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 left, the blades are adjusted so there is slightly more angle of attack on the front and slightly less on the back, resulting in a net upward lift on the right side that rolls the aircraft. The reason the blades are adjusted in the front and back instead of right and left is due to phase lag caused by precession.