Application of the Floquet Transition Matrix to Problems of Lifting Rotor Stability

Interpretation of Floquet Eigenvalues and Eigenvectors for Periodic Systems

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Linear time invariant approximations of linear time periodic systems for integrated flight and vibration control

Many of the critical structural limits on rotorcraft are associated with vibratory loads and fatigue limits. These loads are strongly influenced by higher-harmonic dynamics (i.e., greater than 1-per-rotor-revolution, or 1/rev) in the rotor system. Linear Time-Periodic (LTP) systems are well-suited for representing vibratory loads on rotorcraft and have been used to derive Higher-Harmonic Control (HHC) laws that mitigate the vibratory loads. Despite being a powerful approach, HHC is expensive to implement on production rotorcraft and poses safety/reliability concerns. This talk discusses the development of rotorcraft flight control laws alternative to HHC that minimize rotor loads by acting solely through the primary flight controls (zeroth and first harmonic swashplate control). As opposed to HHC, this strategy could be readily integrated with existing or future Automatic Flight Control Systems (AFCS) on civil and military rotorcraft. The study considers control designs for both conventional and compound rotorcraft configurations. Further, since the rotorcraft manufacturers do not typically rely solely on linear systems obtained from simulation models for the development and integration of flight control laws on production rotorcraft, a novel methodology is proposed to identify LTP systems from flight-test data. The methodology is successfully applied to JUH-60A Black Hawk flight-test data. Broader implications of the methodology are presented in the context of Future Vertical Lift (FVL), flapping-wing Micro Aerial Vehicles (MAVs), and alleviation of aerodynamically-generated noise.

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Rotorcraft flight control design with alleviation of unsteady rotor loads

A theoretical investigation of dynamic response characteristics of rotor blades was carried out with special emphasis on torsional degrees-of-freedom. Coupled equations of motion for flapwise bending and torsion were formulated at varying azimuth positions for rotor blades with noncollinear aerodynamic, elastic and mass axes. Both structural and aerodynamic mass, damping and stiffness coefficients were included. The variations of a sample blade at different flight conditions were investigated from these equations. The obtained numerical results were illustrated. The sensitivity of overall blade vibration characteristics to torsional oscillations was also investigated from the equations of motion for the sample blade. The illustrated results show the importance of torsional degrees-of-freedom in rotor blade analysis. Various possibilities of improving the overall response by tuning blade geometric, structural and aerodynamic characteristics are discussed.

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Sensitivity analysis of torsional vibration characteristics of helicopter rotor blades. Part 2: Aerodynamics and sensitivity analysis

Computer program for determining characteristic functions of aeroelastic instabilities of helicopter rotor in forward flight

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R. P. White

Papers

A method for determining the characteristic functions associated with the aeroelastic instabilities of helicopter rotors in forward flight