Hybrid aerial underwater vehicles (HAUVs) exhibit significant potential due to their ability to operate seamlessly in the air and water domains. However, balancing rapid maneuverability in both media and achieving stability during the cross-domain phase remains a significant challenge. Inspired by the retractable limbs of a tortoise, this paper presented a novel morphing HAUV, Nezha-MB. Nezha-MB utilizes linear actuators combined with a gear and rack system for arm transformation during the transition phase, replacing conventional servos. The transformation mechanism accounts for 11 % of the total weight. In aerial mode, Nezha-MB exhibits flight performance comparable to a quadrotor configuration. In underwater mode, NezhaMB retracts its quadrotor arms into a bullet-shaped shell, significantly reducing drag and energy consumption, while enabling passage through narrow gaps with diameters as small as 134 mm. Simulations and field tests conducted in both aerial and underwater domains demonstrate that NezhaMB combines the swift maneuverability of a streamlined underwater vehicle with the stability of a traditional multirotor vehicle, highlighting its robust and rapid cross-domain capabilities.

Hybrid aerial underwater vehicles (HAUVs) hold great promise but face challenges like air-water integration and high underwater resistance. This paper presents Nezha-Morphing, a bio-inspired HAUV that emulates seabirds’ adaptive wing extension and retraction mechanisms for efficient movement in both aerial and aquatic domains. It has a servo-driven foldable-arm mechanism and is made of high-strength, low-mass materials with a double-system architecture for aerial and underwater control. This paper details the mechanical and electronic system design, dynamic analysis, and experimental results of Nezha-Morphing. The experimental findings are highly impressive: underwater, the folded-arm configuration effectively reduces resistance, enabling a maximum speed of 0.620 m/s and achieving a high average acceleration, significantly enhancing motion efficiency and flexible maneuverability in confined spaces. In the air, with its arms unfolded, the vehicle exhibits exceptional stability and strong wind resistance, maintaining steady flight even under level-4 wind conditions. Moreover, it completes the water-to-air cross-domain transition in just 1.5 seconds. Nezha-Morphing successfully integrates flight stability, cross-domain adaptability, and hydrodynamic efficiency, showcasing substantial potential for diverse applications.

Hybrid aerial-underwater vehicles (HAUVs) are attracting significant interest for their unique capability to operate across both air and water. However, achieving a lightweight design coupled with efficient cross-domain performance remains a formidable challenge. This paper introduces Nezha-T, a novel, ultra-lightweight HAUV featuring a dual-stable floating state capability. This is achieved through an innovative center-of-gravity (CoG) arrangement method, which enables seamless transitions between upright and horizontal floating postures. This ability to stably floating in either orientation is crucial for stable water entry and exit, mitigating impact forces on the vehicle and its payload. Furthermore, to counteract the residual buoyancy inherent in this design, a zero-lift pitch angle is incorporated into the control system, improving depth-keeping and pitch control performance. The proposed design were rigorously validated through computational fluid dynamics (CFD) simulations, pool experiments, and open-water field tests. The results confirm the feasibility of the dual-stable floating states, demonstrate stable cross-domain traversal, verify the effectiveness of the depth-keeping control system, and validate the vehicle’s fixed-wing flight capability.

Tracking a moving unmanned ground vehicle (UGV) with an autonomous Unmanned Aerial Vehicle (UAV) is challenging, particularly in GNSS-denied indoor environments where reacquiring the UGV after losing track poses a significant obstacle. This paper presents a novel learning framework designed to address these challenges, enabling a quadrotor UAV to effectively chase a moving UGV and regain tracking in an indoor environment. The proposed framework encompasses two primary components: the Track-HCL and the Tracking Vision System (TVS). The TVS leverages a lightweight tracker to offer real-time recognition and localization of the UGV. Additionally, the Chronological Ghosting (CG) method is employed to describe the UGV’s motion trend within a single frame. The Track-HCL component introduces a hybrid curriculum strategy to guide policy learning for the Deep Reinforcement Learning (DRL) agent. The Track-HCL enables the agent to learn the tracking policy conducive to target chasing and proficient reacquisition. We demonstrate the effectiveness of the proposed method in both simulation and field experiments.

