| Abstract | Analytical simulation software tools are used to predict the behavior of an aircraft in various situations that may be difficult or undesirable to achieve in flight. This can provide important insight into aircraft behaviors, which can improve pilot proficiency and safety. This is especially true for unconventional or highly modified aircraft, which do not yet have proven performance. This report presents the research and development of a simulation capability at the National Research Council Canada for an unconventional aircraft, namely the twin-engine aircraft procured and modified to support the Hybrid Electric Aircraft Testbed (HEAT) project.
Historically, the NRC has used imperative programming languages to perform aircraft simulations (MATLAB/Simulink, C++, and Fortran 77). Unlike past simulators developed at the NRC, the simulation capability described in this report was developed using Modelica-Dymola; Modelica is an open-source, declarative programming language in which the objective of the program is defined, but not the control flow. This allows the program to be acausal, which allows for multi-domain interactions between the aircraft dynamics, the electrical components of the powertrain, the thermodynamics of the batteries, and the fluid dynamics of the cooling system.
Towards this modelling objective, discharge curves were obtained experimentally for the 120 Ah battery modules used in the HEAT aircraft. The discharge curves informed the development of an open-circuit voltage exponential-polynomial model, which simulated battery voltage as a function of instantaneous current draw and battery state-of-charge when used in an equivalent circuit model. The geometric properties of the module were used to develop a physical model of the battery, allowing for simulations of the battery module (and cell) surface temperatures. These geometric measurements were combined with publically available measurements of certain thermodynamic parameters to further refine the model. The thermodynamics model was validated against preliminary experimental data collected during discharge. The model of the battery cooling arrangement to be used on the HEAT aircraft was developed in Modelica, and the complete battery model was integrated with the existing simulation capabilities. Simulations were performed for the expected flight profile, and the cell core temperatures were within the operational range of the cells used. |
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