Model and Full-Scale Tiltrotor Hover Download Tests

This paper describes recent research at Boeing directed at reducing tiltrotor download, increasing overall hover performance, and reducing hover noise. The research encompassed testing at model scale, using a 0.23-scale powered tiltrotor model, and at full scale on the NASA/Bell XV-15 tiltrotor aircraft. The model tests were conducted to understand the relationship between installed rotor performance and airframe download while seeking to investigate concepts that would reduce overall download-to-lift ratios, as well as lowering impulsive noise produced when the blades cut through the recirculating fountain flow above the tiltrotor midwing region. The most promising device was then fabricated and flight-tested in full scale on the NASA/Bell XV-15 tiltrotor aircraft. The model-scale results closely matched the data from the XV-15 tests and showed that the device reduced the download-to-lift ratio by 1.6%, while also reducing the fountain induced impulsive noise by 5 dB. A Navier-Stokes CFD analysis was used to compute the interaction flow on the wing and was compared to flow visualization measurements from the wind tunnel test. Overall agreement was found.