# Assessment of a Mid-Fidelity Numerical Approach for the Investigation of Tiltrotor Aerodynamics
## 4 Results and Discussion
### Full Vehicle Simulations
#### 4.2.1 Hover Condition  
In hover flight conditions, strong aerodynamic interactions between the proprotors and wing characterise the flow field around the airframe. Figure 13 shows the comparison of the contours of the instantaneous vertical velocity component around the vehicle computed in hover for the blade azimuth \(\psi=270^{\circ}\) over a \(Y-Z\) plane passing through the nacelle axis. Figure 12: Comparison of the contours of the sectional normal force on the wing during the last proprotor revolution, \(\alpha=5^{\circ}\), \(V_{\infty}=220\) kts, \(\Omega=517\) RPM. (**a**) CFD [18]; (**b**) DUST. The DUST representation closely resembles the global behaviour of the flow field around the vehicle that is computed by CFD. In particular, the DUST results obtained over the complete vehicle model show a flow region over the fuselage characterised by a quite moderate upwash. This flow feature typical of tiltrotor configurations is known as fountain flow [6]. The size and strength of the fountain flow is greater in half-span CFD models due to the reflection of the flow caused by the symmetry boundary condition [6]. The hover flight condition is characterised by a not symmetrical behaviour of the flow field as shown both by CFD simulations of Potsdam and Strawn [6] and by flow visualizations of [PERSON] et al. [49]. Indeed, a significant higher upwards velocity region over the fuselage is observed from the half-span CFD solution shown in Figure 13a. Therefore, the lower computational effort required by a mid-fidelity approach enabled us to obtain a better representation of this characteristic flow feature with respect to the CFD simulations over the whole range of Reynolds numbers. Figure 13: Comparison of the contours of the vertical velocity component around the vehicle in hover at \(\psi=270^{\circ}\), \(\theta_{N}=90^{\circ}\), \(\Omega=589\) RPM. (**a**) Half-span model: CFD [17]; (**b**) Complete model: DUST. half-span models. The ability to obtain a quite good representation of the overall flow field around the complete vehicle has to be considered a point of strength of the use of a mid-fidelity approach to aerodynamics for the preliminary design of a tiltrotor aircraft. Considering now the vehicle aerodynamic performance, the interference of the wing on the rotor downwash provides a significant download on the vehicle that must be overcome by the rotor in the hover and climb phases of the tiltrotor flight missions. Figure 14 shows the comparison of the instantaneous surface pressure on the fuselage computed by DUST and CFD in hover for the blade azimuth \(\psi=270^{\circ}\). A large high-pressure region covering almost all of the wing surface can be observed due to the impingement of the rotor wake. The high-pressure region extent and the positive \(C_{p}\) values over the wing are quite high for CFD with respect to the DUST results. Consequently, the download on the wing computed by DUST is lower with respect to CFD. Indeed, the download on the wing computed by DUST resulted in being about 3% of the global proprotor thrust, while CFD provided a download of 10.3%. This difference could be related to an incorrect prediction of the aerodynamic effects that are related to the wing surface that, in this flight condition, is normally invested by the proprotor wake. As a matter of fact, for this flight condition, the complex flow field around the wing is characterised by low velocities and low Reynolds numbers and, particularly, by a massive wing shedding region (see Figure 13a) and flow separations. Consequently, as could be expected, these flow conditions typical of hover have to be considered difficult to represent with potential methods. Moreover, CFD shows a higher-pressure region over the fuselage upper surface (see Figure 14a) due to the higher strength of the fountain flow effect that is related to the use of a half-span CFD model. This feature could negatively affect the evaluation of the global download of the vehicle that was obtained by CFD with respect to the real aircraft.