Integrated Thermal / Power / Propulsion / Vehicle Modeling Issues Related to a More Electric Aircraft Architecture

A new subsystem-based approach to solve aerospace vehicle energy management issues is described. The goal of this approach is to create an “Energy Optimized Aircraft” that will maximize energy utilization for broad capabilities while minimizing complexity. To support this goal, an advanced modeling and simulation ICD process is established. This process addresses several of the current challenges facing modeling and simulation of large integrated system. Two initial integrated models are presented. First at the mission level, AFRL has undertaken the development, integration and demonstration of a tip-to-tail thermal model. The major components of the integrated model include the Air Vehicle System (AVS), the Fuel Thermal Management System, the engine models, and Power Thermal Management System (PTMS). Second at the segment level, a model of the electrical system including the generator, electrical accumulator unit, electrical distribution unit and electromechanical actuators has been developed. Included in the model are mission level models of an engine and aircraft to provide relevant boundary conditions. It is anticipated that the tracking of the electrical distribution through numerical integration of these various subsystems will lead to more accurate predictions of the bus power quality. This tool is used to evaluate two architectures. The first architecture makes use of an electromechanical accumulator unit to handle the regenerative energy created by the electromechanical actuators. In the second architecture power resistors in the actuator electronic units are used for dissipation of regenerative energy. Transient evaluations and energy metrics were used in evaluating capability of the two systems. 1.0 INTRODUCTION/BACKGROUND The US Air Force Research Laboratory‟s Propulsion Directorate initiated the Integrated Vehicle & Energy Technology (INVENT) Program in 2008. Leading up to the INVENT initiative there has been focused interest on the part of thermophysicists and thermal engineers to solve a “new set” of challenging aerospace vehicle thermal management problems. These problems are, for the most part, a result of inefficiencies stemming from a variety of components and systems such as electrical power systems, propulsion systems, and high-energy electronics devices. The idea that these are a “new set” of thermal management problems is a misnomer in that the relative inefficiencies have remained constant while the power and power densities that these devices are expected to consume and/or provide have continued to increase. This has ultimately resulted in the increase in total heat load combined with high-flux heat generating components. One would think this would not be a particularly challenging problem considering the evolution of a variety of thermal management concepts such as high-flux thermal management components developed over the last twenty years. But when one looks at additional constraints such as on-demand requirements driven by duty cycle, operating temperatures, isothermality, total heat load, the availability of suitable heat sinks, and poorly-defined environmental boundary conditions; these “new sets” of challenging thermal management problems can RTO-MP-AVT-178 28 1 UNCLASSIFIED/UNLIMITED UNCLASSIFIED/UNLIMITED Integrated Thermal/Power/Propulsion/Vehicle Modeling Issues Related to a More Electric Aircraft Architecture quickly become a costly thermal management nightmare. However, the implication that only better thermal management concepts and technologies need to be developed ignores the fact that, from an energy perspective, it is only a symptom resulting from the failure to properly take into account the need for improved system integration and optimization. INVENT was established to address these thermal management challenges in modern survivable military aircraft, from a vehicle energy perspective, through new system integration and optimization approaches. These new aircraft have three to five times the heat load of legacy platforms while being limited in their ability to reject heat to the environment. Rejecting heat to the engine cycle through various flow paths has become the preferred approach. The added heat load is the result of modern avionics, advanced mission systems, fueldraulic based vectored thrust control systems, increased use of composite structures, and larger more electric aircraft engine accessories such as generators, gear boxes, or environmental controls. The legacy approach to these systems has been to provide continuous infrastructure (hydraulics, fueldraulics, pneumatics, electricity, cooling, etc.) even though many of the loads are used a small percentage of the mission (low duty cycle). INVENT is addressing the potential use of on-demand, duty cycle based systems that can greatly reduce these heat loads overall by “turning-down” their infrastructure demands during idle periods. The ability to provide on-demand power and cooling may be the key to reducing infrastructure needs as well as reducing energy demand that decreases heat loads as a result. This concept is referred to as the Energy Optimized Aircraft (EOA) by the INVENT program. The main EOA infrastructure subsystems being addressed are the adaptive power and thermal management system (APTMS), robust electrical power system (REPS), and high performance electric actuation system (HPEAS), and their associated load suites as well as the engine system integration. The focus of the INVENT EOA is to make aircraft and vehicle systems more energy efficient by maximizing overall system energy efficiency in lieu of sub-optimized components and subsystems. The ability to solve the thermal challenges requires the knowledge and integration of complex systems to reduce the heat loads by addressing the “entire” vehicle energy picture. INVENT seeks to demonstrate the potential EOA technologies integration using modeling and simulation (M&S) followed by validation testing in the laboratories using systems integration facilities in conjunction with engine and vehicle test laboratories. The complexity of these highly integrated systems necessitates an effective M&S analytical approach to avoid the costs and risks associated with “cut & try” approaches to system integration. The purpose of this paper is to highlight the advancements in M&S that make a virtual approach to complex aircraft systems integration possible and then provide two examples, a thermal and electrical analysis, of integrated M&S investigate. 