An integrated APU-generator constants speed drive system having various features is disclosed. The system may have a gearbox having a differential with a constant speed gearbox output and a hydraulic output controller control shaft. The system may have a hydraulic output speed controller that adjusts the angular velocity of the hydraulic output controller control shaft of the differential. In response, the differential may adjust the angular velocity of the constant speed gearbox output. In this manner, the angular velocity of the constant speed gearbox output may be maintained at a substantially constant angular velocity. A generator may be driven by the constant speed gearbox output. By maintaining the constant speed gearbox output at a substantially constant angular velocity, an alternating current generated by the generator may be maintained at a substantially constant frequency.