### DTC and ATC  
For proper flight control, it is vital to characterize the throttle behavior of a propulsion system. Oklahoma State conducted previous flights with turbine-based, direct throttle control. The pilot supplied throttle inputs directly to the turboprop engine electronic control unit (ECU), leaving the electrical load (the electric motors propelling the aircraft) to respond to the turbogenerator's power output naturally. Without classifying this performance, it is difficult to control the turbine system, and in the case of a turbogenerator, this can lead to extensive electrical system damage. This control style is not desirable because of the lag between throttle input and variation in thrust output. There are also further difficulties at lower power outputs when the pilot desires to throttle the aircraft to a power output more inferior than the idle power output of the turbine. In this idle scenario, when the pilot has the throttle reduced to the minimum, the aircraft may utilize too much power and be unable to slow down sufficiently. In dissimilar motors with differing throttle demands, DTC is incapable of supplying power individually or adequately. An example of a dissimilar motor setup aircraft would be a quadrotor, VTOL aircraft. It is, therefore, necessary to develop an active throttle control scheme, where the pilot or autopilot throttles the electric motors propelling the aircraft, and the turbine responds to any demands of the load. This active controller will significantly improve pilot control and decrease pilot workload compared to the turbine-based throttle control. The active throttle controller will also help ensure proper system voltage and power management utilization throughout the operation during nominal working conditions. One of the benefits of using electric motors as propulsors is the short response time to a given throttle input or power demand. From a pilot's perspective, an electric motor is desirable for its near-instantaneous throttle response compared to the response time of a combustion-based engine. Electric motors can be commanded to make drastic, sporadic throttle changes, and the motors can respond as quickly as the power source can supply current to the motor. A significant example of this need for rapid throttle change is in VTOL aircraft. VTOL electric aircraft use rapid changes in throttle response to balance the aircraft between its points of control, which places a time constraint on the electrical power bank that must respond quickly to a changing load. The time it takes for the turbine engine to throttle from idle to full power is measured in seconds, while the response of an electronic speed controller is measured in milliseconds. So, while the turbine is revving up, there is a need for the system to be supplying instantaneous power. For this reason, having batteries integrated into the power system is essential. Active throttle control, ATC, provides an autonomous method for the turbine to react to any given electric load supplied by the pilot or operator. From an operability standpoint, the ATC, in conjunction with battery augmentation, allows the pilot to control the aircraft as if the power source were solely battery-powered. The turbine has time to react to demands from the fluctuating electric loadings on the turbine output shaft with battery supplementation. The turbine mechanically can take up to five seconds to increase from idle to maximum throttle, while electric motors are capable of a full-throttle sweep in under a second in most conditions. The control schematic can be seen in Figure 3.  
A PID controller was used to control the throttle inputs to the turbine. In this operating mode, the main goal of the turbine is to maintain a constant supply voltage to the electrical load. The voltage supplied by the turbogenerator varies as the demanded load is changed (this can be seen later in this paper). The voltage supplied to the load (i.e., the rectifier voltage) is used as a feedback variable for the PID.