Optimal control of helicopter vibration through cyclic variations inblade root stiffness

Abstract

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