A New Framework For Helicopter Vibration Suppression; Time-Periodic System Identification and Controller Design

In forward flight, helicopter rotor blades function within a highly complex aerodynamic environment that includes both near-blade and far-blade aerodynamic phenomena. These aerodynamic phenomena cause fluctuating aerodynamic loads on the rotor blades. These loads when coupled with the dynamic characteristics and elastic motion of the blade create excessive amount of vibration. These vibrations degrade helicopter performance, passenger comfort and contributes to high cost maintenance problems. In an effort to suppress helicopter vibration, recent studies have developed active control strategies using active pitch links, flaps, twist actuation and higher harmonic control of the swash plate. In active helicopter vibration control, designing a controller in a computationally efficient way requires accurate reduced-order models of complex helicopter aeroelasticity. In previous studies, controllers were designed using aeroelastic models that were obtained by coupling independently reduced aerodynamic and structural dynamic models. Unfortunately, these controllers could not satisfy stability and performance criteria when implemented in high-fidelity computer simulations or real-time experiments. In this thesis, we present a novel approach that provides accurate time-periodic reduced-order models and time-periodic H2 and H infinity controllers that satisfy the stability and performance criteria. Computational efficiency and the necessity of using the approach were validated by implementing an actively controlled flap strategy.;The results show that first, important helicopter aeroelastic features can only be captured using high-fidelity coupled aeroelastic analysis; ignoring these features through uncoupled analysis leads to closed-loop performance degradation and instabilities. Second, time-periodic models are necessary to obtain an accurate map between control actuation and helicopter aeroelastic response; time-invariant models fail to provide accurate prediction.;Third, time-Periodic H2 and H infinity controllers satisfy the stability and design performance criteria when implemented in high-fidelity aeroelastic analysis. Finally, we propose robust H2 and Hinfinity controller design strategies that are capable of modeling variable advance ratios.;In this proposed approach, the reduced-order models were directly identified from high-fidelity coupled aeroelastic analysis by using the time-periodic subspace identification method. Time-periodic H2 and Hinfinity controllers that update the control actuation at every time step were designed. The control synthesis problem was solved using Linear Matrix Inequality and periodic Riccati Equation based formulations, for which an in-house periodic Riccati solver was developed.