Research status of aerodynamic design for integrated propulsion and airframe in near-space high-altitude long-endurance (HALE) UAVs

Near-space long-endurance unmanned aerial vehicles (UAVs) play an important role in persistent reconnaissance, communication relay, wide-area early warning, and emergency response missions. However, due to the extremely low atmospheric density and the cross-altitude flight characteristics of near-space operations, these vehicles encounter prominent low-Reynolds-number effects, reduced propulsion efficiency, and strong coupling between flight and propulsion systems. These challenges underscore the need for integrated flight-propulsion aerodynamic design. This paper reviews recent technical progress in this field for near-space long-endurance UAVs. Based on the configuration characteristics of near-space vehicles, three categories are identified: conventional-layout high-altitude long-endurance UAVs, low-dynamic large-flexibility high-altitude long-endurance vehicles, and stealth-layout high-altitude long-endurance UAVs. For each category, the primary technical challenges and the corresponding focuses of integrated flight-propulsion design are clarified. The theoretical framework of integrated flight-propulsion aerodynamics is then introduced, structured around a systematic research paradigm of modeling-constraints-tasks-evaluation. Following this, the application of robust optimization methods within integrated design is discussed. Specifically, conventional-layout platforms benefit from general integrated robust optimization; low-dynamic platforms require robust design that addresses distributed propulsion and airframe integration; and stealth-layout platforms place emphasis on robust intake and exhaust system design. Building on this foundation, the potential of passive flow-control approaches, such as variable-pitch passive control and bump control, and active flow-control techniques, such as synthetic dual-jet actuation, for enhancing aerodynamic and propulsion performance is further examined, and the distinct research focuses associated with the three platform types are summarized. Finally, the study synthesizes the research paradigm (modeling-constraints-tasks-evaluation) and key technical priorities for integrated flight-propulsion design in near-space long-endurance UAVs, and outlines future development directions involving robust integrated optimization and flow-control enhancement technologies.