Author
Salini S. Nair
Bio: Salini S. Nair is an academic researcher from Indian Institute of Technology Madras. The author has contributed to research in topics: Ground resonance & Fuselage. The author has an hindex of 1, co-authored 3 publications receiving 3 citations.
Topics: Ground resonance, Fuselage, Phase (waves), Noise, Nonlinear system
Papers
More filters
01 Jan 2016
TL;DR: In this article, the phase relations between the fuselage states corresponding to the least damped mode (regressive lag mode) and its correlation with frequency coalescence were analyzed for ground resonance.
Abstract: Ground resonance is a type of aeromechanical instability that occurs when the helicopter is in contact with the ground. It may occur due to coalescence between frequencies of two modes of the system if damping is insufficient. In this paper, we analyze the phase relations between the fuselage states corresponding to the least damped mode (regressive lag mode) and its correlation with frequency coalescence. The phase of fuselage states (attitude or attitude rates), which are easily measurable, is observed to exhibit certain trends with variation in parameters like rotor speed and landing gear stiffness. The phase data can aid in the design of a stability augmentation system for ground resonance. It can serve as a parameter to detect the possibility of instability and in systems with uncertainties in parameters, like landing gear stiffness, it can aid in selecting the appropriate feedback gain for stabilization. The model we have primarily considered has isotropic rotor and anisotropic hub, hence multiblade coordinate transformation is used and the stability analysis is done in fixed frame. The analysis is further extended to incorporate dynamic inflow effects and anisotropy in rotor blades, where Floquet method is used for stability analysis. Air resonance instability is also investigated on similar lines and the proposed method is found to be good for its detection.
2 citations
••
TL;DR: In this article, the authors show that nonlinearity inherent in the lead-lag and flap equations will result in limit cycle oscillations and this is also demonstrated through experimental studies.
1 citations
01 Jan 2016
TL;DR: In this article, the effects of aerodynamic interactions between rotors on acoustics were investigated using an extended form of Peters-Morillo model and the Ffowcs Williams-Hawkings equation was used to determine the acoustic field.
Abstract: A good understanding of rotor noise mechanisms, such as blade-vortex, rotor-turbulence and rotor-wake interactions, is essential in order to devise strategies to control them and help in complying with noise regulations. In this paper, the effects of aerodynamic interactions between rotors on acoustics will be investigated. A quick but relatively accurate computation of main rotor-tail rotor as well as coaxial rotors interactions and the resulting unsteady loads that lead to strong acoustic emissions would be useful in developing optimum designs for rotor blades with low noise generating characteristics. The strategy is to utilize finite state wake models to capture the effects of unsteady aerodynamics that are computationally less intensive compared to CFD or free wake methods. The flow-fields above and below the rotor are computed using an extended form of Peters-Morillo model. The Ffowcs Williams-Hawkings equation is used to determine the acoustic field. Acoustic predictions for a coaxial rotor system and for a main rotortail rotor configuration with the incorporation of rotor-wake interaction effects are presented. These results are compared with the acoustic signatures obtained for the case of non-interacting rotors. The total noise for the aerodynamically interacting coaxial rotor system is found to be higher by 2-3 dB compared to the aerodynamically non-interacting coaxial rotor system. A similar analysis is done for noise levels of tail rotor which also predicts an increase in SPL by about 2 dB with the inclusion of interaction effects.
Cited by
More filters
01 Jan 1974
40 citations
••
TL;DR: In this article, a ground resonance test environment was created for soft-in-plane rotor systems with flexible landing gear models using direct finite-element models and applying modal reduction to embed CAD-derived landing gear model.
Abstract: Soft-in-plane rotor systems are susceptible to a self-induced vibration phenomenon called ground resonance. This dynamic instability results from lag motions of the rotor blades coupling with airframe degrees of freedom, while the helicopter is in ground contact. As an addition to slope landing studies in the past and investigations of non-linear landing gear effects, this work focuses on a systematic study of partial skid contact. A ground resonance test environment was created. It encompasses 3D helicopter models with flexible landing gear models using direct finite-element models and applying modal reduction to embed CAD-derived landing gear models. Both approaches are used in a multibody dynamics simulation. The models showed acceptable results for the simulation of non-linear dynamic behaviour including typical non-linear effects like limit cycles. Special focus is given to different methods of contact simulation, using 3D spring–damper elements and polygonal contact elements for multi-directional contacts. The simulations showed two counteracting effects for partial ground contact and time-variant contact conditions. On one hand, the reduction of restoring forces in partial ground contact should lead to more unstable conditions. On the other hand, energy dissipation shows a larger influence on the system stability behaviour after a sudden disturbance. This effect is of high interest for soft, partial landing conditions.