1,721,602 research outputs found
Finite element analysis with uncertain probabilities
No full textThis paper presents a general method for the finite element analysis of linear mechanical systems by taking into account probability density functions whose parameters are affected by fuzziness. Within this framework, the standard perturbation-based stochastic finite element method is relaxed in order to incorporate uncertain probabilities in static, dynamic and modal analyses. General formulae are provided for assessing the (fuzzy) structural reliability and several typologies of optimization problems (reliability-based design, robust design, robust/reliability-based design) are formalized. In doing this the credibility theory is extensively used to extract qualified crisp data from the available set of fuzzy results, so that standard optimizers can be adopted to solve the most important design problems. It is shown that the proposed methodology is a general and versatile tool for finite element analyses because it is able to consider, both, probabilistic and non-probabilistic sources of uncertainties, such as randomness, vagueness, ambiguity and imprecision. © 2010 Elsevier B.V
Structural Coupling and Whirl-Flutter Stability with Pilot-in-the-Loop
Full text linkThis paper investigates structural coupling problems for tiltrotors, considering not only the interaction of the flight control system with the flexible structure but also the potentially adverse effects on the aeroservoelastic stability that may be caused by the pilot's involuntary, high-frequency, biodynamic response. The investigation is focused on the analysis of the side effects that could appear at high speed in the airplane flight regime, where the whirl flutter boundaries may be significantly reduced. A detailed tiltrotor model, representative of the Bell XV-15 and of a flight control system has been built and joined with a pilot biodynamic model acting on the power-lever and on the center stick, available in the literature. Additionally, a modified version of the XV-15 using differential collective pitch for yaw control in airplane mode instead of rudder has been investigated to show the effect of different yaw control designs. The stability analyses presented demonstrate that the structural coupling analysis and the flutter boundaries for tiltrotors must be evaluated not only considering the closed loop created by the flight control system but also the effect of involuntary pilot response. Sensitivity analyses examine the most critical parameters impacting tiltrotor aeroservoelastic stability. Finally, the employment of notch filters as a means of prevention is discussed
Using MATLAB-simulink Rtw to Build Real Time Control Applications in User Space with RTAI-LXRT
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Turbulent Airwake Estimation from Helicopter–Ship Wind-Tunnel Data
Full text linkThis paper presents a stochastic approach for modeling the turbulent airwake suitable for real-time simulation of the helicopter–ship dynamic interface. This approach relies on the measurements of unsteady loads collected during a wind-tunnel test campaign with a scaled helicopter operating over the deck of simple frigate shape 1. Power spectral densities of the measured aerodynamic loads combined with the estimated frequency response functions are used to find, through an optimization algorithm, a model of airwake spectra over the range of frequencies which mainly affects the pilot workload during shipboard operations. Then, a set of autoregressive filters is designed for every particular rotor position and wind-over-deck condition, so that when driven by white noise, the spectrum of the output will reproduce those obtained from the optimization. This approach is applied to three different tested wind directions and three rotor positions by implementing the autoregressive filters into the multibody model of the experimental rotor. Frequency response analysis of the aerodynamic loads demonstrates that the turbulent airwake model obtained from the experimental data can predict the unsteadiness of loads comparable to those measured in the wind tunnel across the bandwidth of interest for pilot activities. The identified airwake models could be applied to a full-scale model to simulate the unsteady loads effectively experienced by the helicopter during a ship landing flight
Novel L-Shaped Gurney Flap for Rotorcraft Vibration Reduction
No full textThis work concerns an assessment of the rotor blade vibration reduction capabilities of a novel L-shaped trailing edge Gurney Flap. The primary effect of this L-tab is represented by a modification of the reference airfoil mean line shape, both in terms of camber and chord length, this latter being related to the two counter rotating vortical structures developed past the tab vertical prong. Previously validated computational fluid dynamics results are exploited to develop a physically based thin-line reduced order model, which successfully reproduces the mean line modifications induced by the L-tab, in addition to accurately capture the steady aerodynamic forces and the first harmonic of the unsteady loads generated by fixed configurations of the airfoil L-tab system and by oscillating motions of the movable device. A thin-line linear model is also developed for a blade section equipped with a classical trailing edge flap. Comparisons of the aerodynamic loads generated by these two movable devices for equal input oscillation laws, allow to estimate the ranges of reduced frequency where the L-tab is expected to perform better with respect to the trailing edge flap and vice-versa. These two reduced order models are then exploited to build up two separate three degrees of freedom linear aerostructural models for a blade equipped with a partial span L-tab or a trailing edge flap. A higher harmonic control algorithm is then applied and compared between the two devices to reduce separately the JV/rev harmonics of the blade root rotating frame vertical force, flapping and feathering moments. A significant reduction of the vibratory loads is obtained. Moreover, the attainment of similar results with a well known trailing edge device, such the classical flap taken under consideration, is a further confirmation of the potential feasibility of this novel L-tab as an effective alternative mean for vibration reduction on rotor blades
Multiple Input Describing Function for Non-Linear Analysis of Ground and Air Resonance
No full textAeroelastic stability is a key issue that drives the design of modern rotorcraft. The robustness of stability analysis is fundamental to determine the amount of freedom a designer has in defining the key properties in specific rotorcraft problems dominated by stability. The paper presents an effective technique to investigate the effect of nonlinearities on the ground resonance stability, resorting to the multi input describing function. In this way it is possible to investigate the cases when multiple harmonics are injected into a nonlinear component, a typical condition for rotorcraft components. To show the potential of the method, an application that considers a nonlinear model for hydraulic lead-lag dampers is presented
Limit analysis of masonry structures based on fictitious associative-type contact interface laws
No full textWe illustrate an original method for the limit analysis of masonry structures modeled as assemblies of dry rigid blocks with Coulomb-type (non-associative) contact interface laws. The method resorts to a fictitious system characterized by cohesive-type contact interface laws that depend on the axial forces of the real block system. Two theorems establish the connection between the collapse state of the real (frictional) block assembly and that of the fictitious one. Hence, an alternative problem of mathematical programming is presented to evaluate the minimum collapse load multiplier. According to the proposed formulation, the complementarity condition is not introduced as constraint but is obtained as Karush-Khun-Tucker condition. Several numerical results concerning with masonry arches, portals and panels are provided to illustrate the application of the proposed approach, which is also validated through the comparison with some existing methods
Exploration of the Effects of Rotor Blade Twist on Whirl-Flutter Stability Boundaries
Full text linkThis paper investigates the influence of tiltrotor blade twist on whirl-flutter stability boundaries. Preliminary evaluations indicate that the whirl-flutter speed can be increased if the blade twist slope is reduced. This positive effect results from the shift in the overall thrust toward the blade tip, increasing the flapwise bending moment at the blade root and the trim coning angle. This, in turn, generates a positive pitch-lag coupling, increasing the whirl-flutter speed. However, the shift of high sectional thrust forces toward the blade tip sections returns a higher induced drag, showing the tendency to increase the power required. The paper shows that, by using blade twist laws based on piecewise linear functions and adding the wing airfoil thickness as a second design parameter, it is possible to identify aircraft configurations that improve the whirl-flutter stability boundaries without penalizing the power required in airplane and helicopter mode flight. This is possible because the blade twist and the wing airfoil thickness have an impact on both power required and whirl-flutter speed, so a simple optimization algorithm can identify good tradeoffs. A detailed tiltrotor model representative of the Bell XV-15 is used to display the effectiveness of the proposed approach. The examples show that increases up to 21% on the whirl-flutter speed are achievable without penalties in the aircraft power required and with the additional benefit of a benign impact on rotor pitch link loads
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