Advancements in Hydraulic Systems for the More Electric Aircraft

03/02/2015
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Advancements in Hydraulic Systems for the More Electric Aircraft

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Advancements in Hydraulic Systems for the More Electric Aircraft

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Advancements in Hydraulic Systems for the More Electric Aircraft Jeff Skinner, PE (1), Aaron Smith, PhD (2), Stefan Frischemeier (3), Martin Holland (4) 1 : Eaton, 5353 Highland Drive, Jackson MS 39206, JeffreyDSkinner@eaton.com 2 : Eaton, AaronSmith@eaton.com, 3: Eaton, StefanFrischemeier@eaton.com 4 : Eaton, MartinHolland@eaton.com Abstract This paper summarizes a study of two different aircraft architectures for a hypothetical single aisle aircraft including a comparison of the baseline centralized hydraulic system approach found on most commercial aircraft today: a hybrid electro-hydraulic approach with distributed hydraulics in multiple zones of the aircraft, and a fully distributed system including hydraulics or electro-mechanical actuation at each point of use. Advantages and disadvantages of each of the architectures for the hypothetical aircraft size as well as considerations of the impact of other aircraft sizes on the study conclusions are discussed with a brief overview of some of the enabling technologies for future hydraulics and electro-mechanical actuation. Introduction With the advance of More Electric Aircraft (MEA) or Energy Optimized Aircraft (EOA) architectures, the use of traditional hydraulic systems for powered flight controls and utility actuation systems has been threatened by electric actuation systems including Electro-Mechanical Actuation (EMAs) and Electro- Hydrostatic Actuation (EHAs). Despite the historically high power density of hydraulics for conversion, storage, and distribution of energy from the engines to the flight control and utility consumers, hydraulic systems are generally considered unfavourable on aircraft due to: potential for leakage, need for maintenance and repair, and installation complexity and cost. The alternatives described within this paper reduce or eliminate these points of contention; however, as with any system installed on board an aircraft complete trade studies must be conducted with consideration of the key performance characteristics including safety and availability, power consumption and installed power, and system weight. For the selected architectures on a hypothetical single aisle aircraft, the following performance characteristics will be estimated using an existing single aisle aircraft hydraulic system as a baseline for comparison: number of potential leak points (LRU attachment points, bulkhead fittings, firewall fittings), overall reliability, installed power, energy consumption, and system weight. In addition, the following will be described for consideration in future trade studies: 1. Ability to use smart features such as health monitoring and prognostics algorithms 2. Power-on-demand capability 3. Ability for adaptive control and re-configuration in flight 4. Overall life cycle cost impacts Note, with conventional and localized zonal hydraulic systems, hydraulic power is provided and generally maintained at a constant pressure allowing use of existing flight control and utility actuators with the control laws generally remaining independent of the hydraulic power generation system. However, in the case of EHAs and EMAs actuation control is generally integrated within the device with servo-controlled EM machines. For this reason, the trade-studies include devices typically outside of the traditional hydraulic power generation system definition. In addition, for the more electric architectures, the impact to the electrical generation and distribution systems must be included. Baseline Hydraulic System The single aisle aircraft hydraulic system is chosen as the baseline for this study based on its wide use in industry and information available in the public domain. SAE AIR5005[1] provides a summary description of the hydraulic system including the size of the hydraulic power sources and the allocation of the three independent systems to the flight control and utility consumers. Table 1 summarizes the hydraulic power sources installed on this aircraft. Use Pump Qty Power Primary Engine Driven 2 127.34 kW Primary AC Motor Driven 1 10.78 kW Back-up AC Motor Driven 1 10.78 kW Back-up Ram Air Turbine Driven 1 26.14 kW Back-up Power Transfer Unit 1 29.31 kW Total Installed Hydraulic Power: 204.35 kW Table 1: Baseline hydraulic power sources Alternative Architecture Evaluation A top-down systems approach is utilized to design and evaluate the alternative systems as described in [2]. This method focuses on development of an aircraft system which first meets the safety and availability requirements. For civil transport aircraft, probability for a failure leading to a catastrophic event must be less than 10 -9 per flight hour. Other system failure scenarios are also assigned maximum probabilities per flight