CitationDownload as .RIS
Emerald Group Publishing Limited
Copyright © 2001, MCB UP Limited
Advances in more-electric aircraft technologies
Advances in more-electric aircraft technologies
Keywords: TRW Aeronautical Systems, Airbus, Flight controls, Electrical, Design
Advances in aviation technologies are frequently driven by factors like weight reduction, increased reliability, enhanced performance or life cycle cost savings. Put all four together in one concept and you have, says TRW Aeronautical Systems, a powerful argument for change.
The ongoing evolution of aircraft electrical design is generating a new technology road map in which major benefits in the areas of weight, reliability, performance and cost are being combined with increased onboard power demands to create today's "more-electric aircraft" systems. In addition to their myriad operational benefits both today and in the near future, these evolutionary programs are also believed to be serving as the critical foundation for a revolutionary, longer term concept known as the "all electric aircraft".
"The goal of the visionary 'all-electric' aircraft concept of the future is to eliminate as many hydraulic power sources and hydraulic lines as possible", explains Mike Yates, chief engineer for Systems Integration at TRW Aeronautical Systems (Lucas Aerospace). "And the engine, which is currently required to produce thrust, pneumatic power, hydraulic power and electrical power, can now be redesigned and optimized to produce thrust and predominantly electrical power."
Noting that it will take another 10 to 15 years to develop the technologies behind the all-electric aircraft vision, Mike emphasizes that there are intermediate steps that can come before that time and those next steps are taking place right now. Termed more-electric aircraft systems, the intermediate steps involve the evolutionary application of electrical power systems, electronics and distributed architectures to simplify much of the current bulk and complexity inherent in hydraulic and pneumatic aircraft systems. Immediate benefits derived from the wider application of electrical power and electronics include performance and reliability improvements as well as savings in weight, space and overall life cycle costs.
Taking the more-electric aircraft from paper to platform, Airbus Industrie has base-lined both power-by-wire flight controls and variable frequency power generation for its new A380 super jumbo. The A380 will be the next clear platform in which a major technological change toward more-electric systems is implemented.
Current advancements surrounding aircraft systems can be grouped into the areas of flight controls, power generation and management, and engine controls. In each of these fields, TRW Aeronautical Systems considers that it is unique in its breadth of experience and its depth of knowledge for current and future applications. Moreover, because of the interrelated nature of these critical aviation subsystems, TRW also believes that its wide-ranging capabilities provide rare synergistic benefits for present and future systems integration efforts.
Flight control systems
Flight control systems provide several examples of the technologies supporting the evolution from traditional hydraulic actuation controls to fly-by-wire, and then eventually to power-by-wire, to achieve benefits such as enhanced availability and aircraft maintainability, and reduced weight and aircraft ground service time.
"We've been involved with flight control systems since the early days", says Phil Hudson, TRWs chief engineer for flight controls. "The control systems were originally hydraulic/mechanical and then evolved into fly-by-wire designs through the application of digital computer technology. We were involved in the introduction of fly-by-wire on the Airbus A320, the first civil application of a complete fly-by-wire system, and we've also been pursuing electric actuation over the last ten years so that we can move even further into power-by-wire technologies such as electro-hydrostatic actuation (EHA) including our integrated actuation package (IAPTM), and electro-mechanical actuation (EMA)."
Conventional flight control actuation, known as fly-by-wire, is controlled electrically but powered hydraulically. These systems rely on a complicated circuit of high-pressure hydraulics running throughout the aircraft to supply the hydraulic pressure needed to move each control surface. The goal of power-by-wire is to significantly reduce or eliminate altogether the hydraulic connection, and its associated risks, by providing electrical power straight to the actuators. These electrically powered flight control actuation technologies involve a motor and an electronic unit that serves as the motor controller. However, the electronic unit can also be designed to serve more functions than simply motor control. It can serve as a smart actuator controller in its own right and be part of a distributed control system.
Distributed control is a main TRW technology theme and has potential for application in flight controls and engine control systems. According to TRW this technology puts intelligence local to the actuation elements in a control system and can substantially reduce harness weight and improve fault detection and isolation when applied to today's aircraft and engine systems. TRW's main technology themes of "more-electric aircraft systems" and "distributed controls" can be combined to achieve the optimum system design.
