Generalized quantitative stability analysis of time-dependent comprehensive rotorcraft systems

Rotorcraft stability is an inherently multidisciplinary area, including aerodynamics of rotor and fuselage, structural dynamics of flexible structures, actuator dynamics, control, and stability augmentation systems. The related engineering models can be formulated with increasing complexity due to the asymmetric nature of rotorcraft and the airflow on the rotors in forward flight conditions. As a result, linear time-invariant (LTI) models are drastic simplifications of the real problem, which can significantly affect the evaluation of the stability. This usually reveals itself in form of periodic governing equations and is solved using Floquet’s method. However, in more general cases, the resulting models could be non-periodic, as well, which requires a more versatile approach. Lyapunov Characteristic Exponents (LCEs), as a quantitative method, can represent a solution to this problem. LCEs generalize the stability solutions of the linear models, i.e., eigenvalues of LTI systems and Floquet multipliers of linear time-periodic (LTP) systems, to the case of non-linear, time-dependent systems. Motivated by the need for a generic tool for rotorcraft stability analysis, this work investigates the use of LCEs and their sensitivity in the stability analysis of time-dependent, comprehensive rotorcraft models. The stability of a rotorcraft modeled using mid-fidelity tools is considered to illustrate the equivalence of LCEs and Floquet’s characteristic coefficients for linear time-periodic problems

Robust Stability Analysis: a Tool to Assess the Impact of Biodynamic Feedthrough on Rotorcraft

Biodynamic feedthrough (BDFT) may significantly affect the closed-loop behavior of rotorcraft, reducing the stability and increasing the proneness to Rotorcraft-Pilot Couplings (RPC). Leveraging robust stability analysis, the inherently uncertain pilot BDFT can be treated as the uncertain portion of a feedback system, allowing analytical, numerical or graphical determination of proneness to RPC by comparing robust stability margins of helicopter models with BDFT data. The application of the proposed approach to collective bounce is exemplified using simple analytical helicopter and pilot BDFT models, and applied to detailed helicopter models and BDFT measurement data

A Numerical Study of Vibration-Induced Instrument Reading Capability Degradation in Helicopter Pilots

Rotorcraft suffer from relatively high vibratory levels, due to exposure to significant vibratory load levels originating from rotors. As a result, pilots are typically exposed to vibrations, which have non-negligible consequences. Among those, one important issue is the degradation of instrument reading, which is a result of complex human-machine interaction. Both involuntary acceleration of the eyes as a result of biodynamics and vibration of the instrument panel contribute to a likely reduction in instrument reading capability, affecting flight safety. Therefore, being able to estimate the expected level of degradation in visual performance may give substantial benefits during vehicle design, allowing to make necessary adjustments while there is room for design changes or when retrofitting an existing aircraft to ensure the modifications do not adversely affect visual acuity and instrument reading ability. For this purpose, simulation is a very valuable tool as a proper model helps to understand the aircraft characteristics before conducting flight tests. This work presents the assessment of vibration-induced visual degradation of helicopter pilots under vibration exposure using a modular analysis environment. Core elements of the suggested analysis framework are an aeroelastic model of the helicopter, a model of the seat-cushion subsystem, a detailed multibody model of the human biodynamics, and a simplified model of ocular dynamics. These elements are combined into a comprehensive, fully coupled model. The contribution of each element to instrument reading degradation is examined, after defining an appropriate figure of merit that includes both eye and instrument panel vibration, in application to a numerical model representative of a medium-weight helicopter

Prediction and Simulator Verification of Roll/Lateral Adverse Aeroservoelastic Rotorcraft–Pilot Couplings

The involuntary interaction of a pilot with an aircraft can be described as pilot-assisted oscillations. Such phenomena are usually only addressed late in the design process when they manifest themselves during ground/flight testing. Methods to be able to predict such phenomena as early as possible are therefore useful. This work describes a technique to predict the adverse aeroservoelastic rotorcraft–pilot couplings, specifically between a rotorcraft’s roll motion and the resultant involuntary pilot lateral cyclic motion. By coupling linear vehicle aeroservoelastic models and experimentally identified pilot biodynamic models, pilot-assisted oscillations and no-pilot-assisted oscillation conditions have been numerically predicted for a soft-in-plane hingeless helicopter with a lightly damped regressive lead–lag mode that strongly interacts with the roll modeat a frequency within the biodynamic band of the pilots. These predictions have then been verified using real-time flight-simulation experiments. The absence of any similar adverse couplings experienced while using only rigid-body models in the flight simulator verified that the observed phenomena were indeed aeroelastic in nature. The excellent agreement between the numerical predictions and the observed experimental results indicates that the techniques developed in this paper can be used to highlight the proneness of new or existing designs to pilot-assisted oscillation