Electrically Driven General Systems for UAVs

Thursday, October 01 2009

General systems are defined as all of the flight safety critical systems of an aircraft, excluding the engine and flight control computing. Usually, the main subsystems in a general system include the electrical power generation and distribution system, the primary and secondary flight control actuation system, the hydraulic system, the environmental control system, the fuel system, and the landing gear and brake system.

General systems of conventional aircraft are operated with a variety of different sources of energy that include mechanical, thermal, hydraulic, pneumatic, and electric energy. All of these types of energy are generated, transferred, distributed, and consumed by several subsystems in different parts of the flight envelope. Due to high power conversion rates in only a small bandwidth of the overall flight envelope, low efficiency factors apply to most of the systems. In addition, the range of use of dissimilar types of energy results in a complex system design with an extensive need for maintenance.

General Systems Architecture

The design of conventional general systems architecture includes the use of a variety of energies. Three main types of energy can be identified: electric, hydraulic, and pneumatic energy, all converted from mechanical energy provided by the engine via gearboxes. Figure 1(a) shows a simplified conventional general systems architecture with three main types of energy. Basically, a redundant electric power supply is used to support the avionic system including flight control computing and mission management. In addition, control and monitoring functions of general systems including different types of sensors are powered electrically as well. The power supply for the primary and secondary flight control actuation, landing gear, and brake is accomplished with a redundant hydraulic system. The environmental control system used for cooling and pressurization of the avionic compartments runs with pneumatic power extracted from the engine. Fur ther more, some system designs include an air turbine starter (ATS) that uses pneumatic power. For ground operation at an airbase, ground power equipment providing all three types of energy is necessary.

More Electric Aircraft

With the More Electric Aircraft concept, hydraulic and pneumatic systems are replaced by electric systems. Similar to the conventional general systems as described above, a simplified architecture is used to explain the changes on the system. Figure 1(b) shows an example of a system architecture with electrically driven general systems in a singleengine configuration. The power generation and distribution system delivers a redundant high electrical direct current of 270 VDC from an integrated starter generator. The starter generator is incorporated on the main shaft of the engine without additional gearboxes. No other energy source except electric power is extracted from the engine.

All subsystems such as primary and secondary flight control actuation, avionics, and environmental control systems are supplied with 270 VDC for high-power applications; in parallel, a conversion to 28 VDC is provided to run the control electronics. A change from pneumatic to electrical energy for the environmental control system implies the utilization of closed vapor cycles. The landing gear system might still include an electrically driven hydraulic power package to operate the gear during takeoff and landing, but will be switched off during flight. Nevertheless the brake system will be electrically driven.