Logically, it would seem that for a helicopter to roll to the left (see flight dynamics), lift would be required on the right and a downward force would be required on the left. That is, after all, how an airplane rolls to the left: ailerons on the trailing edge of the wings simultaneously increase lift on the right and reduce it on the left. Such is not the case with the helicopter, however, but determining that took some trial and error. When Igor Sikorsky built the first helicopter, he first set up the controls much as for an airplane, as described previously: if he wanted the craft to drift left, the swashplate was tilted left so the blades would be at maximum pitch (and therefore greatest lift) on the right and minimum pitch (minimum lift) on the left. Much to his surprise, however, the helicopter moved backwards in this configuration. When applied forward input with the control stick, the craft moved left. This was because of phase lag: an input to a rotating mass (such as a gyroscope or the rotor of a helicopter, is felt 90° along the plane of rotation after the point of input. More formally, "a system in resonance receives a periodic excitation force sympathetic with the natural frequency of the system. The flapping frequency of a centrally hinged system is equal to the speed of rotation. Therefore, maximum response occurs 90 degrees after maximum periodic excitation." (Navy Helicopter Flight Training Instruction 2004)
To rig the helicopter for controllable flight, Sikorsky moved the point at which the rotors were attached to the rotating swashplate by 90°. With this new rigging, when he moved the cyclic left, the rotors were at minimum pitch off the nose, and maximum pitch off the tail, and he rolled left. Modern helicopters are rigged the same way.This 'rigging' is known as the advance angle and is the difference in the angle from the rotor hub to the blade pitch control arm and the angle from the rotor hub to the axis around which the blade pitch is changed.