Optimization of Extension-Twist Coupled Composite Blades for High-Speed Rotorcraft

A variable stiffness composite optimization methodology is presented to study the extension-twist coupling capability of composite rotor blades to passively vary the elastic twist distribution as a function of the rotational speed of the rotor. To this end, an optimization framework, with composite laminates as design variables, is implemented to optimize an extension-torsion-coupled composite blade based on the UH-60A Black Hawk. The results show that variation in twist angle of up to 9◦ can be achieved by reducing the rotor speed by 20% (from 100%NR to 80%NR) using optimized composite laminates while complying with material strength constraints under both centrifugal and aerodynamic loads in hover. Using this optimized design, a composite blade could be constructed with 13◦ nose-up structural twist. As the rotor is spun up to 100%NR (for hover), the blade elastically twists nose-down to a near-optimum linear twist distribution of −12.5 ◦ . Yet when the rotor speed is reduced to 80%NR to potentially accommodate compressibility effects on the advancing blade tip in high-speed forward flight, the blade elastically untwists to only −3.5 ◦ of linear tip twist along the blade span. This passive twist adaptivity could improve a wide range of rotor performance metrics, including power, hub vibrations, and root bending loads, across both flight regimes.