An integrated auxiliary power unit generator constant speed drive system comprising: an auxiliary power unit comprising an engine having a variable frequency rotational output comprising a shaft turning at a non-constant angular velocity; and a gearbox connected to the variable frequency rotational output and having a constant speed gearbox output comprising a shaft configured to turn at a constant angular velocity; wherein the gearbox transfers energy from the variable frequency rotational output to the constant speed gearbox output, and wherein the gearbox comprises: a differential connected to the variable frequency rotational output of the auxiliary power unit; and the constant speed gearbox output is connected to the differential, wherein the differential is configured to rotate the constant speed gearbox output at the constant angular velocity, and characterised in that the gearbox further comprises a hydraulic output speed controller in mechanical communication with the differential whereby a speed of the constant speed gearbox output may be controlled, and wherein the differential further comprises a hydraulic output speed controller control shaft whereby the differential mechanically is coupled to the hydraulic output speed controller, and wherein the hydraulic output speed controller comprises: a speed sensor proximate to the constant speed gearbox output, and configured to monitor the angular velocity of the constant speed gearbox output, and provide speed control instructions; a servomotor responsive to the speed control instructions; and a swash plate connected to the servomotor and proximate to the hydraulic output controller control shaft, and in that the swash plate engages the hydraulic output controller control shaft and controls the angular velocity of the hydraulic output controller control shaft. an auxiliary power unit comprising an engine having a variable frequency rotational output comprising a shaft turning at a non-constant angular velocity; and a gearbox connected to the variable frequency rotational output and having a constant speed gearbox output comprising a shaft configured to turn at a constant angular velocity; wherein the gearbox transfers energy from the variable frequency rotational output to the constant speed gearbox output, and wherein the gearbox comprises: a differential connected to the variable frequency rotational output of the auxiliary power unit; and the constant speed gearbox output is connected to the differential, a differential connected to the variable frequency rotational output of the auxiliary power unit; and the constant speed gearbox output is connected to the differential, wherein the differential is configured to rotate the constant speed gearbox output at the constant angular velocity, and characterised in that the gearbox further comprises a hydraulic output speed controller in mechanical communication with the differential whereby a speed of the constant speed gearbox output may be controlled, and wherein the differential further comprises a hydraulic output speed controller control shaft whereby the differential mechanically is coupled to the hydraulic output speed controller, and wherein the hydraulic output speed controller comprises: a speed sensor proximate to the constant speed gearbox output, and configured to monitor the angular velocity of the constant speed gearbox output, and provide speed control instructions; a servomotor responsive to the speed control instructions; and a swash plate connected to the servomotor and proximate to the hydraulic output controller control shaft, and in that the swash plate engages the hydraulic output controller control shaft and controls the angular velocity of the hydraulic output controller control shaft. a speed sensor proximate to the constant speed gearbox output, and configured to monitor the angular velocity of the constant speed gearbox output, and provide speed control instructions; a servomotor responsive to the speed control instructions; and a swash plate connected to the servomotor and proximate to the hydraulic output controller control shaft, and in that the swash plate engages the hydraulic output controller control shaft and controls the angular velocity of the hydraulic output controller control shaft.