| Tilt rotor hover aeroacoustics have been studied experimentally and computationally. Flow and far field acoustic measurements were made on a 1/12.5 scale model. Scale factors were derived to extrapolate the model measurements to the XV-15. Noise reduction strategies and basic configuration changes were investigated.;Acoustic predictions were made using Lowson's equation, modeling a rotor blade as a force and source/sink combination at an effective blade radius. Mean aerodynamic models were developed from hover performance predictions, and the interactional flows were modeled from the measurements.;A strong rotor/rotor interaction caused impulsive noise preferentially radiated behind the aircraft. There was a characteristic azimuthal variation in blade loading: it reduced, increased abruptly, then reduced again. Acoustic predictions using this model showed good agreement with full-scale data.;Hot wire measurements detected three length scales in the inflow turbulence: an ambient scale, a fountain turbulence scale, and a tip vortex remnant scale. Flow contraction, multiple length scales, and intermittency caused the turbulence spectra to exhibit non-classical features.;"Diagonal fences" reduced the turbulence intensity in the fountain region by a factor of about 3, and reduced noise by 4 equivalent full-scale dBA behind the aircraft. Operating the rotors out-of-phase reduced the unweighted OASPL behind the aircraft, but cancellation only occurred at the fundamental frequency, which was not significant after A-weighting.;The rotor/rotor interaction was larger on the Variable Diameter Tilt Rotor. The fountain flow was smaller and less intermittent, but had higher turbulence intensity at the rotor tip, and caused more broadband noise. Tip vortex remnants remained in the proximity of the rotor tips, whereas they were carried further towards the hub on the standard tilt rotor. Broadband noise was reduced when the rotor spacing was increased, and when the rotor plane/wing clearance was reduced.;Semi-span models overestimate broadband rotor noise. The image plane causes a high velocity wall jet, and higher inflow turbulence levels because of the boundary layer formed on the image plane. For development of turbulence models and CFD code validation, full-span measurements should be used. |
