Phase lag (rotorcraft)

For the discussion of the shift between phasors or sine waves, see Phase (waves).

In order for a helicopter to roll left (see flight dynamics) intuition would tell us that we require maximum lift on the right and minimum lift on the left. That is, after all how an airplane rolls left: ailerons on the trailing edge of the wings simultaneously increase lift on the right and reduce it on the left. Not so with the helicopter but figuring it out took some trial and error. When Igor Sikorsky built the first helicopter he first built it the same way an airplane works. As the blades in his helicopter rotated his control stick tilted a swashplate. The rotors were attached to a rotating swashplate that sat on this control swashplate. Therefore, as the rotors turned the pitch was changed cyclically. Which is why a helicopter control stick is called a cyclic.

Sikorsky set it up so that if he desired to drift left, then swashplates were tilted left 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 he started moving backwards. He tried a forward input and he went left. That phenomenon is because "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) Simply put, an input something that is rotating like a gyroscope or the main rotor of a helicopter will be felt 90° along the plane of rotation from the point of input. This is known as phase lag.

How then to rig a helicopter for controllable flight? Sikorsky simply moved the point where the rotors were attached to the rotating swash plate 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.

The Phase lag is Delay between cause and effect i.e. after applying maximum pitch to blades the blade inertia will cause it to rise about 90° of rotation.

References

I believe the description here for "phase lag" is incorrect. What it actually describes is a phenomenon known as "gyroscopic precession". Phase Lag is actually the movement of a rotor blade that is allowed to move forward or rearward through the use of a shock absorber to ensure that all blades are "in phase" with each other in flight. This is called the Lead and Lag of a rotor blade. Reference: • Pilots Handbook of Aeronautical Knowledge, FAA-H-8083-25

          •	Rotary Wing Flight TM 1-260 
          •	History of Helicopter Flight, J. Gordan Leishman