With the continuous advancement of radar detection and counter-detection technologies, relying solely on stealth shaping can hardly meet the stealth performance requirements of advanced aircraft in a wide electromagnetic frequency domain. Meanwhile, stealth materials have evolved from the initial large-area spray-on absorbing coatings on the airframe to systematic stealth coatings and multifunctional stealth structures, with increasing specificity of application. Establishing an aerodynamic-stealth optimization design method for aircraft that accounts for the scattering characteristics of radar-absorbing materials can mitigate the adverse effects of shape design driven by stealth requirements on aerodynamic and flight performance, while ensuring stealth performance under wideband conditions. Therefore, to address the wideband omnidirectional stealth challenges of advanced aircraft, this study integrates the impedance boundary condition (IBC) theory and the S-parameter inversion method based on effective medium theory (EMT) to construct a calculation method for electromagnetic scattering characteristics of targets with electromagnetic metamaterials and coated media. Furthermore, an adjoint equation for the electromagnetic field for metal-dielectric hybrid targets is established, and the multi-level fast multipole expansion of the adjoint equation is derived. On this basis, an adjoint optimization method for aircraft aerodynamic-stealth design considering the scattering characteristics of absorbing materials is proposed. This adjoint method enables fast calculation of the gradients of aerodynamic and stealth objectives in the design problem, and the sequential quadratic programming (SQP) algorithm is then applied for optimization search. Optimization design is carried out on a typical flying wing configuration with local absorbing coating. The results show that the pressure drag of the optimized shape is significantly reduced under subsonic conditions at Ma = 0.8 and Ma = 0.85, decreasing by 33% and 35%, respectively. Meanwhile, the mean radar cross section (RCS) within the target angular domain θ = 85°–95°, φ = 135°–225° is reduced by approximately 18%. The proposed method can improve the aerodynamic-stealth performance of the aircraft to a certain extent and can serve as a reference for the shape and coating design of aircraft incorporating radar-absorbing materials.

Stealth coating design is the key technology and necessary measure for stealth performance of advanced combat aircraft. The traditional coating design mainly depends on engineering experience, and lacks systematic research on coating design, leading to the unclear relationship between stealth coating design and aerodynamic characteristics of aircraft. Thus, it is difficult to achieve integrated optimization of aircraft aerodynamics, stealth, weight, use and maintenance. To solve the above problems, this paper takes the symmetrical airfoil NACA65013 as the research object, and comparatively analyzes the effects of coating thickness and position on its aerodynamic, stealth and weight characteristics. It is found that the coating position and thickness significantly influence the aerodynamic, stealth and weight characteristics of the airfoil, with obvious contradictions among them. Then, according to the requirements of the aerodynamic, stealth and weight design, the optimal coating area is selected, the aerodynamic stealth optimization design of the airfoil considering the influence of coating is conducted and compared with that of airfoil shape without coating. The results show that the forward Radar Cross-Section(RCS) of the aerodynamic stealth design results considering coating is one order of magnitude lower than that of the airfoil shape without coating, and more than 90% lower than that of the initial airfoil. This research provides an efficient and reliable design method for the fine design of aircraft aerodynamic stealth shape and coating material, exhibiting high theoretical and engineering value.

The flying wing layout is the most promising aerodynamic layout for future aircraft because of its advantages in aerodynamics, stealth and structure. A large number of design analyses conducted in academia for flying wings reveal that the low-speed takeoff/landing performance and controllability are the main difficulties in the performance and safety of the flying wing layout. For the flying wing layout UAVs and combat aircraft, the low-speed performance and controllability are further deteriorated due to the influence of the stealthy design. Previous research has mainly focused on the design of high-speed cruising, while little research on low-speed design can be found. This paper examines the high and low speed integrated design of a flying wing layout UAV based on the high-performance discrete adjoint optimization design platform. First, the effects of different low-speed design models on the low-speed characteristics of the aircraft were compared. Then, a high and low integrated design model was established, comprehensively improving the high and low speed performance of the aircraft. Finally, the results were analyzed, and the main points and rules of the high and low-speed integrated design were summarized, providing a powerful and effective method for the aerodynamic design of the flying wing layout.