Modern aircraft designs require thermal management using fuel flow to cool increasingly demanding on-board systems. The topology and control of such thermal management systems has been shown to substantially impact the potential flight endurance. Excess fuel flow is employed for temperature regulation. The flow is recirculated from the engine through a cooler. If the flow is insufficiently cooled and directed to a common reservoir, the reservoir temperature may rise excessively. The excessive rise results in the fuel pump saturating at its maximum output and onboard systems being insufficiently cooled. The result is a reduction in flight endurance. Leveraging previous works, a dual tank topology with a reservoir tank, containing the majority of the fuel reserves, and a smaller recirculation tank is used in conjunction with an online fuel flow control scheme to manage the on-board thermal loads. Previous work focused on studying operation with flight mission parameters that allowed for thermal constraints not to limit the flight endurance using proper temperature and flow control. Implicitly, there was no prediction of the flight endurance when the thermal constraints were limiting. The current design extends the control scheme to explicitly address operation when flight endurance is limited by thermal constraints. During overly taxing mission phases, thermal management is achieved by saturating the fuel flow rate and controlling the devision of the recirculation fuel flow between the two tanks. The complementary online control design is provided in a companion paper. Herein, the modeling, design, and analysis yield the ability to characterize and predict the flight endurance subject to thermal limitations using closed form expressions. The closed form expressions allow for efficient online computation of the flight endurance, or offline evaluation of a broad range of vehicle design and mission planning parameters. The main feature is the efficient computation of the Thermal Endurance Gauge ratio, namely the ratio between the flight endurance with and without thermal constraints. The capabilities of these expressions are showcased in a suite of four prototype software tools, simulating online flight endurance evaluation, and offline mission and vehicle evaluation. The utility of these tools is demonstrated in connection with the case study in the companion paper.
Modern aircraft designs require thermal management using fuel flow to cool increasingly demanding on-board systems. The topology and control of such thermal management systems has been shown to substantially impact the potential flight endurance. Excess fuel flow is employed for temperature regulation. The flow is recirculated from the engine through a cooler. If the flow is insufficiently cooled and directed to a common reservoir, the reservoir temperature may rise excessively. The excessive rise results in the fuel pump saturating at its maximum output and onboard systems being insufficiently cooled. The result is a reduction in flight endurance. Leveraging previous works, a dual tank topology with a reservoir tank, containing the majority of the fuel reserves, and a smaller recirculation tank is used in conjunction with an online fuel flow control scheme to manage the on-board thermal loads. Previous work focused on studying operation with flight mission parameters that allowed for thermal constraints not to limit the flight endurance using proper temperature and flow control. The current design both simplifies and extends the control scheme to explicitly address operation when flight endurance is limited by thermal constraints. The control design is divided using time-scale separation into the operation mode controller, tank level controller, and temperature controller. The operation mode controller is simplified to only command two different operation modes, without loss of proficiency compared to previous designs. The tank level controller regulates the recirculation tank level and is unchanged from previous designs. Finally, the temperature controller is redesigned with two modes, saturated and unsaturated. The saturated mode explicitly addresses operation under conditions where flight endurance is limited by thermal constraints. The steady-state implications of employing the control design are investigated in a companion paper, producing novel results in analyzing thermal endurance and mission profile evaluation. The new control scheme is implemented in simulation with a dynamic model of the thermal management system and is demonstrated to support the claimed capabilities of the online control design and the associated analysis.
The laser powder bed fusion AM process has been used to manufacture beams with unique internal geometries that are capable of increasing inherent damping in a part. The concept of the internal design is to have densely packed, unfused powder pockets that dissipate energy via particle interaction. Four Inconel (IN) 718 beams have been tested and all demonstrated the capability to suppress vibration 10X more effectively than a fully fused beam. The mechanism presumed to dissipate energy and thus suppress vibration is the sliding of unfused particles. This mechanism has been associated with a crack opening under Mode II fracture. Based on this assumption, a proportional expression has been developed as a criterion for optimizing unfused powder locations for vibration suppression effectiveness and was validated with 3.175 mm thick beams. This study investigates five uniquely designed IN-718 beams created via the optimizing criterion to assess accuracy of the expression. The intent of this study is to investigate the predictability of the unfused pocket optimization criterion. The results of this study will lead to a more robust design criterion for more complex 3D structures with improved damping capability.
A modular vibration-based fatigue test capability that significantly minimizes the effects of boundary conditions has been developed. The system utilizes a specimen with boundary conditions on its node lines, which isolates the gage section for deflection as a free-free beam in its first bending mode. The thin specimen is suspended inside an electromagnet by 6 lbs. monofilament fishing line, and permanent magnets are bolted to the bottom of the specimen. The alternating current inside of an electromagnet attracts and repels the permanent magnets rapidly (~55 Hz for a 0.016? thick specimen), causing the beam to cycle in first bend. This testing capability is ideal for generating and assessing fatigue life of thin specimens requiring large deflections for failure. Understanding and characterizing fatigue behavior of thin components is important, especially since the emergence of additive manufacturing (AM) for small, fatigue susceptible components. An alternate solution is to suppress the vibratory susceptibility of the component. Therefore, the importance of thin coatings capable of providing damping to components is on par with fatigue characterization. In this study, 0.4 mm cold-rolled Titanium (Ti) 6Al-4 V specimen were fatigued and compared to published data. Also, a thin damping coating (titanium nitride, or TiN) was applied to a few Ti 6Al-4 V specimens to assess the performance. Both the fatigue and damping assessments are necessary to validate the free-free test method.