"You get the benefits of distributed control along with the benefits of electric actuation and you get those benefits without adding any major cost because you have to have the electronics there anyway for local control", says Yates.
The benefits of moving to more-electric flight controls are believed to be substantial. To put it into context, a large civil aircraft, such as the A380, would have three channels of hydraulic circuits to ensure adequate system redundancy. The likely first step is to replace one hydraulic circuit with a series of electrohydrostatic actuation, realizing more than an estimated 1,000 pounds of weight savings, and increasing the pressure of the two remaining hydraulic channels to 5,000 psi, realizing an additional estimated 1,000 pounds of weight savings.
Maintenance benefits are also thought to be substantial. Power-by-wire actuation units are line-removable with only mechanical and electrical connections to the aircraft, which eliminates the need to refill or bleed systems of hydraulic fluids as is required with central hydraulics. Since power-by-wire actuators are self-contained and remotely located at the surfaces, the area exposed to damage is greatly reduced. Additionally, power-by-wire actuators can be designed as position sensitive, which means that the actuators provide only the flow and pressure necessary to move and hold the actuator in a desired position. Conventional central hydraulic systems are configured to produce continuous pressure. Flow is metered at each actuator, which can lead to a large consumption of power and generate unwanted heat.
TRW has been developing these flight control technologies for several years and has substantial experience in proving these technologies as they move through the development process. The company's IAP™achieved 1,000 hours of in-service flight experience over two years as part of the US Air Force's Electric Starlifter program. The program reportedly has been successful in demonstrating the effectiveness and benefits of the IAP™with systems running on both wings and both flight control channels providing full roll control authority. In TRW's opinion, no other company has gained that amount of proven flight experience on power-by-wire technology.
The company has also been developing EHAs for large aircraft as part of the COVAN (Commandes de Vol Avions Nouveaux or Flight Controls for Future Aircraft) program. The Company's EHAs have begun initial flight testing on an A340 aileron and A320 aileron after having completed 1,000 hours of pre-flight testing for flight qualification.
The increase in usage of electrical power for primary and secondary actuation and ultimately replacing pneumatic services on the aircraft will place demands on the electrical system, both in terms of total power draw and power quality.
Noting that the next generation A380 large surfaces will require multiple electrical actuators, Yates acknowledges that, "The electric power demand will be increased substantially. The generating system has to be designed to cope with all power demands including multiple surface transients. Additionally, the actuator motor control is by short, high-current pulses. So power quality is obviously an issue".
Clearly, any change from hydraulic to electric flight control actuators places increased demands on the aircraft electrical power generation system. One of the parallel "more-electric" technology themes where TRW Aeronautical Systems has addressed these requirements for power generation and power quality involves variable frequency generators. Variable frequency generation is said to provide the key to a higher output power management system, which increases power without significantly increasing weight.
Traditional aircraft designs utilize a constant frequency generator system in which the speed variations of the engine are cancelled out within the generator itself through a complex integrated drive generator subsystem. The variable frequency design eliminates this complex subsystem and allows generator output to be variable over the engine speed range, resulting in significant improvements in both reliability and maintainability. The first civil jet-engine all-variable frequency system is now flying on Bombardier's Global Express® business jet.
"We have clearly demonstrated that this system concept enables successful operations of all the aircraft's systems", says Peter Crouchley, TRW Aeronautical Systems' chief engineer for electrical systems. "In addition to successful operation on Global Express®, Airbus has evaluated this technology and has base lined the variable frequency system for its next-generation A380 aircraft."
The Global Express® utilizes TRWs 40 kVA variable frequency generators. To offer the same benefits to large aircraft, TRW Aeronautical systems engineers have targeted higher power capabilities for variable frequency and are moving swiftly to prove the technology. The company's 90 and 120 kVA variable frequency generators have undergone rigorous testing over the past two years, reportedly producing excellent power quality throughout. The 120 kVA unit is said to have already successfully achieved more than 1,000 hours of accelerated life testing, and a 150 kVA generator is currently in development.
Another important issue surrounding power generation involves the ability to generate aircraft power during emergency situations. TRW is developing fan-driven generators that could be used to replace conventional ram air turbine (RAT) designs. In addition to providing adequate emergency power under windmill engine conditions, the new design gives the generator the capability to supplement the main generating system by operating as a main generator over the whole engine speed range. This capability creates the opportunity to reduce the size and weight of the other main generators.
