FABRICATION, WIND TUNNEL TESTING, AND FREEWHEELING ANALYSIS OF 4.75-FT DIAMETER COMPOSITE BLADES
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The Maryland Tiltrotor Rig (MTR) is a new test facility for the development of next-generation high-speed tiltrotors. This thesis describes the development of the first set of Froude-scale tiltrotor blades for the MTR. The blades have a −37◦ twist/span, a VR-7 profile, a D-spar, and ±45◦ quasi-isotropic plies of carbon fiber. Titanium leading-edge weights bring the center of gravity to near the pitch axis at 0.25 c. The root cutout is until 0.263 R. The stiffness properties loosely follow the NASA-Bell XV-15 aircraft. The blades were instrumented and then integrated on the MTR in the Glenn L Martin Wind Tunnel and powered check out tests were conducted up to 2400 RPM (Mtip = 0.53) to test for tracking, balance, and structural integrity. Zero torque freewheeling tests were conducted to simulate future whirl flutter conditions. These tests produced 0-1500 RPM for θ75 = 0 − 8◦ at various tunnel speeds. A flexible flapping rotor analysis was developed to understand the freewheeling condition and to predict the test data. The freewheeling condition is unique to proprotors and is where wind tunnel tests are traditionally performed for whirl flutter, so it was the principal focus of the analysis. Proprotor freewheeling, unlike helicopter autorotation, occurs at high inflow but zero thrust. There were two collectives for a given RPM, and it was discovered that the collectives tested during the initial check-out were the lower set of collectives, which was not representative of a full-scale tiltrotor in cruise. Thus, the analysis provided guidance for proper operating collectives in future tests. In addition, the low collective set provided a unique, interesting, and challenging validation case where the airfoils operated in negative stall. Accurate negative angle of attack stall data was crucial to predict- ing these collectives. It was shown that the in-house 2D C81 deck extracted with TURNS code in fact gave more consistent data predictions than the US government C81 deck from NASA Ames, likely due to differences in Reynolds number. The flexible flapping analysis also predicted blade bending moments and strains, but correlation with test data was cut short due to the COVID-19 shutdown. There is a vast and broad range of research to be conducted in the next five to ten years. It is hoped that the method developed and the blades fabricated here will provide a good baseline to assess all future advances.