Suppression of ground resonance in rotorcraft using active landing gear

This article examines the fundamental aspects of controlling ground resonance in rotorcraft equipped with actively controlled landing gear. Ground resonance is a mechanical instability affecting rotorcraft on the ground. It occurs at certain rotor speeds, where the lead–lag motion of the rotor couples with the motion of fuselage creating a self-excited oscillation. Typically, passive or semi-active lag dampers are used to avoid instability; however, these are undesirable from a design and maintenance perspective. Innovations in active landing gear for rotorcraft, such as articulated robotic legs, have provided an alternate approach to avoid the instability, eliminating the need for lag dampers with respect to ground resonance. This article extends classic ground resonance to include movable landing gear and identifies key physical parameters affecting dynamic behavior. Applying LQ optimal control to this model, it is shown that ground resonance instability can be eliminated using active landing gear as the control mechanism, even when there is no lag damping present in the rotor. In addition, while superior performance is achieved when landing gear movement can occur both longitudinally and laterally, it is still possible to stabilize ground resonance with inputs in a single direction, albeit with reduced performance.

Nonlinear dynamics and control of helicopter ground resonance

Ground resonance is an aero-mechanical instability in helicopters that use soft in-plane rotors. Traditionally, ground resonance is mitigated by using passive lead–lag dampers that provide sufficient in-plane damping. However, these dampers because of their passive nature cannot adapt to all operating conditions. In this work, a magnetorheological fluid–based semi-active lead–lag damper is proposed to offer controllable damping. Two nonlinear control strategies are reported to operate the voltage to be supplied to the magnetorheological damper. The first strategy is a model-based control using dynamic inversion. The second is a fuzzy logic control integrated with a particle swarm optimization algorithm to optimize the control parameters. Both control strategies are shown to be effective in eliminating ground resonance. Unlike bang–bang control, the prescribed control algorithms can make use of complete voltage level available in the magnetorheological damper with smooth voltage updates. A comparative study of the controller performances is made through appropriate performance indices and system responses. Finally, the most optimum control strategy to mitigate ground resonance is inferred.

Modeling and Testing of Fluidic Flexible Matrix Composite Lead–Lag Dampers

A new lead–lag damper concept using a fluidic flexible matrix composite (F2MC) tube is presented in this paper. A model is developed for an articulated rotor blade integrated with an F2MC damper consisting of an F2MC tube, an inertia track, an orifice, and a hydraulic accumulator. Benchtop tests using a 4.5-ft rotor blade demonstrate the performance of a smallscale F2MC damper. The blade–damper system model predictions are verified by comparing experimentally measured and model-predicted frequency response data. In benchtop tests, the model predicts blade damping ratios of up to 0.34 with the F2MC damper. A simplified articulated blade based on the UH-60 rotor is simulated to assess the feasibility of a full-scale F2MC damper. Simulation results predict that the damper can generate blade damping ratios of over 0.30 at low blade lag angles.

Helicopter Ground Resonance Investigation With Dissimilar Nonlinear Lead-Lag Dampers

This work describes the analysis of helicopter ground resonance when nonlinearity and non-isotropy of the problem are taken into account. Ground resonance is a dynamic instability caused by the interaction between the rotor and the airframe of a helicopter. Sources of nonlinearity can be geometrical (finite blade lead-lag motion) and constitutive (hydraulic lead-lag dampers and shock absorbers). Standard methods use special coordinate transformations that make it possible to cast the problem in linear, time invariant form when considering small oscillations of an isotropic rotor about a reference solution. However, potential non-isotropy of the rotor (e.g. resulting from degraded performance of lead-lag dampers) may turn the problem into linear, time periodic. In such cases, the Floquet-Lyapunov method is normally used to study the stability of the coupled system. In this work the problem is investigated using Lyapunov Characteristic Exponents (LCE). The analysis shows that in some cases, characterized by a marked contribution of the nonlinearity of the blade lead-lag dampers, the problem assumes a nearly chaotic behavior. The stability of the system is investigated, and the sensitivity of the LCEs with respect to system parameters is determined, in an attempt to provide a consistent analysis framework and useful design guidelines.

Load Reduction in Lead–Lag Dampers by Speed-Scheduled Aperture and Modulated Control of a Bypass Valve

Existing hydraulic dampers are passive devices, and their damping characteristics cannot be adapted to the varying damping needs of each different flight condition of a rotorcraft. Therefore, dampers and their interfaces to the rotor system are constantly subjected to large damping forces, which contribute to their damage and wear. In this work, we consider the possible use of a bypass valve as a means to reduce loads while still providing the required amount of damping for safe flight. Two different policies for the use of the bypass are considered: In the first and simplest of the two, the valve is opened of a constant amount, which varies only as a function of the vehicle flight speed, whereas in the second the valve aperture is modulated as a function of blade azimuth using a higher harmonic feedback law. The investigation on the load-reduction capabilities of the bypass augmented damper is conducted by using validated mathematical models of the A109E Power helicopter and of its damper. It is found that both policies are capable of delivering the required level of damping at substantially reduced loads, which for the speed-scheduled aperture drop at values that range between 30% and 60%, depending on the flight condition, of the loads generated by the standard passive device, and for the higher harmonic modulated law at values around 20%–25%.