# Assessment of a Mid-Fidelity Numerical Approach for the Investigation of Tiltrotor Aerodynamics
## 4 Results and Discussion
### Wing-Prorotor Simulations
#### 4.1.2 Proprotor Effect on Wing  
The effect of the proprotor on the normal force acting on the wing is investigated in the following. The DUST simulations showed a decrease in the average lift acting on the wing of 2.5% due to the proprotor installation, while a slightly positive increase of 0.7% was calculated by CFD [18]. Thus, in order to obtain more detailed insight into the capabilities of a mid-fidelity approach to capture the flow physics related to the aerodynamic interaction effect of the proprotor on the wing, Figure 11a shows a representation of the flow field that is computed by DUST at \(\psi=310^{\circ}\) by means of the iso-surface of the Q-criterion and the contours of the slipstream perturbation velocity on a horizontal \(X-Y\) plane over the wing. This flow field representation clearly shows the interference of the proprotor helical system on the wing that mainly provides an upwash due to the blade tip vortex encountering the inboard region of the wing. This feature provides an increase in the slipstream velocities over the wing with respect to freestream in the inboard region of the wing between \(2Y/b=0.2\) and \(2Y/b=0.4\). This augmented velocity in the streamwise direction provides a local increase in the dynamic pressure and, consequently, an increase in the wing lift at the inboard region. The comparison of the span-wise sectional normal force distribution computed at \(\psi=310^{\circ}\) shown in Figure 11b reflects this flow physics effect. Indeed, an increase in the wing loading at the inboard region is observed when the proprotor is installed. This effect is captured by DUST simulations, even if the positive peak loading at inboard region is slightly lower with respect to CFD. Moreover, the interaction of the vortices that are issued by the proprotor blade root, as clearly observed from the Q-criterion iso-surface shown in Figure 11a, provides a decrease in the normal force with respect to the single wing configuration at the outboard region of the wing. This effect is well captured by the DUST simulations, as shown by the quite good agreement of the normal force distribution with the CFD results in the range between \(2Y/b=0.6\) and \(2Y/b=1\). Therefore, the comparison of the span-wise behaviour of the normal force acting on the wing that is shown in Figure 11b indicates that the mid-fidelity approach implemented in DUST is able to capture the physical effects of the aerodynamic interaction of the proprotor on the wing, while the slight decrease in the global normal force acting on the wing calculated by DUST is mainly due to the slight underestimation of the peak loading occurring at the inboard region. The reduced peak loading that is computed by DUST could be related to the spatial discretisation of the surface panels used to model the wing. Nevertheless, a further investigation of the possible dependence of the solution from the meshing properties of the numerical model is out of the main scope of the work, consisting of the assessment of a light numerical solver aimed at the investigation of the interactional flow physics that are involved in the tiltrotor aerodynamics used for the preliminary design of novel vehicles. Figure 12 provides a confirmation of DUST's ability to capture the flow physics related to this aerodynamic interaction, showing a comparison of the sectional normal force contours acting on the semi-span wing calculated for the last proprotor revolution. The impulsive positive peak loading provided by the blade tip vortex impact is clearly captured by the DUST simulations, showing a 3-per-rev footprint that is characterised by the same intensity and position at the inboard span-wise region of the wing (\(0.2<2Y/b<0.4\)) highlighted by the CFD results. Moreover, the representation of the DUST results also captures the extent and intensity of the loading decrease occurring toward the wing-outboard due to the effect of the interaction of the vortices that are issued at the blade root.