Jig shape optimization for desired shape of a high-altitudelong-endurance class unmanned aerial vehicle underaeroelastic effects

Abstract

The field of aviation is continuously developing, and with advancements in technology, it has established a strong foundation. The desire to integrate air travel into everyday life has accelerated the design and production processes of aircraft. In civil aviation, aircraft are designed with the aim of reducing fuel costs, flying over long distances, and carrying the maximum number of passengers. The military sector, on the other hand, emphasizes features such as flying faster, reaching higher altitudes, and carrying weapons and missiles. Meeting these diverse requirements is possible through the most efficient design of power systems, structures, and aerodynamics in the aircraft. Since engine design falls outside the scope of this thesis, this thesis focuses on aerodynamic and structural design. The desire to make aircraft lighter to achieve better performance has increased. With the advancement in material technology, stronger and lighter aerospace materials have been developed. These lightweight materials are more flexible compared to traditional materials, making them ideal for modern day designs. While aircraft produced with these materials are 30% lighter than those produced with conventional materials, they are also more susceptible to elastic effects. Therefore, to fully realize the benefits of weight reduction, it is crucial to examine the aeroelastic effects on the aircraft in more detail. The field of aeroelasticity focuses on understanding and addressing the combined effects of inertial, elastic, and aerodynamic forces. Its popularity, within the aircraft design community, is constantly increasing as aircraft are becoming more and more flexible. Aeroelasticity is generally divided into two main categories: static aeroelasticity and dynamic aeroelasticity. Some of the common phenomena associated with static aeroelasticity include control reversal, effectiveness, and divergence, while the phenomena associated with dynamic aeroelasticity include buzz, buffet, gust, and flutter. In the aircraft design process, once the conceptual design is finalized during the preliminary design stage, the aerodynamicists start to work on the external geometry to achieve an optimal design. This aerodynamically optimized wing is then handed over to structural engineers, who manufacture it within specified production tolerances. These manufacturing constraints cause the aircraft's external surface to deviate slightly from its optimized design. In summary, there are geometrical differences between the optimal aerodynamic design and the manufactured geometry. These differences result in discrepancies between the calculated performance values of the optimized design and the actual performance values of the manufactured geometry. When the the loss due to geometrical difference is added the aerodynamic performance values will significantly decrease. The external geometry of an aircraft is a dynamic, living cycle. It undergoes many changes from design to production. These external shapes are generally divided into two groups: theoretical shapes and practical shapes. Examples of theoretical shapes include the 1G flight shape, jig shape, and engineering shape. Examples of practical shapes include the manufacturing shape, parking shape, actual flight shape, and operation shape. The aim of this study is to arrive at a more effective design during the preliminary design stage of the aircraft design process by incorporating a multidisciplinary approach. Due to time constraints in aircraft design processes, designers often avoid complex and expensive analyses. This study proposes a method to mitigate these challenges by providing a quick solution for integrating multidisciplinary analysis into the preliminary design stage, thereby enabling a more effective design process. In this study, the RQ-4 Global Hawk, a HALE class unmanned aerial vehicle, is selected. The reason for choosing the RQ-4 Global Hawk is that it has very large aspect ratio making the elastic effects more apparent. Initially, the point cloud data available in the literature for the RQ-4 Global Hawk is acquired. A structured mesh capable of creating this point cloud is generated using a Python coded. This mesh is employed to establish a ZONAIR aerodynamic model, a 3D panel method that uses high-order panels. The results obtained using the ZONAIR aerodynamic model was validated against the available flight data in literature. The same grid are used for the structural analysis of the RQ-4 Global Hawk. The material density is chosen based on the real-life weight of the RQ-4 Global Hawk. The weight distribution is made proportional to the volumes of the RQ-4 Global Hawk's components. The FEM analysis of the RQ-4 Global Hawk is performed with composite materials, using stiffness values found in the literature. A modal analysis is then conducted to determine the natural modes and frequencies related to the wing. After preparing the ZONAIR aerodynamic model and FEM model of the RQ-4 Global Hawk aircraft, the ZONAIR model is made ready for aero-structural coupling. For specific Mach numbers and angles of attack, both the rigid (desired) and elastic (flight shape) results are obtained. Since the RQ-4 Global Hawk is a subsonic aircraft, the chosen angles of attack and Mach numbers show linear values in the graphs. This is advantageous for UAV because improving one design point aeroelastically will automatically improve other design points as well. In light of this information, the differences in lift coefficients between the flight shape and the rigid shape are measured. The results indicate a 5.5% aerodynamic loss between the flight shape and the rigid shape. This difference is a significant loss for aircraft. This difference arises due to the elastic structure of the RQ-4 Global Hawk. To minimize the loss caused by elastic effects, a solution method is developed in this thesis. Defining and applying the jig shape in the design process is crucial to prevent the loss caısed by elastic effects. There are two ways to address the difference caused by elastic effects: the first is structural reinforcement, which is generally undesirable. As increasing stiffness automatically increases weight. In this study, the other method, managing deflections and twists, is exemined. The importance of jig shape design has increased in the aircraft design process. In this study, the main goal is to incorporate jig shape design into the preliminary design process and develop a methodology. A methodology for rigid, elastic, and jig shape design is developed and used iteratively for design optimization. Generally speaking, the procedure involves taking an aerodynamically optimized wing as the target shape. Next, rigid and elastic solutions are obtained at specific design points for this target shape. In the elastic solution, the loads are extracted. These extracted, inverted and applied to the aircraft to obtain an aero-structural solution and identify the initial jig shape. The initial jig shape of the aircraft is found. This initial jig shape is then subjected to the same conditions in an aero-structural solution, and the new flight shape is measured. In the next step, the new flight shape is compared to the target shape. If the difference between the new flight shape and the target shape is below a certain limit, the iteration ends. If it is exceeds the limit, the process starts over. The loads from the new flight shape are inverted, and the jig shape for the second iteration is found. This is then subjected to the ZONAIR aero-structural solution to obtain the flight shape, which is compared again to the target shape. The methodology developed in this study is both fast and practical, requiring many iterations are needed to find the most optimal jig shape. Optimization methods have been used to make this process more intelligent. Especially, a stable and widely used optimization method has been selected. The aim of this study is to simplify complex models to achieve faster solutions, so a fast-working gradient-based optimization method, SQP, has been chosen as the optimization method. A effective optimization model has also been established for the RQ-4 Global Hawk aircraft, automating the jig shape optimization procedure. This procedure enhances the aircraft design process by enabling raid jig shape optimization during the preliminary design stage. This jig shape optimization increases the efficiency of the aircraft, making it more effective. Jig shape optimization is a process that contributing to reaching the targeted range and achieving more successful observations and weapon firings in the field.