1.1 Thermal Analysis Traditionally, the aircraft thermal, power, propulsion, and vehicle systems have been designed and optimized at a subsystem level with little consideration toward the design of the thermal management system (TMS). Such a design philosophy was sufficient due to the low thermal resistance of the airframe skin, the addition of ram inlet heat exchangers, and the relatively small amount of power required by the electrical loads. Aircraft TMS design was conducted through analysis of the anticipated worst case steady state operating points [1, 2]. This approach has been satisfactory for traditional aircraft designs. Modern aircraft made with composite skins have a high thermal resistance thereby greatly reducing convective cooling. In Integrated Thermal/Power/Propulsion/Vehicle Modeling Issues Related to a More Electric Aircraft Architecture 28 2 RTO-MP-AVT-178 UNCLASSIFIED/UNLIMITED UNCLASSIFIED/UNLIMITED addition, the cross-sectional areas of ram inlet heat exchangers have also been reduced. At the same time, the size of the power system has increased by nearly an order of magnitude to support numerous high-power loads that increase the internal heat generated within modern/future aircraft. These factors have led to the current thermal challenges facing modern/future aircraft. The INVENT program was established, in part, to address the thermal challenges of modern, survivable military aircraft. These new aircraft have three to five times the heat load of legacy platforms while being limited in the ability to reject heat to the environment. Rejecting heat to the engine cycle through various flow paths has become the preferred approach. The added heat load is the result of modern avionics, advanced mission systems, fueldraulic based vectored thrust control systems, and larger more electric aircraft engine accessories (generators, gear boxes, environmental controls, etc.). The complexity of these highly integrated systems necessitates an efficient M&S analytical approach to avoid the costs and risks associated with hardware based approaches to system integration. The goal of this thermal effort is to develop, integrate and demonstrate a mission level tip-to-tail thermal model. The major components of the integrated model include the aircraft six degree of freedom model (6DoF) and the vehicle management system (VMS), the engine aerodynamic and engine thermal models, the vehicle thermal model (fuel tanks), the power thermal management system (PTMS), and various representative aircraft level heat loads. The integrated model is then flown over a complete aircraft flight mission from ground idle thru take-off, climb, cruise, landing and post-flight ground hold. Having established a baseline level of performance for the aircraft PTMS system over the full mission length, the PTMS model is then exercised to investigate some possible design space trades. The design trades are an effort to highlight the potential application of the integrated system model. This analysis is not constrained to actual hardware components. The components in this system are representative of modern/future aircraft. The motivation is to stimulate additional dialog and discussion as to the benefits of integrated aircraft system analysis with the long term goal of achieving a design system capable of analyzing future energy optimized aircraft. 1.2 Electrical Analysis The widespread use of more electrical equipment on an aircraft is driving the need to evaluate electrical stability of the system [3, 4]. Military Standard 704F defines requirements for the behavior of such systems [5]. However, evaluation of these systems typically has been done with hardware and this late evaluation can lead to increased costs. However, if such evaluation can take place earlier in the development process before pre-production hardware is built considerable cost savings can be achieved. A tool is being developed that can better quantify electrical stability through simulation of integrated electrical systems. This modeling tool makes use of models developed with bandwidth up to one hundred kilohertz. These models are defined in the INVENT Modeling Requirements and Implementation Plan (MRIP) as segment level models [6]. The MRIP document further defines the interfaces between the various electrical systems. This document is not limited to electrical, but includes the entire aircraft systems (thermal, mechanical, aerodynamic, etc.). In addition to the increase in power usage, there is an additional issue with regenerative power of the actuators. With the move to the more electric aircraft, the actuators have changed from conventional hydraulic actuators to electro-hydrostatic actuators (EHA) and electro-mechanical actuators (EMA). Use of these actuators forces a need to deal with large regenerative power to the bus. Most applications with Integrated Thermal/Power/Propulsion/Vehicle Modeling Issues Related to a More Electric Aircraft Architecture RTO-MP-AVT-178 28 3 UNCLASSIFIED/UNLIMITED UNCLASSIFIED/UNLIMITED actuators are using resistors to “burn off” energy regenerated by the actuator and prevent the energy from returning to the bus. However, the regenerative energy dissipated in the actuators is a concern for thermal dissipation and additional energy required to cool the actuators. Recently in the research field, electrical accumulator units have been identified as a potential technology for solving this problem [7]. An electrical accumulator unit (EAU) is a battery or capacitance technology that can absorb energy and store it for future usage. This technology can be used to store the regenerative energy, however, power stability of the bus needs to be examined and evaluated before use. Lastly, there is a desire to understand the overall system integration impact of the EAU. There has been identified the potential of not just dealing with the regenerative energy produced by the actuators but also to handle some of the peak loads of the actuators thus reducing requirements on the generator. Therefore any weight increase due to the EAU may be partially offset by reduction in generator weights. If the regenerative resistors are removed and the actuators no longer require cooling out to the actuators, further benefits could be realized. This paper will not go into the evaluation of weight savings but will look at overall energy usage of the EAU versus the use of local resistors. Further, the integration testing can begin to ensure the design points that the individual component designers are using are correct. 