Greg,
The X-axis represents the Azimuths about a rotorcraft; 0 being aft.
The Y-axis represents Force.
Yes, the input is a sinusoid. It represents a pulsing force at a given azimuth, at the top of the rotorcraft's mast.
However, the sinewave can slowly vary either way on the X-axis. In addition, the amplitude of the sinewave will slowly vary between 0 and some maximum value.
The speed of the rotor's rotation may vary +/-10%, but it is essentially fixed.
The objective is to convert this 1/rev positive and negative force into a 2/rev positive force. Then output the 2/rev positive force 90-degrees of rotor rotation later.
Hope this makes sense.
Dave