The Bölkow Bo 46 was a West German experimental helicopter built to test the Derschmidt rotor system that aimed to allow much higher speeds than traditional helicopter designs. Wind tunnel testing showed promise, but the Bo 46 demonstrated a number of problems and added complexity that led to the concept being abandoned. The Bo 46 was one of a number of new designs exploring high-speed helicopter flight that were built in the early 1960s. Background Helicopter rotors operate in a much more challenging environment than a normal aircraft propeller. To start with, helicopters normally use the main rotor both for lift and manoeuvrability, whereas fixed-wing aircraft normally use separate surfaces for these tasks. Pitch and yaw are operated by changing the lift on different sides of the rotor, using a system of bell cranks to adjust the blades to different angles of attack as they rotate. To roll to the right, the blades are adjusted so there is slightly more angle of attack on the front and slightly less on the back, resulting in reorienting the rotor thrust to roll the aircraft in the desired direction. The reason the blades are adjusted in the front and back instead of right and left is due to phase lag caused by precession. In forward flight, the rotor system is subject to various forms of differential loading. Imagine a rotor system where the tips of the blades rotate at 300 km/h relative to still air. When that helicopter is hovering in still air, the blades see the same 300 km/h relative wind throughout their rotation. However, when the helicopter starts to move forward its speed is added to the speed of the blades as they advance towards the front of the aircraft, and subtracted as they retreat. For instance, if the helicopter is flying forward at 100 km/h, the advancing blades see 300 + 100 km/h = 400 km/h, and for the retreating ones its 300 – 100 km/h = 200 km/h. In this example, the relative airspeed changes by a factor of two during every rotation. Lift is a function of the angle of the airfoil to the relative airflow combined with the speed of the air. To counteract this change in lift, which would normally pitch the aircraft, the rotor system has to dynamically adjust the angle of attack of the airfoils to ensure they generate a steady amount of lift