2.0 MODEL DEVELOPMENT The model integration effort for the thermal and electrical analysis employs the commercial Matlab/Simulink software package as a top level modeling environment. Many of the subsystem models are developed entirely within the Simulink environment. Simulink offers a wide range of numerical integration solvers well suited for transient problems. As a graphical programming environment, Simulink allows for a model development that can have the look of traditional flow schematics. This allows end-users to translate from a schematic layout to a Simulink model with relative ease. 2.1 Thermal Model The thermal analysis is accomplished by simulating the following components and sub-systems: Aircraft 6-DoF and VMS Aircraft fuel thermal management system (tanks, etc.) Engine performance (thrust, fuel burn rate, etc.) Engine fuel thermal management system (fuel pumps, etc.) Power thermal management system (PTMS – power turbine, heat exchangers, etc.) A schematic of the model interconnectivity is given in Figure 1, followed by brief descriptions of the individual component and sub-system models. Integrated Thermal/Power/Propulsion/Vehicle Modeling Issues Related to a More Electric Aircraft Architecture 28 4 RTO-MP-AVT-178 UNCLASSIFIED/UNLIMITED UNCLASSIFIED/UNLIMITED Figure 1: Top level schematic: integrated thermal model 2.1.1 Aircraft Models – Air Vehicle System (AVS) The aircraft six degree of freedom model is a variable mass, rigid body model representative of a blended wing-body long range aircraft. The aircraft 6-DoF model is intended to serve as a mission level analysis tool with sufficient fidelity to enable relevant trade studies (e.g., accounting for additional ram air drag associated with a vapor cycle PTMS as compared to an air cycle PTMS) and yet with sufficient execution speed that full length mission performance metrics can be produced rapidly. The primary modeling objective for the 6-DoF model is to dynamically update the following data as a function of the aircraft flight condition: ambient atmospheric data, required engine thrust, and a coordinated set of control surface actuator loads. The model that has been developed for the present air platform is a MATLAB/Simulink application with the following features and capabilities:  Trim and IC capability for steady level flight at any point within the flight envelope.  Easily specified mission legs in terms of altitude, Mach number, roll angle and course.  Aircraft weight, inertia tensor and cg location dynamically updated throughout the mission.  Control effectors: wing-tip “clamshells” for directional and braking control; outboard elevons for roll control; beavertail and inboard elevons for pitch control.  Symmetrical engine thrust – no differential thrust control, no thrust vectoring.  Vehicle aerodynamics based on table look-up scheme; aerodynamic database developed using a vortex-lattice method.  In standalone mode, inclusion of a very simple dynamic engine thrust model.  Ability to integrate instantaneous data over the length of the mission to arrive at performance metrics such as range, endurance, total fuel burn, etc.  Flight control loop closure providing cruise regulation and tracking of altitude, airspeed or Mach number, bank angle and heading. Integrated Thermal/Power/Propulsion/Vehicle Modeling Issues Related to a More Electric Aircraft Architecture RTO-MP-AVT-178 28 5 UNCLASSIFIED/UNLIMITED UNCLASSIFIED/UNLIMITED  Feedback gains scheduled throughout the flight envelope as a function of gross weight and freestream dynamic pressure. The top-level feedback architecture for the aircraft flight control computer is presented below in Figure 2. Figure 2: Aircraft stabilization and tracking feedback design 2.1.2 Aircraft Models – Fuel Thermal Management System (FTMS) The air vehicle fuel thermal management system (FTMS) is modeled using an AFRL developed Simulink based toolset comprised of various FTMS components. Temperature of the fuel inside the fuel tank drives constraints throughout the system and therefore particular focus is given to the heat transfer model for each tank. The toolset is comprised of material heat transfer models capable of capturing solar loading, infrared radiation, and aerodynamic convection on the aircraft surface. The conductive heat transfer to the internal surface of the fuel tanks establishes heat transfer between the fuselage/fuel and external surfaces. Finite volume methods are implemented in the wall material along with standard lump capacitance approaches for fuel volumes [8]. Numerical integration in Simulink allows for time-domain analysis of the temperature response resulting from highly variable boundary conditions applied to fast responding (fuel tank wall) and slow responding (full fuel tanks) physical components. In addition to the fuel tanks, the additional heat load attributed to flow through the fuel pumps is captured downstream of the fuel tanks, Figure 3. A variety of aircraft subsystems utilize fuel as a viable heat sink, and depending on the subsystem a significant temperature rise could be observed. Eventually the fuel temperature can rise to levels incapable of cooling temperature constrained components, and therefore increased fuel flow beyond engine demand is required to maintain component temperatures within appropriate operating ranges. The FTMS model utilizes circuit temperatures to determine if additional flow is needed, and any excess flow is returned to the tanks. The resultant „temperature runaway‟ conditions under high levels of return flow and/or low fuel tank mass can be analyzed effectively using the FTMS models. Such mission critical responses will be highly dependent on the complicated interaction between all coupled subsystems, and therefore the appropriate hooks are in place to couple other essential aircraft subsystems. The detailed analysis of the interdependent subsystems will be essential in developing an EOA. cruiseCmndBus