## 4 Aerodynamic Analysis of UAV Based on the Centroid Self-Trim Deformation
### Multivariable Influence Analysis on Pitching Trim Characteristics of UAV  
The wing configuration is symmetrical when the UAV is in a straight and level flight. In this condition, the UAV is not subject to any external interference and only needs to be trimmed for the longitudinal pitch motion at this time. This section explores the influence of the configuration and distribution of the wingspan ratio of the inner and outer sections on the trim status of the UAV. This research considers the influence of three main parameters, the angle of attack \(\alpha\), the equivalent elevator deflection angle of the tail \(\delta_{e}\), and the engine thrust \(T\) on the trim results. The resultant force and moment of the UAV are equal to zero under the combined influence of the three parameters. According to the aerodynamic calculation results, the lift of the wing and tail have a linear relationship with the angle of attack, and the drag has a quadratic polynomial relationship with the angle of attack. The increments of \(\Delta C_{L}\) and \(\Delta C_{D}\) produced by the elevator have a quadratic polynomial relationship with the deflection angle. Therefore, for the longitudinal pitch motion, the following relationship needs to be satisfied at the trim point:  
\[ \begin{array}{l}T\cos\alpha+\frac{1}{2}\rho V^{2}S(C_{D0}+C_{D,\alpha}\alpha^ {2}+C_{D,\alpha}\alpha)=0\\ T\sin\alpha+\frac{1}{2}\rho V^{2}S(C_{L0}+C_{L,\alpha}\alpha+C_{L,\beta_{e}} \delta_{e}^{2}+C_{L,\delta_{e},1}\delta_{e})-G=0\\ \frac{1}{2}\rho V^{2}S(C_{m0}+C_{m4}\alpha^{2}+C_{m2}\alpha+C_{m,\delta_{e},2} \delta_{e}^{2}+C_{m,\delta_{e},1}\delta_{e})=0\end{array}  \tag{8}\]  
where \(\rho\) represents the air density, \(V\) represents the flight speed, \(S\) represents the reference area, and \(G\) represents the weight of the aircraft. The direction of the engine thrust \(T\) is along the longitudinal axis of the fuselage. Because of the smaller wing area of the V-tail, the influence of the increase in drag is ignored, and only the effect of the increase in lift and  
Figure 10: The change in the lift/drag ratio \(L/D\) of the pre-selected model under different angles of attack when Ma = 0.3. The angles of attack equals the following: (**a**) AOA = 0\({}^{\circ}\), (**b**) AOA = 6\({}^{\circ}\), (**c**) AOA = 10\({}^{\circ}\), respectively. the increase in pitch moment caused by its deflection is considered in the longitudinal trim modeling. Table 6 shows the state value of the trim, corresponding to some configurations of the pre-selected models when Ma = 0.3. The following can be observed from the trim results:  
1. With the increase in \(\Lambda_{\text{out}}\), corresponding to the configuration, the angle of attack \(\alpha_{\text{trim}}\) first decreases and then increases, and the engine thrust \(T_{\text{trim}}\) first increases and then decreases. The configurations corresponding to the minimum drag coefficient \(C_{\text{Dmin}}\) have the largest value of \(\alpha_{\text{trim}}\) and the smallest value of \(\delta_{\varepsilon,\text{trim}}\);
2. The state value of \(\delta_{\varepsilon,\text{trim}}\) increases approximately linearly with an increase in \(\Lambda_{\text{out}}\), corresponding to the configuration. With an increase in the wingspan length of the inner section, the value of \(\delta_{\varepsilon,\text{trim}}\) gradually decreases, and the slope of the deflection curve of the rudder also gradually decreases;
3. As the wingspan proportion of the inner section of the pre-selected model increases, the state value of \(T_{\text{trim}}\) gradually decreases. Because \(\Lambda_{\text{in}}\), corresponding to the same change in \(\Lambda_{\text{out}}\), gradually decreases, the total lift of the UAV increases and the corresponding state value of \(\alpha_{\text{trim}}\) gradually decreases. When \(\Lambda_{\text{out}}\), corresponding to the configuration, changes from 0\({}^{\circ}\) to 10\({}^{\circ}\), the aerodynamic change in the UAV is relatively gentle.