First flight of the Bölkow Bö 46 V1
Thursday, January 30, 1964
The BO 46 V1 was a built to test a full-scale version of the Derschmitt lead-lag rotor concept (the advancing blade lags to reduce tip Mach no. in forward flight and the retreating blade leads to reduce the effect of blade stall by increasing tip speed). The aircraft, piloted by test pilot Wilfried von Englehardt, managed to achieve hovering flight but was not able to establish stable forward flight.
The project was eventually abandoned due to lack of sales prospects and improvements in conventional rotor design and analysis.
Source: "MBB's Helicopter History - Based on a Solid Heritage", Christina Gotzhein, VERTIFLITE May/June 1984, Page 96