# Effects of Aft-Body Extension Design on the Fundamental Characteristics of HWB Civil Aircraft
## 2 MDO Optimization Platform
### Aerodynamic Module  
The main function of the aerodynamic module is to construct the aerodynamic model of the studied configuration, and the built aerodynamic model is then used for the aerodynamic calculation in the trim module and stability and control module. The optimization calculation in this paper has the following characteristics: first, the aircraft planform changes very drastically for the need of exploring different planform layout aircraft; second, the optimization process requires aerodynamic calculations for many cases. Based on the above two characteristics, this paper adopts the low-order method based on the potential flow theory, i.e., the vortex lattice method, for aerodynamic calculations. AVL (Athena Vortex Lattice), a general software for aerodynamic analysis based on the vortex lattice method, is used, which can realize fast and automatic calculation of aerodynamic characteristics of aircraft of different configurations in the form of batch files. Since the AVL calculation of aerodynamics requires the meshing of the aircraft geometry, it needs the whole aircraft's  
Figure 4: Structure of the weight module. shape information, which can be provided by the geometry module. Due to the limitation of theoretical assumptions, the vortex lattice method is only available for the calculation of lift and lift-induced drag, but not the calculation of friction drag and form drag. Therefore, this paper needs to introduce the FRICTION code based on the classical flat plate theory as a supplement to the vortex-lattice-method aerodynamic calculation software to realize the calculation of friction drag and form drag. This part of the calculation does not require the aircraft planform information; it requires only the average aerodynamic chord length and the airfoil parameters of three segments of the HWB, as well as the reference area of entire aircraft. As mentioned before, the AVL calculation requires aircraft-shape information, which consists of two parts. The first part is the planform-shape information, which can be concretely reflected in the mesh model of AVL software, as shown in Figure 5. The planform shape is determined by the coordinates of the leading and trailing edges of several chordal sections, which are derived from the aircraft's planform curves' output by the geometry module. The second part is the airfoil, which corresponds to the three fuselage segments of the HWB civil aircraft--the center body, the blending area, and the outer wing. Each segment of the fuselage selects the corresponding airfoil according to its own functional characteristics Figure 6 [24]. The center fuselage airfoil is selected based on the following considerations: The first consideration was to choose an airfoil with both a suitable lift coefficient and the ability to achieve longitudinal aerodynamic moment balancing. For this purpose, an airfoil with a reflected camber design was selected, which allows the aerodynamic loads to be unloaded at the trailing edge of the airfoil to achieve pitch moment balancing. The second is the demand for a reasonable span-wise distribution of lift on the aircraft. Since the chord length of the center fuselage is longer than the other two segments, if the lift coefficients of the three airfoils are similar, the lift contributed by the center fuselage per unit span length will be excessive, and such a lift distribution is not conducive to drag reduction. So, the lift coefficient of the center fuselage airfoil should not be too large if the configuration is to obtain a reasonable lift distribution along the span. The transition-part airfoil, which is close to a symmetric airfoil, is used to achieve the airfoil transition between the outer wing section and the center fuselage. For the outer wing, a supercritical airfoil with a positive camber trailing-edge is used to ensure the transonic performance of the aircraft. In addition, the relative thickness of each airfoil is set with reference to the airfoil thickness distribution  
Figure 5: AVL mesh model of the HWB aircraft.