# Assessment of a Mid-Fidelity Numerical Approach for the Investigation of Tiltrotor Aerodynamics
## 4 Results and Discussion
### Full Vehicle Simulations
#### 4.2.2 Conversion Mode Conditions  
Three flight configurations were considered by the DUST simulations throughout the conversion phase of the tiltrotor aircraft, as reported in Table 4. The results of the DUST simulations obtained over the three aircraft flight configurations characterised by a different nacelle angle and freestream velocity are thoroughly compared in the following with the CFD results from the recent work by [PERSON] and [PERSON] [16]. In particular, differently from hover and cruise conditions, the comparison of the polar representation of the loads acting on the blade in conversion mode are not presented, since they are not available from the work by [PERSON] and [PERSON] [16]. **Nacelle angle \(\theta_{N}=75^{\circ}\), freestream velocity \(V_{\infty}\) = 40 kts. **  
The combination of the high incidence angle of the nacelle with the low freestream velocity provides complex vortex interactions. Indeed, the vortices that are issued by the propropt blades quickly wrap up into a pair of \"disk vortices\" starting particularly close to \(\psi=90^{\circ}\) and \(\psi=270^{\circ}\). This feature, which was well described in the CFD activities that were reported in [16; 17], was clearly captured by the DUST simulation results, as can be observed from the instantaneous flow field computed at \(\psi=270^{\circ}\) represented by the iso-surface of the Q-Criterion in Figure 16. In particular, at \(\psi=90^{\circ}\) and \(\psi=270^{\circ}\), a propropt blade interacts significantly with the vortices that were issued by the second and third blade of the propropt passing simultaneously on both the upper and lower surface of the rotor blade. The flow physics of this interaction is quantitatively captured by the DUST simulation, as shown by the contours of vorticity illustrated in Figure 16b at the azimuthal angle \(\psi=270^{\circ}\). The coherent representation of the flow field computed by DUST confirms the suitability of the implemented physics-based model for proproptor wakes to correctly simulate the complex interactions that characterise the conversion flight conditions with a computational cost that is greatly lower with respect to CFD. The interactions that are related to the formation of disk vortices create large variations in the normal force acting on the propropt blade. This effect is clearly visible from the comparison of the blade \(M^{2}c_{n}\) computed at \(r/R=0.95\), as shown in Figure 17a. The DUST simulations capture well the occurrence and amplitude of the normal force variations around \(\psi=90^{\circ}\) and \(\psi=270^{\circ}\) computed by CFD. Moreover, the amplitude of the sudden fluctuations in the pitching moment are not well captured by DUST, even if the mean value along the rotor revolution is quite similar to the CFD results. The incorrect representation of the pitching moment peak loading could also be affected by the prediction of a chord-wise force component that is not available for comparison from CFD. Figure 16: Instantaneous flow field computed by DUST in conversion flight condition at \(\psi=270^{\circ}\), \(\theta_{N}=75^{\circ}\), \(V_{\infty}=40\) kts, \(\Omega=589\) RPM: (**a**) iso-surface of Q-Criterion, (**b**) iso-contours of vorticity on a slice cutting the nacelle axes. In order to evaluate the aerodynamic effect between the proprotor wake and wing, Figure 18 shows the comparison of the instantaneous flow field on a slice cutting the wing longitudinally at midspan. A quite good agreement is observed between the CFD and DUST flow field representations, showing, for this flight condition, that the proprotor wake is advected downstream, completely missing the wing. Nevertheless, the DUST results do not show the limited separated flow region over the upper surface of the deflected flap, as can be expectable from potential methods.