# Longitudinal Wind Tunnel Tests of the PROSIB 19-Pax Airplane
## 3 Results and Discussion
### Wing-Body Contribution  
The wing-body configuration was evaluated with the three available flap deflections, as shown in Figure 6. Results are reported in Figure 7. As the flap was of a Fowler type, the increase in the slope of the lift curve in addition to its upward shift is apparent. The deflection of the flaps increased the lift coefficient at the zero angle of attack \(C_{L_{0}}\) by 0.35 with \(\delta_{\rm f}=15^{\circ}\) and by a further 0.40 with \(\delta_{\rm f}=30^{\circ}\). The wing lift curve slope \(C_{L_{0}}\) increased from 0.073 deg\({}^{-1}\) in clean configuration to 0.092 deg\({}^{-1}\) in a flapped configuration. The stall was anticipated at a lower angle of attack, whereas drag and pitching moment magnitude increased.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **SD (\(\alpha\), deg)** & **SD (\(C_{L}\))** & **SD (\(C_{D}\))** & **SD (\(C_{M}\))** \\ \hline
**avg-** & 0.034 & 0.0051 & 0.0007 & 0.0006 \\
**min** & 0.015 & 0.0029 & 0.0002 & 0.0002 \\
**max** & 0.047 & 0.0062 & 0.0010 & 0.0010 \\ \hline \hline \end{tabular}
\end{table}
Table 6: Standarddeviation (SD) values of the longitudinal aerodynamic coefficients, evaluated on the wing–body configuration.  
Figure 6: Wing body configurations: (**a**) flap deflected at \(0^{\circ}\); (**b**) flap deflected at \(15^{\circ}\); (**c**) flap deflected at \(30^{\circ}\).  
The slope of the pitching moment coefficient with respect to the angle of attack is related to the measure of aircraft longitudinal stability. The derivative \(C_{M_{s}}\) was equal to \(0.007\,\mathrm{deg}^{-1}\) for the clean configuration, whereas it was reduced to about zero with the  
Figure 7: Aerodynamic curves for the wing–body configuration: (**a**) lift curves; (**b**) drag polars; (**c**) pitching moment curves.  
flaps deflected. The values achieved by the wing-body combination indicated its unstable contribution to aircraft longitudinal stability with respect to the chosen reference point.  
From the acquired data, it was possible to estimate the non-dimensional location of the wing-body aerodynamic center \(\overline{x}_{\text{a.c.WB}}\) for the clean configuration, whose shift from the moment reference point was evaluated as the opposite of the ratio of the pitching moment curve slope and the lift curve slope:  
\[\overline{x}_{\text{a.c.WB}}=\overline{x}_{\text{ref}}-\frac{C_{M_{\text{a,WB }}}}{C_{L_{\text{a,WB}}}}=0.25-\frac{0.007}{0.073}=0.153 \tag{1}\]  
indicating that it was located at a distance of 15.3% m.a.c. length ahead of the chosen reference point.  
Although an estimation for the flapped configurations could be made with Equation (1), since their pitching moment curves were quite flat, it was apparent that the flap deflection shifted the aerodynamic center back to the 25% of the m.a.c. As previously stated, the deflection of the flap also caused a shift of the pitching moment at the zero angle of attack \(C_{M_{0}}\) by \(-0.11\) for \(\delta_{\text{f}}=15^{\circ}\) and by a further \(-0.15\) for \(\delta_{\text{f}}=30^{\circ}\). These additional values of the pitching moment coefficient have to be balanced by the horizontal tailplane in the full aircraft configuration to keep the longitudinal equilibrium at the desired lift coefficient.