Mud motor or progressive cavity pump with varying pitch and taper

A mud motor comprising:

• a rotor comprising one or more rotor lobes extending helically and defining a rotor pitch;
• a stator comprising two or more stator lobes extending helically and defining a stator pitch;
• wherein the rotor and the stator together define a tapered profile of the mud motor that varies proceeding from a top end of the mud motor to a bottom end of the mud motor;
• wherein the rotor pitch and the stator pitch vary as proceeding from the top end of the mud motor to the bottom end of the mud motor;
• and
• wherein the mud motor has a pressure differential between the top end and the bottom end that is consistent between the top end and the bottom end;
• and
• wherein the rotor and the stator define a plurality of cavities, wherein rotation of the rotor within the stator is configured to progress each cavity of the plurality of cavities from the top end of the mud motor to the bottom end of the mud motor, and a volume within each cavity of the plurality of cavities varies less than 1% between the top end of the mud motor and the bottom end of the mud motor.