Fabrication and Wind Tunnel Hover Tests of Untwisted Proprotor Blades

Abstract

The Maryland Tiltrotor Rig (MTR) is one of three recently built tiltrotor researchrigs outside industry in the world investigating whirl flutter. The interchangeability of its blades, hub, and spar, however, makes it unique. Although highly twisted blades are the norm in tiltrotors, untwisted blades are also of research interest at very high cruise inflow λc. They provide a baseline benchmark for hover performance and, additionally, help decouple flap and lag stiffness, simplifying the measurement of cross-sectional stiffness. Therefore, the objective of this thesis was to fabricate untwisted composite tiltrotor blades of 2.375 ft radius and to carry out hover tests in the Glenn L. Martin Wind Tunnel (GLMWT). A repeatable process was developed for the in-house fabrication of the blades. The materials used were Boeing-donated IM7/8552 ±45◦ twill weave prepreg carbon fiber, in-house machined root insert, Hysol PL7000 adhesive, tungsten-carbide leading-edge weights, and Rohacell IG-31 foam. The blade was formed within an aluminum 6061 mold. The blades were instrumented with flap gauges at the 1/4c location. Non-rotating frequencies were measured. The blades were then statically balanced and integrated onto the MTR. Hover data was acquired at collective angles from 9◦−18◦. From thrust and torque data the rotor Figure of Merit and modified Figure of Merit* were calculated. The modified Figure of Merit accounts for tunnel recirculation and is the correct metric for hover performance. The induced power factor κh, mean drag coefficient cdo and lift-curve slope a were extracted from the data. A Blade Element Momentum Theory (BEMT) with exact angles and C81 airfoil decks was used to verify the quality of the data. The results tracked the data, including the stall regions. The inclusion of tunnel recirculation was crucial for the accuracy of hover prediction within the wind tunnel. The data and analysis were then compared with twisted blades from a companion work. The untwisted blades were predictably less efficient than the straight blades.