Soft Aerial Robots: The Development of Adaptive, Multi-Functional and Multi-Terrain Aerial Robots

Abstract

Classical aerial robots have demonstrated precise control and high speeds through the use of highly optimized rigid body structures. However, when compared to their biological flying counterparts, they still struggle to match performance parameters in energy efficiency, multi-functionality, multi-terrain accessibility, scalability (to micro-scale), agility, close contact interaction, robustness, and maneuverability in cluttered environments.

The recent emergence of soft aerial robotics has seen an increased interest in the utilization of smart and functional materials to develop aerial robots with innovative and morphologically adaptive structures. Through the interpretation and study of biological flyers, soft aerial robots leverage their morphological features, to exploit dynamic and biomechanical effects to achieve better aerodynamic performance, interact safely with unknown and cluttered environments, sensing itself and its surroundings better, and expand their range of capabilities. Taking another bold step towards autonomous operation in real-world environments.

We organized this special issue to highlight top-tier work of leading researchers in the field of bio-inspired, reconfigurable, adaptive, and soft aerial robotics, in order continue the ongoing conversation about the current state of the art, as well as technical and conceptual obstacles, and to examine the difficulties and prospects for the future of soft aerial robotics.

In this issue, we observe several research thrusts currently in focus. For example, De Petris et al. demonstrate how morphological compliance, embedded sensing, and collision resilience converge in Morphy. A quadcopter developed with sensorized elastic joints that withstand high-speed impacts, provide real-time angle deflection feedback, and compress to squeeze through narrow openings.^[1] Abazari et al. investigated optimal compliance in MAVs for collision resilience. Their tests showed that softer propeller guards led to more elastic collisions without improving energy dissipation, while softening the inner frame enhanced energy damping and extended collision time, reducing impact accelerations.^[2]

Kubota et al. explored a learning-based wind classification method using multiple strain sensors that detect wing deformation in flexible wings on a hummingbird-mimetic mechanism. The setup highlighted high accuracies in distinguishing wind direction under varying flow conditions. This highlights the potential of wing strain sensing for enabling quick responses to flow disturbances in aerial robots.^[3]

Shape morphing to extend the flight envelope of aerial robots was another pivotal focus. Tang et al. developed PairTilt, a quadcopter with a two pair of tiltable, coupled rotors that minimizes gyroscopic torque and reduces overall mechanical complexity compared to standard omnidirectional aerial robots. It essentially decouples forward translation from pitch rotation for enabling enhanced agility for compact maneuvering and improved energy efficiency.^[4]

Xu et al. present a novel passive way to achieve bimodal flight (between quadcopter and revolving flight) through two propeller and motor pairs vertically mounted on revolute joints that lock in place as the yaw rate threshold of the system is reached. Thus, enabling tilted hovering capabilities during revolving flight by controlling the position and roll-pitch angles of the robot.^[5] Wilcox et al. highlight a terrestrial-aerial robot capable of reversible morphing through their Leveraged Actuation and Self-Assembly (LaMSA) system, driven by shape memory alloy muscles and linear springs. The setup enabled the transformation of the SMA's small stroke into larger displacements with lower power consumption.^[6] Finally, Girardi et al. developed their shape morphing drone by utilizing multistable composite laminate airframes made of carbon-fiber grids and pre-stretched elastic membranes. Actuated by soft pneumatic systems, the design achieves a 48% reduction in width-span (to 122 mm) for navigating narrow spaces without compromising flight stability.^[7]

Saikot et al. highlighted a SMA-based tunable perching mechanism that could adapt to various landing impacts and orientation misalignments at contact speeds up to 2.16 m s⁻¹ and misalignments up to 45°.^[8] Li et al. featured a new approach to aerial-aquatic transitions and perching with bio-inspired mechanoreceptors to detect how well the remora-inspired suction cups are attached.^[9]

Finally, the issue concludes with two exciting forward-looking roadmap review papers. Lau et al. analyze the drive, power, and the role of elastic components for energy and thrust enhancements. They analyze the challenges of scaling up ornithopters and the crucial role of wing flexibility and elastic energy storage in reducing power consumption and boosting performance for hovering flight. It concludes that optimizing wing design, incorporating elastic mechanisms, and potentially exploring wing morphing are vital for improving the hover efficiency and agility of future ornithopters.^[10] To conclude, Ramirez et al. studied the development of aerial-terrestrial robots. They highlight that combining the advantages of aerial and terrestrial locomotion can lead to efficient energy consumption and robust environmental interaction. Future work should focus on understanding dynamic coupling, improving system robustness, and integrating manipulation capabilities.^[11]