Optimal control of helicopter vibration through cyclic variations in blade root stiffness

This study demonstrates that optimal multi-cyclic variations of the blade root flap and lag stiffness can produce simultaneous reductions in all of the components of vibratory hub loads of a four-bladed hingeless rotor helicopter. Both gradient- and non-gradient-based optimization schemes are successful in reducing the hub vibrations. The required stiffness variations can be reduced (without significantly compromising performance) by introducing a penalty on the input in the objective function used for minimization. Reductions in the vibration performance index of over 90% were seen with optimal 2/rev and 3/rev flap and lag stiffness variations. The concept was effective in reducing vibrations over a range of variations in configuration (fundamental flap, lag, and torsion frequencies) and operational parameters (forward speed). Furthermore, it was shown that stiffness variations of discrete flap and lag springs introduced to the blade root region are effective in reducing vibratory hub loads. Thus, the introduction of discrete controllable stiffness elements (devices) is a viable method for varying the stiffness of the blade root region.