A Thermodynamic analytical program has been developed to help investigate the impact of power system requirements on Tactical Aircraft (TA) and Unmanned Aerial Vehicle (UAV) performance. The Visual Basic for Applications (VBA) program has a user interface that operates in MS-Excel, linking several subsystem analysis programs for execution and data transfer in the power systems analysis. The analytical model is configured to include a UAV encoded propulsion engine cycle code, which allows the inspection of power extraction effects on engine performance. The model was benchmarked by validating the results of the encoded engine program for a TA with an investigation of the separate effects of shaft power extraction and pneumatic bleed air power extraction of power. The selected engine cycle for this purpose was that of the standard TA fighter. The altitude was varied from sea level (SL) to 40,000 ft with a constant Mach number of 0.9. This analysis was then repeated with an encoded UAV engine cycle code to investigate UAV Secondary Power System (SPS) performance in the range of SL to 67000 ft. As in the case of the TA, it was found that the change in SFC increases with altitude and shaft horsepower extraction but the interfacing of the UAV engine code with the main shell requires additional refinements.