# Effects of Aft-Body Extension Design on the Fundamental Characteristics of HWB Civil Aircraft
###### Abstract  
The extension of aft-body fuselage has become an important trend in the design of Hybrid Wing Body (HWB) civil aircraft due to its improved effect on flight control authority of aircraft. However, it is still unclear how efficient the extension of atf-body will be in improving flight control authority and how it affects other aspects of the flight performance of HWB. To address these problems, this paper evaluated and compared the flight performances of four HWB configurations with different aft-body lengths. A physics-based Multidiscipline Design Optimization (MDO) platform was firstly constructed, and four optimal design works were developed based on this platform, with different static margin constraints. The configurations to be studied came from the results of the four optimization design works, which presented different aft-body lengths under the influence of correspondence between static stability margins and aft-body layout. By investigating these four HWB configurations with the weight, basic takeoff and landing performance, and flight control authority, we can determine the influence of the aft-body extension design on the performances of the HWB civil aircraft and provide advice for the layout design of the HWB aircraft. HWB civil aircraft; Multidiscipline Design Optimization; planform design; aft-body extension +
Footnote †: journal: Applied Sciences  
concept design of HWB, which come from its highly integration of lifting fuselage, wings, and control surfaces, this paper needs to establish an MDO design platform for the HWB and study the characteristics mentioned above based on this platform. The MDO method has long been used in the design of new-concept civil aircraft. Boeing used the WingMDO tool for early BWB multidisciplinary optimization, which uses the traditional vortex lattice method for aerodynamic estimation and the single-beam simplification assumption for structural weight estimation. Through a multidisciplinary optimization approach, adjusting the planform, center body airfoil shape, and torsional distribution, Boeing optimized the design of the BWB450 to achieve a statically stable trim (static stability margin = 5%) [17]. [PERSON] et al. established a high-fidelity multidisciplinary optimization platform based on the CFD (Computational Fluid Dynamics) method. Optimization constraints involve lift, trim, static stability margin, and the allowable bending moment of the central fuselage. In the aerodynamic module of MDO, a high-order model based on the Navier-Stokes equation is used. Due to the high-order aerodynamic calculation method, the shape of the aircraft does not change drastically during the optimization process, as the plane shape of the aircraft is not involved; only the spanwise torsion distribution of the wing is changed [18]. [PERSON] comprehensively studied the influence of various stability constraints on a 100-seat BWB layout design. In this author's research, a multi-objective optimization design framework was adopted. The optimization objective is not limited to cruise aerodynamic drag; it also combines the maximum/minimum takeoff weight [19]. [PERSON] et al. discussed the impact of two key constraints --static stability margin and thrust-specific fuel consumption (TSFC)--on the HWB optimization results [20]. According to previous studies on MDO aircraft, there are two conclusions. First, the multidisciplinary optimization design for an aircraft with blended wing and body can be divided into two types: planform change or not. The former includes the change of aircraft planform, and accordingly, its design space consists of parameters such as wing swept-back angle, wingspan, fuselage length and width, transition part width, etc., which can be used to realize the exploration of various potential configurations in the aircraft conceptual design stage. Due to the drastic changes in the aerodynamic shape, the kind of optimization is more suitable for using low-order aerodynamic models.