Ultra-long endurance unmanned aerial vehicles are advanced aircraft with excellent sustained flight capability and have the characteristics of low-cost, long-term, large-scale, and high-density intelligence collection and monitoring tasks. Due to these advantages, the ultra-long endurance unmanned aerial vehicle has become an important direction for future air vehicles.

 Undergraduates from Beihang University have proposed a scheme for a highly efficient ultra-long endurance unmanned aerial vehicle, “Feng Ru 3.” This scheme contains the ultra-high-aspect-ratio wing, low-drag drop-shaped fuselage, and high-placed empennage. These compose a highly efficient aerodynamic configuration. In addition, the flying platform structure is constructed from a novel composite material with light weight and high strength. This project has innovatively designed a dual-loop power system consisting of an inner loop electronic fuel injection system and an outer loop engine health monitoring system. The team has developed a durable loitering flight control system involving gust alleviation technology and PID parameter self-adaptation technology based on open-source flight control equipment. In addition, this project abandons the traditional takeoff and landing method of the landing gear and shows an innovative vehicle-based takeoff method.

Ground testing (including wind tunnel test, structure static test, propulsion system test) and flight test (including autonomous takeoff and landing test, dolly takeoff test, and long-endurance flight test) are performed on “Feng Ru 3”. The experimental results reveal that (1) the highly efficient aerodynamic configuration vastly increases the aerodynamic efficiency, resulting in a maximum lift-to-drag ratio of ~18.36. (2) The wing and tail made by ultra-light and high-strength composites can withstand a load of 4G, and their flexural changes are within an acceptable range. The stiffness of the tail support carbon tube also meets the design requirements. (3) After multiple 120-h experiments, it is proved that the dual-loop power system can drive the propulsion system super efficiently, enduringly, reliably, and cost-effectively. (4) The introduction of the adaptive PID algorithm in the flight control system of this project can effectively support stable, energy-efficient, and reliable completion of long-endurance flight missions. (5) The project adopts a takeoff method using a vehicle. This approach can reduce about 5 kg of the weight of the landing gear and its additional structures present a higher aerodynamic efficiency, reducing approximately 10% of aerodynamic drag.

Due to the highly efficient aerodynamic configuration, ultra-light and high-strength composites, a highly reliable dual-loop power system, a durable loitering flight control system, and the vehicle-based takeoff method, this developed unmanned aerial vehicle has achieved a comprehensive breakthrough in the capability of executing ultra-long endurance missions. Despite extreme and adverse weather conditions, including wind gusts of 7–8 levels, “Feng Ru 3” successfully achieved an uninterrupted flight of 80 h, 46 min, and 35 s. This achievement of “Feng Ru 3” has set the world record for “the endurance of 25–100 kg class oil-powered fixed-wing unmanned aerial vehicle” certified by the International Aviation Federation.