Advances in Folding-Wing Flying Underwater Drone (FUD) Technology

Highlights: What are the main findings? This review compares two main folding-wing configurations—lateral and longitudinal—assessing their trade-offs in terms of performance, efficiency, and structural complexity. It also evaluates dual-use and hybrid propulsion systems, emphasizing the challenges of integration, mass, and energy efficiency across different media. What are the implications of the main findings? The study presents a comprehensive approach for High-Altitude Aerial Vehicles (FUDs), focusing on advanced materials, intelligent control systems, and innovations in energy to address existing challenges. It emphasizes the significant transformative potential of FUDs for both military and civilian use, urging targeted efforts to tackle critical bottlenecks and explore emerging technologies such as swarm operation and energy recovery. The evolution of modern warfare and civil exploration requires platforms that can operate seamlessly across the air–water interface. The folding-wing Hybrid Air and Underwater Vehicle (FUD) has emerged as a transformative solution, combining the high-speed cruising capabilities of fixed-wing aircraft with the stealth characteristics of underwater navigation. This review thoroughly analyzes the advancements and challenges in folding-wing FUD technology. The discussion is framed around four interconnected pillars: the overall design driven by morphing technology, adaptation of the propulsion system, multi-phase dynamic modeling and control, and experimental verification. The paper systematically compares existing technical pathways, including lateral and longitudinal folding mechanisms, as well as dual-use and hybrid propulsion strategies. The analysis indicates that, although significant progress has been made with prototypes demonstrating the ability to transition between air and water, core challenges persist. These challenges include underwater endurance, structural reliability under impact loads, and effective integration of the power system. Additionally, this paper explores promising application scenarios in both military and civilian domains, discussing future development trends that focus on intelligence, integration, and clustering. This review not only consolidates the current state of technology but also emphasizes the necessity for interdisciplinary approaches. By combining advanced materials, computational intelligence, and robust control systems, we can overcome existing barriers to progress. In conclusion, FUD technology is moving from conceptual validation to practical engineering applications, positioning itself to become a crucial asset in future cross-domain operations.