"The all-electric engine of the future will require electrical power to be able to operate", explains Yates. "It's vitally important that we have an element of power which is independent from the rest of the aircraft and that is available when you need to restart the engines. This design not only provides emergency aircraft power under windmill conditions, it also provides the electrical power required to power the pump that pumps the fuel that gets the engine started. Otherwise you would not have that engine restart capability, so it's quite crucial in that respect."
In addition to their activities surrounding flight control systems and power generation and management, a significant portion of TRW Aeronautical Systems' more-electric aircraft activities has involved the evolutionary application of engine control systems.
Jet engine control systems raise the pressure of the fuel to enable combustion and deliver the required fuel quantity for starting, accelerating and steady state running the engine at all engine operating conditions. Originally the control systems were fairly simple but became more and more complex as time went on to incorporate features like altitude compensation and control limiters for pressure, temperature and speed. This complication in hydro-mechanical controls mandated the introduction of supervisory electronics and led to today's digital electronic controls, including full authority digital electronic engine control (FADEC) systems. TRW Aeronautical Systems has been in the forefront of these engine control developments since the early days of flight.
"We produced the Concorde FADEC demonstrator program back in the 1970s", says Brian Farman, chief engineer, Electronics. "We went from hydro-mechanical engine control technology to digital FADEC technology, which we've been working on for many years now. That was a major change in engine control technology. Today, FADEC technology is now the norm across the industry, and we are working to exploit the step changes in commercial technologies to reduce the size and cost of our systems, including their application to distributed engine controls."
This step in more-electric aircraft design involves incorporating some electronics in each of the engine actuators and connecting them via a digital data bus to the network of nodes located around the engine. TRW has demonstrated a three node distributed engine control system incorporating a "smart" fuel valve. Embedded in that fuel valve is a two-channel electronic controller taking fuel flow demand information from the digital data bus and controlling valve position to the fuel flow that is required. The company believes that the application of distributed control technology will have the added benefit of facilitating fault isolation with 100 percent certainty, not only improving in-service reliability but also providing an estimated 20-30 per cent savings in the cost of maintenance of that system over a typical engine system. Additionally, it is claimed that a weight saving of approximately 110 pounds can be achieved on a large civil engine with a distributed architecture through simplification of the harnesses.
Slightly further down the "more-electric" evolutionary path is the "smart" electrically driven fuel pump. Based on technology that has already been demonstrated on a 15,000lb-thrust engine by TRW Aeronautical System engineers in collaboration with the Defence Evaluation and Research Agency (DERA), the smart pump will directly modulate fuel flow to the engine. The engine control will demand a fuel flow and the smart fuel pump will reportedly adjust to the desired speed, driving the pump to deliver the required fuel flow and eliminating the valve in the fuel metering system.
The all-electric engine
The benefits provided by the smart electric fuel pump are believed to be realized when used as an enabler for the all-electric engine. By reducing the need for hydraulic, pneumatic, or mechanical power takeoff, the new engine can be redesigned and simplified to focus on producing thrust and predominantly electrical power. In addition to obvious reliability enhancements stemming from the simplified design, theoretical design projections feature a smaller overall outer shell for a given thrust range with a corresponding improved drag coefficient.
Having, according to TRW, clearly proven the utility of fly-by-wire technologies and established the technology groundwork for follow-on power-by-wire applications, as well as variable frequency power generation, its aeronautical system engineers are looking toward the next major milestone development of the systems and equipment for the all-electric engine.
Bringing it all together
But even before the "all-electric" aircraft concept takes to the skies, the numerous examples of more-electric aircraft technologies provided by TRW Aeronautical Systems point to the complex interaction between system requirements of flight controls, power management and engine controls in more-electric aircraft designs. TRWs systems expertise in all three of these areas enables the company to be fully aware of the impact that more-electric transitions in one of these areas has in the others and, more importantly, understand the system integration issues as the industry moves further down the road toward the all-electric aircraft. In the meantime, TRW informs us that it anticipates seeing many of its new technologies being proven in commercial service well before the all-electric aircraft vision becomes a reality.
Details available from: TRW Aeronautical Systems. Tel: +44 (0) 121 451 5975 (UK); Tel: +1 703 648 0831 (USA).