Modern aircraft are aerodynamically designed at the edge of flight stability and therefore require high-response-rate flight control surfaces to maintain flight safety. In addition, to minimize weight and eliminate aircraft thermal cooling requirements, the actuator systems have increased power-density and utilize high-temperature components. This coupled with the wide operating temperature regimes experienced over a mission profile may result in detrimental performance of the actuator systems. Understanding the performance capabilities and power draw requirements as a function of temperature is essential in properly sizing and optimizing an aircraft platform.
Under the Air Force Research Laboratory's (AFRL's) Integrated Vehicle and Energy Technology (INVENT) Program, detailed models of high performance electromechanical actuators (HPEAS) were developed and include temperature dependent effects in the electrical and mechanical actuator components. These models couple directly to dynamic models of the aircraft bays that enclose the actuators and the electrical power system (EPS). Heat transfers into/out of the bays are calculated based on the aircraft's operating condition (altitude, Mach number), environmental conditions (day type, solar, ground), and the actuator losses. Traditional analysis for modeling actuators in a bay focused on two primary modes of heat transfer, convective and conductive, with the primary path being that of convection. Due to the high temperature nature of some operating conditions, radiative transfer may become a significant mode, thereby decreasing the accuracy of a convection/conduction only solution.
Although radiative flux is captured using computational fluid dynamics (CFD) solvers, the solutions typically require detailed CAD drawings of the actuators and bays, and significant computational time for short time-duration simulations. Since INVENT is focusing on mission-level capabilities with the objective of achieving full and continuous dynamic predictions, a reduced-order enclosure radiation model was developed, wherein a Monte Carlo analysis tool accepts simple geometric representations of the actuators and bays, and determines the appropriate view factors via isotropic emission of discrete energy packets. High run-time speeds exceeding hundreds to thousands of times faster than real-time are achieved through the reduced-order approach. To further the computational efficiency without loss of fidelity, the bay thermal model is implemented in a discrete time domain while the actuator response is calculated using variable time step methods. This decoupling yielded a five-fold increase in integrated system simulation speed.
In this paper, details are provided on the mathematical and numerical approaches taken for this integrated modeling effort. In addition, results illustrating the associated thermal effects on the actuators along with impacts of enclosure radiation are described. Comparisons between the decoupled and coupled reduced-order internal radiation and CFD analysis are presented.