Extension-twist coupled tiltrotor blades using Flexible Matrix Composites

Extension- twist coupling behavior has been investigated as a passive means for improving flight characteristics of a tiltrotor aircraft. Previous studies, focused on employing carbon epoxy materials for blade spar designs. Limitations were observed based on strain capability for moderately large twist, as well as coupling authority to generate the required twist with a 20% change in rotor speed. Previous studies used non-structural weights to generate more centrifugal force to achieve significant amount of twist change. In the present investigation, the feasibility of using Flexible Matrix Composites (FMC) to improve the performance of extension-twist tiltrotor blades is explored. Analyses are performed on a thin walled box beam model with rectangular closed cross-section. Extension-twist coupling is achieved by making the laminated walls antisymmetric with respect to the cross-section mid-planes. Stiffness values are altered by varying only the fiber angle and without changing the geometry of the box beam. FMC has very high values of strain at failure compared to carbon epoxy, making it an ideal candidate for applications requiring large elastic twist. Effects of nonclassical behavior such as bending-shear coupling, torsion related out-of-plane warping and warping restraint effects are investigated. A carbon epoxy (CE) design from previous studies is selected as the baseline. Since some designs of FMC can exhibit very low values of stiffness, constraints are imposed on magnitude of twist under torque, extension under axial load, and bending under tip bending load and compared with that of the CE baseline design. Number of plies on each wall and the geometry of the beam are kept the same for both FMC and baseline. Designs with these fixed torsional, bending or axial stiffness constraints for FMC are then analyzed for twist under axial load. Detailed stiffness and strength analyses are conducted on these designs. The present study shows that FMC produces much higher values of twist, ranging from 2-4 times that of carbon epoxy designs without compromising the strength or adding non-structural weights into the beam. Twist deformations thus passively generated of this range are close to the previously predicted values for optimum performance of a tiltrotor. Preliminary studies performed on FMC show that the effect of torsion related warping in FMC is significant where as when considering global deformation, warping restraint effect is not significant.