Design evaluation of model and full-scale flexible matrix composites tiltrotor blades with extension-twist coupling

Structural tailoring is the distribution of parameters such as stiffness, strength, etc. in a predetermined fashion to meet certain design requirements. Findings published over the years stand proof in exploiting these unique characteristics of advance composites. Aeroelastic tailoring utilizing the extension-twist coupling behavior is of high interest especially in improving the flight performance of a tiltrotor aircraft by passively twisting the blades between the hover and cruise flight modes. Conventional carbon epoxy materials are limited by their strain capability in achieving sufficient twist distribution to obtain the predicted improved performance. A different material system called Flexible Matrix Composites (FMC) is employed to analyze two different tiltrotor blade models. FMC is highly anisotropic and the high strain to failure ratio makes them a good candidate for applications requiring high elastic deformation. An aeroelastically scaled model of XV-15; WRATS, is selected as the scaled model baseline. A rectangular box-beam with uniform cross-section is used for FMC scaled model analysis. A boxbeam model representing the spar, similar to the geometry of XV-15 and stiffness properties similar to BA609 is used to study the full-scale FMC tiltrotor blade spar designs. The design procedure involving stiffness, strength, and deflection is employed to derive various candidate stacking sequences. In both model designs, it is shown that FMC laminates have high design flexibility to achieve large values of twist under axial loading without compromising their strength characteristics. Twist deformation of more than 15 degrees is obtained by just changing the various stiffness parameters while avoiding any weight penalty and design complication issues. The effects of various constraints on the design process are discussed. The present investigation demonstrates that the FMC materials are good candidates for tiltrotor blade application.