# Longitudinal Wind Tunnel Tests of the PROSIB 19-Pax Airplane
## 3 Results and Discussion
### Effects of the Empennage Configuration  
The scope of the horizontal tailplane is to introduce longitudinal stability and provide longitudinal control in a conventional aircraft layout. In this section, a comparison between the two tailplane configurations shown in Figure 8 is made. The configurations analyzed were the WBVH--the complete aircraft without the battery pod--with three flap deflections. These are illustrated in Figure 9.  
The results of the wind tunnel runs are shown in Figure 10. These were untrimmed conditions with the elevator deflection \(\delta_{\text{e}}=0^{\circ}\). The lift curve slope \(C_{L_{\alpha}}\) was about 10% higher than that of the wing-body configuration due to the presence of the horizontal tail. There was no appreciable difference between the lift provided by the two tail configurations. With respect to the wing-body configuration, the drag coefficient has been increased due to the additional contribution of the empennage. The body-mounted tail was slightly more efficient than the T-tail with the flap deflected for landing (\(\delta_{\text{f}}=30^{\circ}\)). However, the aerodynamic efficiency--the lift-to-drag ratio--is not important in landing conditions, where the aircraft has to slow down. Figure 8: Photographs of the two tailplanes: (**a**) body-mounted configuration; (**b**) T-tail configuration. The pitching moment curves presented larger differences, with the body-mounted tail slightly more stable than the T-tail with increasing flap deflection. The values for \(C_{M_{0}}\), \(C_{M_{a}}\), and \(C_{M_{\delta_{\mathrm{f}}}}\) for the two empennage with different flapped deflections are reported in Table 7. These values have been obtained with a linear regression on the angle of attack range \(-2^{\circ}\) to \(10^{\circ}\). The reported quantities have been defined as follows:  
\(C_{M_{0}}\) is the intercept of the regression line;  
\(C_{M_{a}}\) is the slope of the regression line;  
\(C_{M_{\delta_{\mathrm{f}}}}\) is the shift between the \(C_{M_{0}}\) at \(\delta_{\mathrm{f}}\
eq 0^{\circ}\) and the \(C_{M_{0}}\) at \(\delta_{\mathrm{f}}=0^{\circ}\) scaled by the value of deflection \(\delta_{\mathrm{f}}\). More generally, a control power derivative was evaluated as the incremental ratio of \(C_{M_{0}}\) with the deflection angle \(\delta\), with zero as the initial point. The expression is as follows:  
\[C_{M_{\delta}}=\frac{C_{M_{0}}(\delta)-C_{M_{0}}(\delta=0^{\circ})}{\delta} \tag{2}\] \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & \multicolumn{3}{c}{**Body-Mounted Tail (Hb)**} & \multicolumn{3}{c}{**T-Tail (Ht)**} \\ \(\delta_{\mathrm{f}}\) & \(\mathbf{0^{\circ}}\) & \(\mathbf{15^{\circ}}\) & \(\mathbf{30^{\circ}}\) & \(\mathbf{0^{\circ}}\) & \(\mathbf{15^{\circ}}\) & \(\mathbf{30^{\circ}}\) \\ \hline \(C_{M_{0}}\) & \(0.0441\) & \(-0.0460\) & \(-0.1592\) & \(0.0383\) & \(-0.0231\) & \(-0.1385\) \\ \(C_{M_{\delta_{\mathrm{f}}}}\) & \(-0.0181\) & \(-0.0206\) & \(-0.0241\) & \(-0.0157\) & \(-0.0205\) & \(-0.0218\) \\ \(C_{M_{\delta_{\mathrm{f}}}}\) & n.a. & \(-0.0060\) & \(-0.0068\) & n.a. & \(-0.0041\) & \(-0.0059\) \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Values of pitching moment characteristics for the WBVH configuration at different flap deflections. Elevator deflection \(\delta_{\mathrm{e}}=0^{\circ}\). Derivatives in deg\({}^{-1}\). Figure 9: Complete aircraft configurations with different tailplanes and three flap deflections: (**a**) WBVH_b_F0; (**b**) WBVH_F0; (**c**) WBVHb_F15; (**d**) WBVH_F15; (**e**) WBVHb_F30; (**f**) WBVHt_F30. The configurations of the complete aircraft model with clean wing (\(\delta_{\rm f}=0^{\circ}\)) and several elevator angles \(\delta_{\rm e}\) were also analyzed to evaluate the horizontal tail control power. Examples of elevator deflections are illustrated in Figure 11 for the two horizontal tail configurations. The main effect of a negative elevator deflection was to shift upward the pitching moment curve.