scholarly journals An Analytical Method for Coaxial Helicopter Ground Resonance

2020 ◽  
Vol 316 ◽  
pp. 04005
Author(s):  
Xiangyi Liu ◽  
Shuyan Liu ◽  
Jing Wu ◽  
Shansong Song ◽  
Guocai Hu
Keyword(s):  
Analytical Method ◽  
Dynamic Instability ◽  
Instability Mode ◽  
Time Frequency ◽  
Ground Resonance ◽  
Body Roll ◽  
Time Domain Response ◽  
Mode Energy ◽  
Coaxial Helicopter ◽  
Equations Of Motions

A time-frequency analytical method is presented to analyze physical mechanism of coaxial helicopter ground resonance. Eigenvalue calculation and numerical integration of disturbance equations of motions are used to obtain modal characters and time-domain response characters of coaxial helicopter ground resonance, and the interaction between rotors and body is revealed according to response of various DOFs. The analysis results show that regressive lag mode with upper rotor character is the most instability mode. In dynamic instability region, coaxial helicopter ground resonance is mainly due to energy transferred between periodic lag motion of upper rotor and body roll rotation. For this instability mode, energy transferred between periodic lag motion of lower rotor and body roll rotation is also existed, and it can enhance ground resonance instability of coaxial helicopter.

The Optimization of Time-Frequency Control: Using Non-Autonomous Time-Delay Feedback Systems as Example

2016 ◽  
Author(s):  
Chi-Wei Kuo ◽  
C. Steve Suh
Keyword(s):  
Time Delay ◽  
Dynamic Instability ◽  
Frequency Control ◽  
Frequency Resolution ◽  
Control Technique ◽  
Time Step ◽  
Time Frequency ◽  
Control Scheme ◽  
Fxlms Algorithm ◽  
On Line

A novel time-frequency nonlinear scheme demonstrated to be feasible for the control of dynamic instability including bifurcation, non-autonomous time-delay feedback oscillators, and route-to-chaos in many nonlinear systems is applied to the control of a time-delayed system. The control scheme features wavelet adaptive filters for simultaneous time-frequency resolution. Specifically Discrete Wavelet transform (DWT) is used to address the nonstationary nature of a chaotic system. The concept of active noise control is also adopted. The scheme applied the filter-x least mean square (FXLMS) algorithm which promotes convergence speed and increases performance. In the time-frequency control scheme, the FXLMS algorithm is modified by adding an adaptive filter to identify the system in real-time in order to construct a wavelet-based time-frequency controller capable of parallel on-line modeling. The scheme of such a construct, which possesses joint time-frequency resolution and embodies on-line FXLMS, is able to control non-autonomous, nonstationary system responses. Although the controller design is shown to successfully moderate the dynamic instability of the time-delay feedback oscillator and unconditionally warrant a limit cycle, parameters are required to be optimized. In this paper, the setting of the control parameters such as control time step, sampling rate, wavelet filter vector, and step size are studied and optimized to control a time-delay feedback oscillators of a nonautonomous type. The time-delayed oscillators have been applied in a broad set of fields including sensor design, manufacturing, and machine dynamics, but they can be easily perturbed to exhibit complex dynamical responses even with a small perturbation from the time-delay feedback. These responses for the system have a very negative impact on the stability, and thus output quality. Through employingfrequency-time control technique, the time responses of the time-delay feedback system to external disturbances are properly mitigated and the frequency responses are also suppressed, thus rendering the controlled responses quasi-periodic.

On Controlling Non-Autonomous Time-Delay Feedback Systems

2015 ◽  
Author(s):  
Chi-Wei Kuo ◽  
C. Steve Suh
Keyword(s):  
Time Delay ◽  
Vibration Amplitude ◽  
Controller Design ◽  
Dynamic Instability ◽  
Spectral Response ◽  
Feedback System ◽  
Frequency Resolution ◽  
Qualitative Behavior ◽  
Time Frequency ◽  
Route To Chaos

Time-delay feedback oscillators of non-autonomous type are considered in the paper. These oscillators have been studied extensively for many decades in a broad set of fields such as sensor design, manufacturing, and machine dynamics. A time-delay model system having one time-delay constant and several nonlinear feedback terms in the governing differential equation is first studied. Many researches have demonstrated that a time-delay feedback even in the form of a small perturbation is able to perturb the oscillator to exhibit complex dynamical responses including bifurcation and route-to-chaos. These motions are harmful as they have a very negative impact on the stability, and thus output quality, of the system. For example, manufacturing processes that are characterized by time-delay feedback all have an operation limit on speed because the chaotic behaviors which are unpredictable and extremely unstable are difficult to control. With a viable control solution, the performance, quality, and capacity of manufacturing can be improved enormously. A novel concept capable of simultaneous control of vibration amplitude in the time-domain and spectral response in the frequency-domain has been demonstrated to be feasible for the control of dynamic instability including bifurcation and route-to-chaos in many nonlinear systems. The concept is followed to create a control configuration that is feasible for the mitigation of non-autonomous time-delay feedback oscillators. Featuring wavelet adaptive filters for simultaneous time-frequency resolution and filtered-x least mean square algorithm for online identification, the controller design is shown to successfully moderate the dynamic instability of the time-delay feedback oscillator and unconditionally warrant a limit cycle. The controller design that integrates all these features is able to mitigate dynamical deterioration in both the time and frequency domains and properly regulate the responses with the desired reference signal. Specifically the qualitative behavior of the controlled oscillator output follows a definitive fractal topology before settling into a stable manifold. The controlled response is categorically quasi-periodic and of the prescribed vibration amplitude and frequency spectrum. The control scheme is novel and requires no linearization. By applying wavelet domain analysis approach to the nonlinear control of instability, the true dynamics of the time-delay feedback system as delineated by both the time and frequency information are faithfully retained without being distorted or misinterpreted. Through employing adaptive technique, the high sensitivity of the time-delay feedback system to external disturbances is also properly addressed.

