Development and Whirl Flutter Testing of Swept-Tip Tiltrotor Blades

Abstract

This thesis describes the development and whirl flutter testing of swept-tip tiltrotor blades. The blades are tested on the Maryland Tiltrotor Rig (MTR). The MTR is also developed as part of this thesis. MTR is a new test facility developed to support the research and development of next-generation high-speed tiltrotors. It is a parametric test bed for developing a fundamental understanding of high speed tiltrotor flight and acquisition of test data to validate advanced simulation tools. The baseline MTR is a Froude-scaled, 4.75 feet diameter, 3-bladed, semi-span, floor-mounted, optionally-powered, gimballed flutter rig. In this work, the rotor blades are designed with the objective of gaining a fundamental understanding of the impact of a swept-tip on tiltrotor whirl flutter. Two sets of blades are fabricated for wind tunnel testing -- straight and swept. The blades have a VR-7 profile, chord of 3.15 inches, and linear twist of -37° per span. The blades have a uniform cross section with mass and stiffness properties loosely based on a 1/5.26 Froude scale XV-15 rotor properties. The swept-tip blades are identical to the straight blades up to 80% radius, where a 20° sweep back angle is introduced to assess the impact on whirl flutter. The cross-section was designed using in-house 2-D section analysis tools. The blade inertial and structural properties were carefully measured. A novel method was developed to attain more reliable measurements of the blade cross-sectional stiffness using accelerometers as tilt sensors. Full 3-D models of the blades were also developed concurrently. These models were built in CATIA, meshed in Cubit, and analyzed with X3D. These models were validated with test data of the measured blade properties. Experiments in a vacuum chamber were carried out to measure frequencies and strains. The 3-D model was validated with this data. Tests and predictions proved the blades have sufficient structural integrity and stress margins to allow for wind tunnel testing. The first whirl flutter test of the MTR was completed in the Naval Surface Warfare Center Carderock Division (NSWCCD) 8- by 10-ft large subsonic wind tunnel. Testing was performed in four configurations for both blade geometries. The configurations progressed step-by-step from the baseline configuration of a gimballed rotor, freewheeling flight, with wing fairings installed; the second configuration removed the wing fairings; the third then locked the rotor gimbal; the fourth configuration then operated in powered flight. The frequency and damping of the wing beam and chord bending were collected at a rotor speed of 1050 RPM and wind speeds up to 100 knots. The 100 knots speed was a limitation posed by NSWCCD. In freewheel flight, the swept-tip blades increase the damping and stability of the wing chord bending mode, even at lower flight speeds. However, in powered flight, the opposite effect is observed. The parametric testing results in a comprehensive data set for evaluating the effect of swept-tip blades on whirl flutter. Richer data is expected at higher speeds where the aerodynamic and inertial couplings of the swept-tip blades are expected to be more pronounced. Nevertheless, it is hoped that the results presented in this work will inspire further investigation using advanced computational tools to develop a more complete understanding of tiltrotor whirl flutter and ultimately eliminate it.