CFD Calculations of the XV-15 Tiltrotor During Transition

The focus of this work is to develop an understanding of the complex aerodynamics of the Bell XV-15 tiltrotor aircraft throughout its conversion from hover to forward flight. To accomplish this, high fidelity CFD simulations using HELIOS are done at six different stages of the maneuver: hover mode, conversion mode (75° , 60°, and 30° nacelle angles), and cruise (160 and 220 knots). Good agreement is found when comparing the CFD results against OARF experiments, GTR flight simulator data, and flight test data. The HELIOS predictions show that significant interactional effects on the rotor occur during hover and cruise due to fountain reingestion/ground effect and wing proximity, respectively. These interactions lead to sudden, impulsive fluctuations in normal force and pitching moments on the blades as they pass over the wing. Fluctuations in the normal force with peak-to-peak amplitudes on the order of the mean blade loading are observed. The conversion mode is characterized by large unsteady blade loading due to the highly asymmetrical flow although the interactional effects are almost nonexistent. Overall the trimmed CFD approach used in this work offers a high-fidelity approach to analyze full vehicle tiltrotors in all configurations of flight.