A passively adaptive variable rotor diameter is proposed to improve the flight performance of variable-speed rotors. A validated helicopter model to predict rotor performance is utilized. The analyses focus on three typical flight states (i.e., hover, cruise, high speed) to explore the potential of the passively adaptive variable rotor diameter in reducing rotor power. During hover, the variable rotor diameter can optimize the distribution of lift and then reduce the rotor power, especially at lower rotor speeds. At a cruise speed of 150 km/h, the variable rotor diameter can delay stalling and reduce the rotor power, especially at lower rotor speeds. At a high speed of 300 km/h, the rotor speed needs to be increased, and the reduced rotor diameter is beneficial reducing power. The reduced power comes from decreasing the parasitic power of the fuselage due to decreasing its longitudinal tilt, and the rotor profile power and induced power increase with relatively small magnitudes. The passively adaptive variable rotor diameter is more suitable for larger take-off weights, and can effectively delay stalling at lower rotor speeds. The strategy of varying the rotor diameter can be designed according to the performance improvement requirements.

The flight controllability and safety of unmanned compound rotorcraft are closely related to their aerodynamic characteristics. During forward flight, complex aerodynamic interference effects arise among the rotor, propeller, wing, fuselage, and horizontal–vertical tail. These interactions change dramatically with variations in forward speed, which may have a substantial impact on flight performance. This paper investigates aerodynamic interference related to the rotor, propeller, and fuselage of a sample unmanned compound rotorcraft with a novel configuration. On this basis, a flight dynamics model that incorporates the identified aerodynamic interference is formulated. Firstly, an analysis of rotor/propeller/fuselage aerodynamic interference is performed using the momentum source method (MSM). Subsequently, the aerodynamic models for the wing, fuselage, and horizontal–vertical tail are updated by integrating aerodynamic interference factors, leading to the development of a nonlinear flight dynamics model for the sample unmanned compound rotorcraft. Finally, to validate the updated flight dynamics model, numerical simulation results are systematically compared against wind tunnel test results. The results reveal a significant correlation between the numerical simulation data and wind tunnel test results, which indicates that the updated flight dynamics model possesses high accuracy and reliability and can characterize the dynamic characteristics of the sample unmanned compound rotorcraft within the flight speed envelope.

High-speed capability is a defining feature of next-generation helicopters, enabling time-sensitive missions. This paper compares three high-speed configurations: tiltrotor, coaxial rigid rotor, and compound conventional rotor. Based on existing technology and operational needs, the study focuses on the aerodynamic layout of a compound conventional rotor high-speed unmanned helicopter. With key parameters, including a 300 kg takeoff weight and a maximum speed of 240 km/h, iterative optimization was conducted using theoretical analysis, numerical simulation, and flight dynamics evaluation. A feasible aerodynamic layout based on a “dual-side propulsion concept” was developed, followed by flight performance assessment and full-scale prototype flight tests. The results show: (1) the final layout comprises a two-blade hingeless rotor, three-blade pusher propellers, wings, skid landing gear, an H-tail, and a horizontal stabilizer; (2) flight performance meets all design targets, achieving maximum and cruise speeds of 260.48 km/h and 180 km/h at 1500 m altitude; and (3) full-scale prototype tests confirm the rationality of the aerodynamic layout and the reliability of the design process, achieving a high-speed flight of 242.6 km/h at an altitude of 1280 m. This work provides a valuable configuration reference for high-speed unmanned helicopter development.