 15, 1969
The Bell 533 HPH (High Performance Helicopter) was a compound research helicopter. The speed was achieved in level flight with the 2-blade main rotor configuration. This is an unofficial speed record that remains unbeaten.
The sole example of the Model 533 is displayed outside the main building of the U.S. Army’s Aviation Applied Technology Directorate (AATD) at Ft. Eustis, Virginia
Source: "Hybrid Helicopters: Compounding the Quest for Speed", Vertiflite, Summer 2006, page 44