Large-slenderness-ratio aircraft in which a large portion of the aircraft’s mass is concentrated along its centreline face the problem of severe lateral-directional coupling. Previous research has not fully considered the inertia product when conducting stability analyses of such aircraft. However, neglecting the inertia product may threaten the safety of the aircraft during high-speed flight. This paper investigates the effect of the inertia product on the lateral–directional stability. Two methods, one based on eigenstructure assignment (EA) and the other based on an extended state observer (ESO), are developed for the lateral–directional decoupling of a research large-slenderness-ratio aircraft. Simulation results show that both methods can realize decoupling control. The EA-based control achieves better decoupling performance, whereas the ESO-based control has an advantage in terms of disturbance rejection. Finally, the ESO-based attitude controller is validated through a high subsonic flight test. The decoupling and tracking performance of the attitude control demonstrate the effectiveness and practicality of the ESO-based controller.

To address the strong aerodynamic coupling and insufficient control robustness of fixed-wing unmanned aerial vehicles (UAVs) under near-envelope conditions, this study focuses on three-axis attitude stabilization and develops a two-level control architecture consisting of an attitude/load-factor stability-augmentation loop and an angular-rate stability-augmentation loop. The outer loop adopts an attitude-angle dynamic-inversion and load-factor correction structure, whereas the inner loop employs linear active disturbance rejection control (LADRC). An improved non-dominated sorting genetic algorithm II (NSGA-II) integrating hierarchical pre-optimization and a threefold adaptive mechanism is proposed for multi-objective controller-parameter tuning. Hierarchical single-objective pre-optimization is followed by multi-objective coordinated optimization to reduce premature convergence in high-dimensional parameter optimization. A threefold adaptive mechanism comprising parameter-range adaptation, crossover and mutation probability adaptation, and objective-weight adaptation balances global exploration and local convergence. Nonlinear simulations and field tests show that hierarchical pre-optimization accounts for the main fitness reduction relative to standard NSGA-II (approximately 4.41%), whereas the adaptive mechanism lowers the post-convergence coefficient of variation from 0.0020 to 0.0006 and adds a 0.083% reduction beyond hierarchical pre-optimization alone. Under combined multi-source perturbations in aerodynamic, inertial, and actuator parameters, the optimized three-channel closed-loop controller remains stable within the tested ranges and maintains attitude-tracking performance. This study supports offline tuning under the tested near-envelope conditions.

To enhance the propulsion efficiency of near-space high-altitude unmanned aerial vehicle under low-density conditions and to gain a deeper understanding of the aerodynamic characteristics of contra-rotating propellers under complex interference, this study focuses on a high-altitude contra-rotating propeller propulsion system. A systematic investigation is conducted on the influence of design variables and flow characteristics. Considering the distinctive features of high-altitude environments, including low Reynolds numbers, high induced velocity ratios, and strong mutual interference between front and rear rotors, a numerical simulation method for contra-rotating propellers is established. The aerodynamic performance and typical flow structures are analyzed and compared with conventional propeller configurations to elucidate the aerodynamic advantages of contra-rotating propellers. Furthermore, key design variables such as axial distance, pitch angles of the front and rear propellers, and rotational speed matching are systematically examined to assess their effects on aerodynamic characteristics. Comparative analysis of axial velocity distributions reveals the interaction mechanisms between front and rear rotors under different parameter combinations and identifies the dominant factors influencing aerodynamic performance. The results indicate that rational matching of geometric parameters between front and rear rotors can effectively mitigate adverse interference, optimize wake structures, and improve the overall aerodynamic performance of contra-rotating propellers at high altitudes. These findings provide theoretical guidance and engineering references for the aerodynamic design and parameter selection of high-altitude contra-rotating propeller systems.