Structural Testing of Full-Scale Flap-Bending/Twist Composite Coupled Blades

Structural testing of full-scale blade geometries with flap-bending/twist composite coupling was performed to evaluate the impact of coupling. Full-scale spar geometries were first fabricated with three different coupling distributions, including two with a uniform positive flap-bending/twist coupling, in which a flap up deformation induces a nose down elastic twist. The third spar geometry incorporated a mixed coupling, with a uniform positive coupling at the inboard end and a uniform negative coupling at the outboard end, where the negative flap-bending twist coupling produces a nose up elastic twist when experiencing flap up deformation. A full-scale blade was then fabricated with a positive flap-bending/twist coupling. Measurements of the structural twist distribution of the cured spars were taken to ensure the coupling did not result in any hygrothermal instabilities. Tip twist and strains were then measured under various combinations of flatwise bending and torsional bending loads and correlated with Rotorcraft Comprehensive Analysis System (RCAS) and Variational Asymptotic Beam Sectional Analysis (VABS) predictions for both the spars and blade. Using the strain measurements, stiffness properties of the test articles were also calculated to confirm the predicted coupling levels. Generally good agreement was observed between the measurements and analytical predictions for both the full-scale spars and the full-scale blade.