Mechanical Fault Diagnosis Using Numerical Hilbert Analysis

2002 ◽  
Author(s):  
Baozhong Yang ◽  
C. Steve Suh
Keyword(s):  
Failure Modes ◽  
Dynamic Instability ◽  
Negative Impact ◽  
System Response ◽  
Short Time Scale ◽  
Time Frequency ◽  
Long Time ◽  
Mechanical Faults ◽  
Temporal Events ◽  
Instantaneous Frequencies

Dynamic instability induced by the initiation and development of mechanical faults in a rotary element is known to have a large negative impact on the reliability and operation safety of an entire system. This type of nonlinear system response is generally perturbed by shock impulses of extremely short time scale and amplitude. Thus difficulty presents itself in identifying and isolating features indicative of the presence and progression of faults possibly leading to mechanical deterioration. The perturbed and deteriorated states of a bearing-shaft system subjected to the actions of various types of commonly seen mechanical faults are investigated using the Numerical Hilbert Transform. The presented approach characterizes and realizes temporal events of both short and long time scales as instantaneous frequencies in the joint time-frequency domain. Examples are given to demonstrate the feasibility of applying the approach to the characterization of various deteriorating bearing states and the identification of parameters associated with several failure modes.

Helicopter Ground Resonance Investigation With Dissimilar Nonlinear Lead-Lag Dampers

2015 ◽  
Author(s):  
Aykut Tamer ◽  
Pierangelo Masarati
Keyword(s):  
Dynamic Instability ◽  
Chaotic Behavior ◽  
Linear Time ◽  
Coupled System ◽  
Design Guidelines ◽  
Linear Time Invariant ◽  
Shock Absorbers ◽  
Ground Resonance ◽  
The Stability ◽  
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.

Experimental Identification of Linear and Nonlinear Dynamic Characteristics of a Foam Aircraft Model for Morphing Applications

2006 ◽  
Author(s):  
Raymond R. Joshua ◽  
Douglas E. Adams
Keyword(s):  
Frequency Response ◽  
Nonlinear Dynamic ◽  
Dynamic Instability ◽  
Impact Testing ◽  
Scale Model ◽  
Strong Nonlinearity ◽  
Frequency Range ◽  
Time Frequency ◽  
Model Aircraft ◽  
Linear And Nonlinear

The genesis of so-called morphing aircraft has been mandated by the need for an expanded flight envelope to fulfill a variety of flight objectives. In order to address the need for expanded flight envelopes, researchers must consider the possibility of dynamic instability due to morphing associated with complex aero-structural interactions during design, development, and flight. The linear and nonlinear dynamic response characteristics of a model aircraft structure are examined in this paper. To identify the linear modal characteristics, impact testing was conducted and the nominal frequency response functions were extracted. Nonlinear characteristics are identified using time-frequency, restoring force, and higher-order frequency response analysis of swept sine response data. The scale-model aircraft with reconfigurable pre-stressed components exhibits strong nonlinearity in the 20-40 Hz frequency range. A cubic stiffness nonlinearity is identified in one portion of the aircraft. In the 40-60 Hz frequency range, stiffness nonlinearities dominate and damping characteristics are primarily linear in nature. Effective characterization of the nonlinearity is a prerequisite for efficient reduced-order models that accurately predict dynamic instability.

scholarly journals Dynamic Instability of MRE Embedded Soft Cored Sandwich Beam with Non-Conductive Skins

2011 ◽  
Vol 18 (6) ◽  
pp. 759-788 ◽  
Author(s):  
S.K. Dwivedy ◽  
M. Srinivas
Keyword(s):  
Dynamic Instability ◽  
Parametric Instability ◽  
Core Loss ◽  
Sandwich Beam ◽  
Beam Theory ◽  
Skin Thickness ◽  
Axial Loads ◽  
Parametric Resonances ◽  
Complex Coefficients ◽  
Equations Of Motions

In this work the governing temporal equations of motions with complex coefficients have been derived for a three-layered unsymmetric sandwich beam with nonconductive skins and magnetorheological elastomer (MRE) embedded soft-viscoelastic core subjected to periodic axial loads using higher order sandwich beam theory, extended Hamilton's principle, and generalized Galerkin's method. The parametric instability regions for principal parametric and combination parametric resonances for first three modes have been determined for various end conditions with different shear modulus, core loss factors, number of MRE patches and different skin thickness. This work will find application in the design and application of sandwich structures for active and passive vibration control using soft core and MRE patches.

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