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.

Figure 1. (a) The conventional general systems architecture with three types of energy, and (b) the More Electric Aircraft electrically driven general systems architecture in a single-engine configuration.
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.

Especially with the use of modern, high-efficiency electrical drives, a less complex system design with declined power conversions is achievable. One benefit of higher efficiency will pay off in reduced infrared signatures of the overall aircraft.

As high direct currents cannot be switched with standard contactors and relays, the use of solid-state power contactors (SSPC) is essential. Those SSPCs can be mounted into a dedicated cabinet with a digital interface to the utility control computer to serve as a power distribution center (PDC). This PDC will also replace conventional circuit breakers and their over-current protection functionality. De pend ing on the phase of flight or on failure detection, a prioritization of the power distribution is possible to offer a complex load management system. From the operational point of view, less ground equipment, fewer operational supply items, and less maintenance personnel are some of the benefits of the introduction of electrically driven general systems, which leads to an improved relocation capability. In addition, maintenance efforts, detect ability of failures, and long-term storage will be improved.

Research and Technology Projects

Two research and technology initiatives related to electrically driven general systems were conducted: the Barracuda M-05 UAV Demonstrator and the 270 VDC More Electric Aircraft project launched by the German Federal Office of Defense Technology and Procurement (BWB).

The Barracuda M-05 is an unmanned demonstrator vehicle, which serves as an integration platform for several UAVrelated technologies at EADS Military Air Systems. With a wing span of 7 meters, a takeoff weight of 3 tons, and the capability to carry approximately 300 kg of different payloads, the vehicle is suitable to demonstrate a variety of different UAV missions.

With respect to electrically driven general systems, the aircraft includes the main aspects of the More Electric Aircraft architecture. A redundant electrical power generation and distribution system serves as a backbone for the flight control, avionics, and general systems. The control of the power distribution system is ensured by a digital utility control system with two remote interface units. Due to the lack of com - mercial/military off-the-shelf components, 28 VDC was chosen for the main supply. The flight control actuation system consists of electromechanical actuators, a separate power control unit, and a signal control unit. All primary actuators operate in an active-active mode with both motors adding forces. To ensure a reliable fail-operate-mode switch, electrical motors were selected that can be switched off completely in failure cases.

Beside the primary flight control system, the fuel, environmental control, and landing gear system were driven electrically. The landing gear system contains a hydraulic pump package that transfers electrical energy into hydraulic energy for extension and retraction of the landing gear. After retraction of the gear, the hydraulic system is switched off; therefore, in flight, the aircraft uses just electric power to run all systems. For deceleration after landing, an electrical brake with carbon heat packs is installed on aircraft. Design, development, and integration testing for the Barracuda M- 05 UAV showed that increased attention to power supply characteristics has to be paid to guarantee a stable electrical generation and distribution.

The 270 VDC More Electric Aircraft project was launched in 2003 by the German Federal Office of Defense Technology and Procurement (BWB) to boost high-power, direct-current technology on the equipment and system integration level. Three major system aspects are covered: electrical 270 VDC onboard power generation and distribution; electromechanical actuation system; and environmental and fuel system. Only high-power application in UAVs will be operated with 270 VDC; standard avionic equipment will still require a 28 VDC power supply.

To supply the engine with fuel flow, a tank-mounted boost pump for 270 VDC supply was developed. The maximum power consumption of the single-stage pump does not exceed 3 kW, and it features built-in test software with failure detection.

For the next generation of UAVs, it is assumed that off-the-shelf engines will be used without an integrated starter generator. Therefore, a 270 VDC starter generator was developed that can be integrated into a standard gearbox flange. The starter generator operates as a motor during engine start and switches automatically to generator mode when the engine is running stably. A generator control unit includes the control algorithms, continuous built-in test, and a digital interface to the utility control computer. The power rating of the generator under continuous full load is at 12 kW.

One of the new key elements of the More Electric Aircraft configuration is the introduction of electromechanical actuation systems for the primary flight control. High dynamic requirements under extreme aerodynamic loads are the design driver for the electric motors utilized in this application. A duplex actuator working in an active-active mode capable to provide operational forces up to 18 kN was developed. The system contains an electromechanical actuator, and a power and control electronics box. For the 270 VDC More Electric Aircraft project, a 6kW generator was developed. Again, this generator includes a control unit with a digital interface to the utility control computer. The control unit covers all built-in tests, failure detection, and automatic online/ offline switching.

This article was written by Fokke Mentjes of EADS Deutschland GmbH Defence & Security Military Air Systems in Munich, Germany. For more information, Click Here 

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This article first appeared in the October, 2009 issue of Defense Tech Briefs Magazine.

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