A fundamental study of vibration reduction in rotorcraft using active control of structural response approach (ACSR

This study has several objectives. First, to demonstrate the capabilities of the coupled rotor/flexible fuselage aeroelastic response model by comparing results for the vibration levels at various fuselage locations for both the soft-in-plane and stiff-in-plane rotor blades, with both four and five-bladed rotors, for the cases when the controller is engaged or disengaged. The control forces and power requirements for vibration suppression are also studied, together with their sensitivity to a number of parameters. These comparisons clearly indicate the need for a refined aeroelastic response model, such as developed in this research for studying vibration levels at specific fuselage locations.;The second objective is to present two disturbance rejection control approaches for vibration reduction in a rotorcraft. Numerical results demonstrating the effectiveness of the vibration reduction algorithms for a variety of rotor/flexible fuselage configurations are presented.;The ACSR approach is based on the idea that in a linear system one can superimpose two independent response quantities such that the total response is zero. When applying this scheme to the helicopter vibration reduction problem, the fuselage, at selected locations, is excited by controlled forcing inputs, such that the combined response of the fuselage, due to rotor loads and the applied excitations, is minimized.;A mathematical model for the ACSR system including sensors and four actuators has been developed. A controller based on a disturbance rejection algorithm has been combined with the aeroelastic model of the coupled rotor/flexible fuselage system. Two disturbance rejection algorithms are used to reduce vibrations in the helicopter. The first algorithm is denoted the basic active control of structural response (BACSR) algorithm; and it is an implementation of a similar algorithm developed by Petry. The second algorithm is based on the so called internal model principle (IMP) and it is denoted as an IMP algorithm. This is a more refined algorithm which improves vibration suppression and reduces sensitivity to variation in system parameters.;The helicopter model developed is capable of representing flexible hingeless rotor combined with a flexible fuselage, a platform for the ACSR system and four high frequency force actuators located at the corners of the platform. A concise description of the principal features of the aeroelastic response model is provided next.;The harmonic balance technique is used to calculate the coupled trim and steady state solution in a single pass. A propulsive trim procedure is implemented by simultaneously satisfying trim equilibrium and the vibratory response of the helicopter for all the rotor and fuselage degrees of freedom.;After the trim and response solutions have been found, the rotor vibratory hub loads are determined. Summation of the contribution from all the blades yields the total vibratory hub loads.;A linear control approach can be implemented for our weakly nonlinear aeroelastic response model of a coupled rotor/flexible fuselage system, where the nonlinearity is geometric, due to moderate blade deflections. The nonlinearity manifests itself only in the hub loads. Accelerometers placed at specific fuselage locations to measure these baseline quantities are subsequently used as inputs for generating the control signals. These control signals drive the actuators which induce only small motion between any two points.;The numerical simulations conducted show clearly that the model developed is a valuable and versatile tool for simulating a realistic helicopter behavior. (Abstract shortened by UMI.).