hour [3]. Next, the system weight is evaluated including the piping, electrical wiring, and impact to the electrical generation system. The final criterion for evaluation is the power consumption and the installed power. Note, it is typical that the installed power is generally greater than the maximum power consumption due to redundancy and commonality requirements. Description of Alternate System 1: Zonal Hydraulics The first architecture to be evaluated is a fully zonal system in which the hydraulic pumps are removed from the engine and replaced with hydraulic power- packs distributed throughout the aircraft. In this architecture, multiple hydraulic power sources may be utilized in each zone in order to achieve the system safety requirements described above. Further, in this architecture each hydraulic power pack will provide power to the flight control and other consumers in a manner similar to the baseline where the hydraulic pressure is maintained and traditional servo-valve controlled actuators are utilized. This architecture has the advantage of reduced tubing, elimination of hydraulic components from the rotor burst zone, and simplified aircraft assembly and test by replacing the hydraulic tubing with wiring. Disadvantages of this approach include reduction of heat transfer area for the hydraulic fluid, and an increased number of higher weight electric machines and power conversion elements. Description of Alternate System 2: Fully Distributed Hydraulics An alternate architecture evaluated is a fully distributed system utilizing EHA’s for the primary flight controls and EMA’s for the secondary and utility actuation systems. This approach requires an electric motor and power conversion at each actuator set, but eliminates the hydraulic servo-actuators by building this functionality and the associated control laws within the EHA/EMA motor controller. This architecture shares the advantages and disadvantages described for the Zonal Hydraulic Systems. A key difference is that EHAs and EMAs due to being associated with single consumers operate as power-on-demand devices which minimize the energy consumption; however, this comes at the price of requiring high weight electric machines and power conversions elements as well as thermal management solutions at each actuator. The complexity of EHAs and EMAs can also increase direct maintenance costs and dispatch interruption rates due to the highly integrated nature of the assemblies in which the entire EHA or EMA has to be replaced if or when a sub-component fails. Technology Areas for More Electric Hydraulic Systems Several areas of research and technology development will further advance the advantages of the more electric hydraulic systems including: 1. High speed motor pumps – motor size can be reduced by operating the pumps at increasing speeds 2. Robust Control Methodologies – energy consumption will be reduced by operating with variable speed / variable pressure devices, but the electronic control must be robust and fail safe to meet the system safety requirements 3. Health Monitoring and Prognostics – electronic control enables monitoring of motor input and sensor signals for inference of system health and future prognostics 4. Improved Power Density Electric Conversion / Alternate Motor Drive Topologies – BLDC motor design allows intelligent control and health monitoring; however, it is desirable to reduce the weight of the drive electronics 5. Thermal Management – efficient removal of heat from the hydraulic fluid, electric motor, and electronics 6. Fault Tolerant Motor and Drive Design – allows safe operation of electric motor driven devices even when faults are present without requiring full redundancy 7. Advanced design for assembly and maintainability with a focus given to highly integrated assemblies which can still be maintained in a cost efficient manner. 8. Enhanced integrated control strategies – the integration of aircraft flight control, power management, and actuation control methods has the potential to further mitigate some of today’s disadvantages of zonal hydraulics and distributed actuation systems Conclusions A comparison of two potential more electric hydraulic systems with a single aisle aircraft hydraulic system has been completed. The advantages of zonal and fully distributed hydraulic systems, primarily relating to reduction of tubing, hydraulic connection points, and reduced assembly time are significant. Further activity is required to reduce the weight incurred with more electric hydraulic systems, particularly for future aircraft with wild frequency electrical power distribution. A summary of technology areas to improve the performance of the more electric architectures is also provided. References 1 SAE AIR5005, Aerospace – Commercial Aircraft Hydraulic Systems, Revised 2010 2 Frischemeier, Stefan, Aircraft Hydraulic System Design and Evaluation – A Top-Down Approach, The Ninth Scandinavian International Conference on Fluid Power, SICFP’05, 2005 3 14CFR25 Code of Federal Regulations, Part 25 “Airworthiness Standards: Transport Category Airplanes”