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.
Proceedings of the 74th American Helicopter Society Annual Forum, Phoenix, Arizona, May 15-17, 2018.