Deep Reinforcement Learning Achieves Multifunctional Morphing Airfoil Control

Smooth camber morphing aircraft offer increased control authority and improved aerodynamic efficiency. Smart material actuators have become a popular driving force for shape changes, capable of adhering to weight and size constraints and allowing for simplicity in mechanical design.

University of Michigan, Ann Arbor, MI

Uncrewed aerial vehicles (UAVs) are growing in popularity for both civilian and military applications, which makes improving their efficiency and adaptability for various aerial environments an attractive objective. Many studies pursue this goal using morphing techniques that incorporate shape changes not typically seen in traditional aircraft. Due to weight and volume constraints consistent with smaller flight vehicles, smart materials, such as macro fiber composites (MFCs), have been used to achieve the desired shape changes. Macro fiber composites are low-profile piezoelectric actuators which have gained substantial attention within the morphing aircraft community. Piezoelectric actuators operate by generating strain when voltage, and hence an electric field, is applied to the electrodes. Piezoelectric actuators are also well known for their capabilities to produce high force-output and a high-speed actuation response. Unlike traditional piezoelectric actuators, which are composed of solid piezoelectric material, MFCs are manufactured using a series of thin piezoceramic rods in a composite laminate layup allowing them to exhibit excellent flexibility while still maintaining the performance benefits attributed to traditional piezoelectric actuators. Furthermore, MFCs exhibit large out-of-plane curvatures when bonded to a thin inextensible substrate, like steel shim, which shifts the structure's neutral axis. This behavior is attractive for camber morphing airfoil applications and has spurred a large subset of research in the field of morphing aircraft.

Though the field of morphing aircraft is brimming with novel morphing mechanisms, camber morphing wings and airfoils have proven to be especially beneficial due to their ability to increase control authority and improve efficiency. MFC actuators have been widely used in camber morphing wings, in part because they are capable of seamlessly generating cambered actuation allowing them to serve both as the airfoil skin and actuator. Furthermore, the lightweight nature of MFCs and their rapid actuation response are advantageous in UAV applications. Reductions in aircraft weight lead to greater fuel efficiency and rapid actuation allows for greater maneuverability. MFC-driven camber morphing has been applied to several UAV control problems including localized optimization for adverse aerodynamic disturbance and stall recovery, as well as improved efficiency and control effectiveness in roll and pitch for a rudderless aircraft. Finally, pitch and yaw control effectiveness and yaw stability were also demonstrated in an avian-inspired rudderless UAV with a camber morphing MFC tail actuator.