A greater need to address energy optimization during the aircraft design process has recently developed due to the transition to more electric systems, a continued drive toward lower fuel consumption, and more stringent thermal management requirements. To facilitate this growing need, it is necessary to model the energy exchanges between the various subsystems and to predict the detailed distribution of power within the vehicle. As a result, the utilization of multidisciplinary modeling and simulation is a crucial element that needs to be addressed to enable a system-level energy optimization.
The modeling and simulation environment must include all relevant subsystems that comprise the aircraft power architecture. Each of the models should be capable of capturing the complex, transient subsystem interactions and relating them to the overall performance of the vehicle. It is of upmost importance that the integrated simulation is capable of effectively linking the vehicle performance to quantifiable changes in the individual subsystem design parameters. In particular, when tackling the challenge of energy optimization, there must be a major focus on the propulsion and thermal management subsystems. The efficient use of engine energy extraction and the efficient management of wasted energy are key players in the system-level optimization problem.
The MathWorks’ Simulink platform was used to create a system-level simulation of the tactical fighter aircraft platform. Simulink was ideally suited for this task due to the object oriented nature of the software and its numerical integration and optimization capabilities. Several important subsystem models were created within this system-level model. Currently, propulsion, power, and thermal management subsystem models are included and integrated together with an air vehicle model and mission profile.
The Simulink model contained six subsystem level blocks: Mission Profile, Air Vehicle, Propulsion Subsystem, Thermal Management Subsystem, System Heat Loads, and System Controller.
The Mission Profile block handles all of the mission level data for the simulation and passes it to the other subsystems at each time step. The Air Vehicle model keeps track of important vehicle parameters such as weight, drag, and lift. Additionally, this subsystem uses an energy balance to determine the thrust required for the vehicle to operate throughout the flight envelope.
The Propulsion Subsystem block is responsible for calculating the engine thrust and fuel burn throughout the mission. This is accomplished using information from both the mission profile and air vehicle as well as important information from the Thermal Management Subsystem (TMS) model. There are several important and intimate connections between the propulsion subsystem and TMS in the model. This is particularly important since shaft power extraction and compressor bleed requirements from the thermal management system affect the engine performance.
The final blocks in the system-level model are the System Heat Loads block and the System Controller. The Systems Heat Loads block keeps track of the heat loads and temperature requirements of the various components over the mission profile. The System Controller uses this temperature information to control the propulsion and thermal management systems throughout the mission.
The six subsystem blocks are directly linked together through various data buses in Simulink. Also, many of the important parameters of interest such as thrust, weight, thrust specific fuel consumption, and thermal management temperatures and flow rates are saved as variables in the MATLAB workspace.
The actual engine is modeled separately using Numerical Propulsion System Simulation (NPSS) software. NPSS is the industry standard gas turbine cycle analysis software and has many capabilities in the domain of engine component modeling in addition to a very robust solver. This NPSS engine model is then directly linked to the Simulink model to enable its seamless functionality in the system-level simulation.
The NPSS engine model is directly linked to the system-level Simulink simulation through the use of an S-function in Simulink. The appropriate variables can easily be passed between the two programs at each time step of the simulation.
The system-level tactical fighter Simulink model was combined with the NPSS engine model and exercised throughout a 2-h mission profile. At the beginning of the simulation, the engine model was run in on-design mode to size the engine. After the engine was sized at the design point, the engine model was switched to off-design mode, and the system-level simulation was started. At each time step of the simulation, the Simulink and NPSS models were executed simultaneously.
The focus of the current research has been on establishing a baseline modeling and simulation environment from which to support future studies into energy optimization. Future tasks will focus on the transition to higher fidelity modules in the integrated model when necessary. Then, the research will emphasize the system-level energy optimization and the capability of trading off various TMS architectures and concepts.
This article is based on SAE Technical Paper 2010-01-1787 by Adam C. Maser, Elena Garcia, and Dimitri N. Mavris of Georgia Institute of Technology.