# 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
Figure 17: Comparison of the time histories of the non-dimensional normal force \(M^{2}c_{n}\) and pitching moment \(M^{2}c_{m}\) on the proprotor blade in conversion flight condition at \(r/R=0.95\), \(\theta_{N}=75^{\circ}\), \(V_{\infty}=40\) kts, \(\Omega=589\) RPM. **Nacelle angle \(\theta_{N}=60^{\circ}\), freestream velocity \(V_{\infty}\) = 100 kts. **  
The decrease in the nacelle angle combined with the increase in the free-stream velocity allows for the distance between the tip vortices to increase with respect to the previous flight condition, thus preventing the interactions between the vortices and the formation of disk vortices. This flow behaviour, in good agreement with the CFD results [16], is clearly  
Figure 18: Comparison of the instantaneous flow field at wing midspan in conversion flight conditions at \(\psi=270^{\circ}\), \(\theta_{N}=75^{\circ}\), \(V_{\infty}=40\) kts, \(\Omega=589\) RPM. (a) CFD-[16]; (b) DUST. visible in Figure 19, showing a representation of the instantaneous flow field at \(\psi=270^{\circ}\) by means of the iso-surface of Q-Criterion. The three-dimensional flow representation also shows the massive shedding that occurs behind the wing control surfaces and the nacelle. Furthermore, in this flight condition, the lower angle of the nacelle causes the blades to pass in front of the wing. Consequently, the rotor downwash invests the wing and accelerates the flow over the upper surface. This flow behaviour around the wing is well captured by the DUST simulations, as shown in Figure 20 by the instantaneous flow field that is illustrated on a slice cutting the wing longitudinally at midspan. Nevertheless, also for this case, the DUST results do not show the limited separated flow region that is outlined by the CFD results over the upper surface of the deflected flap. Figure 19: Instantaneous flow field that is computed by DUST in conversion flight condition at \(\psi=270^{\circ}\), \(\theta_{N}=60^{\circ}\), \(V_{\infty}=100\) kts, \(\Omega=589\) RPM; iso-surface of Q-Criterion. Figure 20: Comparison of the instantaneous flow field at wing midspan in conversion flight condition at \(\psi=270^{\circ}\), \(\theta_{N}=60^{\circ}\), \(V_{\infty}=100\) kts, \(\Omega=589\) RPM. **(a)** CFD-[16]; **(b)** DUS. The comparison of the non-dimensional blade normal force \(M^{2}c_{n}\) and pitching moment \(M^{2}c_{m}\) at \(r/R=0.95\) in Figure 21 shows a quite good agreement between the DUST and CFD results. Indeed, the DUST simulations capture the large oscillations of both the blade normal load and pitching moment throughout the proprotror blade revolution well. Nevertheless, concerning pitching moment representation, a smaller amplitude and a small phase shift in the curve behaviour is observed. These small differences could be explained analogously to the results discussion of the previous configuration while also taking the lower accuracy of the numerical model with respect to CFD into account. **Nacelle angle \(\theta_{N}=30^{\circ}\), freestream velocity \(V_{\infty}\) = 140 kts. **  
In this flight condition, the low nacelle angle and high freestream velocity prevent interactions between the tip vortices that show a quite coherent helical path. This flow behaviour can be observed from the instantaneous velocity field that is computed by DUST at \(\psi=270^{\circ}\), as shown in Figure 22. The flow field representation  
Figure 21: Comparison of the time histories of the non-dimensional normal force \(M^{2}c_{n}\) and pitching moment \(M^{2}c_{m}\) on the proprotror blade in conversion flight condition at \(r/R=0.95\), \(\theta_{N}=60^{\circ}\), \(V_{\infty}=100\) kts, \(\Omega=589\) RPM. shows that a massive shedding behind the wing control surfaces and nacelle is still present, even if it decreased with respect to the analysed flight conditions in the first stages of the conversion phase.