The air-sea interface is vital in studying heat and energy exchange between the sea and air. The field observation technology of the air-sea interface is an effective way to explore the nature of the air-sea interface. This paper presents an observation system called DANDELION for the air-sea interface environment. The system includes an automatic surface vehicle (ASV), a launching device using a pair of high-speed rotating friction wheels, and eight dandelion-like micro profilers. This system can implement the environmental observation of the air-sea interface in an extensive range through micro profilers' rapid and multi-point placement. The DANDELION system was characterized by establishing the friction wheel launching mechanism model and summarizing the effects of different wings on the profiler performance. A series of experiments were conducted in Qiandao Lake, China, to characterize the DANDELION system. We demonstrate the developed system with data from field experiments, which show very high flexibility and feasibility to observe the air-sea interface, implying potential applications in ocean transient phenomena observation.

The agile and safe transdomain in waves is a promising feature but the primary bottleneck of the hybrid aerial underwater vehicle (HAUV). In this article, the surfing algorithm is proposed for Nezha-mini, our predeveloped HAUV prototype, to search for the dynamic window facilitating takeoff in waves and avoiding hazardous waves. For the first time, the cross-domain window, i.e., the vehicle is at the wave crest and heading downstream, is characterized and defined through the vehicle-wave coupled dynamic model. The novel surfing algorithm consists of the gradient perceptron, time-limited momentum gradient search, heading server, and initial conditions. Nezha-mini senses, searches, and tracks the dynamic window in real-time, until the takeoff decisions are triggered. Numerical simulations and experiments in regular and irregular waves reveal the effectiveness of the algorithm. The vehicle maintains a healthy initial attitude and inaccessible wave disturbance during takeoff, thus alleviating the thrust distraction from stability recovery and uncertainty. The average transition time and energy cost are reduced by 59.2% and 26.1% compared with random takeoff cases, and the locomotion is smooth, graceful, and low-risk. The computation and cost are low as the algorithm only requires the basic flight controller and the data from the inertial measurement unit instead of the prior parameters of the HAUV and waves. In comparison with the adaptive robust controller, which resists wave disturbance directly, this article provides an enlightening strategy from the perspective of harnessing waves.

Underwater thermocline, common in the lakes and ocean, plays a vital role in meteorological forecasting in the ocean and lakes dynamics research. This letter proposes a method for rapid and multipoint observation of thermocline variations with time and space using an airdropped micro-profiler array, named the DRAGONFLY system. It comprises specially designed disposable low-cost micro-profilers, a general unmanned aerial carrier platform, and a ground control system. This system can conduct periodic profile observations at a single point or quickly survey a large area. A series of experiments to characterize the micro-profiler and the DRAGONFLY system were conducted in Qiandao Lake, China. We demonstrate the developed system with data from field experiments, which show very high flexibility, and feasibility to observe the lake thermocline, implying potential applications in ocean transient phenomena observation.

A new solution to improving the poor endurance of the existing hybrid aerial underwater vehicle (HAUV) is proposed in this paper. The proposed multimodal hybrid aerial underwater vehicle (MHAUV) merges the design concept of the fixed-wing unmanned aerial vehicle (UAV), the multirotor, and the underwater glider (UG) and has a novel lightweight pneumatic buoyancy adjustment system. MHAUV is well suited for moving in distinct medium and can achieve extended endurance for long distance travel in both air and water. The mathematical model is given based on Newton-Euler formalism. Necessary design principles of the vehicle's physical parameters are obtained through different gliding equilibrium points. Then, a control scheme composed of two separate proportional-integral-derivative (PID) is employed for the vehicle's motion control in multi-domain simulation. The simulation results are presented to verify the multi-domain mobility and the mode switch ability of the proposed vehicle intuitively. Finally, a prototype, NEZHA, is introduced to be the experimental platform. The success of the flight test, the hovering test, the underwater glide test, and the medium transition test all contribute to prove the feasibility of the proposed concept of the novel MHAUV.