Showing papers on "Helicopter rotor published in 2018"

Unbalance Compensation for Active Magnetic Bearing Rotor System Using a Variable Step Size Real-Time Iterative Seeking Algorithm

Abstract: It is necessary to reduce the vibration of rotors supported on active magnetic bearings (AMBs) with high displacement precision requirement in operation. An unbalance compensator for an AMB-rigid rotor system is studied. The dynamic model of an AMB-axially symmetric rigid rotor system was first established. Then, the specific form of compensating currents in the AMBs was obtained. At last, an unbalance compensator based on real-time variable step size (VSS) polygonal iterative seeking algorithm for equivalent unbalance coefficients of the rotor at the AMBs positions was proposed. With the compensator, the AMBs produce additional forces that are of same magnitudes but opposite phases as the equivalent unbalance forces at the AMBs positions to reduce the rotor vibration. Since the equivalent unbalance coefficients, which represent the equivalent imbalance of the rotor at the AMBs positions, are independent of the rotating speed, thus, the compensator is suitable for application in both steady state and varying-speed operations. With the VSS method, both the accuracy and the convergence rate of the identification algorithm could be realized. Numerical simulations and experiments confirmed the effectiveness of the proposed method for suppressing the unbalance vibration of the AMB-rotor system in steady states and varying-speed operations.

Nonlinear vibration control of a horizontally supported Jeffcott-rotor system:

Abstract: A positive position feedback (PPF) controller is proposed to control the nonlinear vibrations of a horizontally supported Jeffcott-rotor system. A nonlinear restoring force and the rotor weight are...

Nonlinear response analysis of a rotor system with a transverse breathing crack under interval uncertainties

Abstract: Parametric uncertainties are present in complex mechanical systems due to various reasons such as material dispersion and wear. This paper investigates the effects of interval uncertain parameters on the dynamic behaviors of a rotor system with a transverse breathing crack in the shaft. The uncertainties are modeled as uncertain-but-bounded interval inputs on the basis that no sufficient prior information is available to define their precise probabilistic distributions. A finite element rotor model is formulated and the harmonic balance method (HBM) is employed to solve the deterministic dynamic problem. Based on the Chebyshev approximation theory, a surrogate model for the uncertain problem is established and then used to determine the bounds of the nonlinear dynamic responses. The accuracy verification is performed by comparing with the scanning method. Simulations are carried out considering different uncertainties with several uncertain degrees. It will provide some references for early crack fault detection and condition monitoring in rotor systems with uncertainties included.

Design, control, and visual navigation of the DelftaCopter VTOL tail-sitter UAV

Abstract: To participate in the Outback Medical Express UAV Challenge 2016, a vehicle was designed and tested that can autonomously hover precisely, takeoff and land vertically, fly fast forward efficiently, and use computer vision to locate a person and a suitable landing location. The vehicle is a novel hybrid tail‐sitter combining a delta‐shaped biplane fixed‐wing and a conventional helicopter rotor. The rotor and wing are mounted perpendicularly to each other,and the entire vehicle pitches down to transition from hover to fast forward flight where the rotor serves as propulsion. To deliver sufficient thrust in hover while still being efficient in fast forward flight, a custom rotor system was designed. The theoretical design was validated with energy measurements, wind tunnel tests, and application in real‐world missions. A rotor‐head and corresponding control algorithm were developed to allow transitioning flight with the nonconventional rotor dynamics that are caused by the fuselage rotor interaction. Dedicated electronics were designed that meet vehicle needs and comply with regulations to allow safe flight beyond visual line of sight. Vision‐based search and guidance algorithms running on a stereo‐vision fish‐eye camera were developed and tested to locate a person in cluttered terrain never seen before. Flight tests and a competition participation illustrate the applicability of the DelftaCopter concept.

Fractional-order modeling and dynamic analyses of a bending-torsional coupling generator rotor shaft system with multiple faults

Abstract: Unexpected vibrations induced by the crack fault and other unbalance factors in rotor system seriously affect the health and reliability of the generator. Here, to explore the vibration performances, a bending-torsional coupling model of the generator rotor shaft system is established, in which electromagnetic malfunction (unbalanced magnetic pull) and mechanical failures (fractional-order damping, crack and contact-rubbing) are considered. Then, the simulation is done by a modified Adams-Bashforth-Moulton algorithm. Based on the simulation, the correctness of the new coupling model is verified by comparing with previous model and experimental data. At the same time, the new coupling model is analyzed to obtain the dynamic evolutions of the generator rotor shaft system with the changes of crack depth ratio, the fractional order of damping, rotational speed ratio and mass eccentricity of rotor. In addition to this, some critical values and ranges are proposed. Finally, these results can efficiently provide a theoretical reference for the design of generator rotor system and be applied to forecasting and diagnosing vibration faults in generator rotor shaft system.

On the Effect of Rotational Forces on Rotor Blade Boundary-Layer Transition

Abstract: Laminar-turbulent boundary-layer transition is investigated on the suction side of Mach-scaled helicopter rotor blades in climb and analyzed in view of the effect of rotational forces. Transition p...

A tunable dynamic vibration absorber for unbalanced rotor system

Abstract: A tunable dynamic vibration absorber for unbalanced rotor system which is made up of coil springs and magnetic spring is presented. The structure of the absorber is introduced and the stiffness tuning mechanism of the magnetic spring is explained. A finite element model of the rotor-absorber system was built and the influencing factors to the appearance of the absorber were studied numerically. Finally, experiments were carried out to verify the numerical results, and PID control strategy was tested. The numerical and experimental results show that the present absorber is effective for vibration suppression of an unbalanced rotor system, and the control strategy is effective.

Sommerfeld effect in a two-disk rotor dynamic system at various unbalance conditions

Abstract: Near resonance behavior of a shaft-rotor system driven by a motor with limited power gives rise to the Sommerfeld effect wherein the rotor speed gets stuck till a specific level of excitation power is reached. In this paper, a semi-analytical method is used to observe the Sommerfeld effect of a two-disk rotor system driven through a direct current motor. The effect of relative unbalance position on vibration amplitude of a two disk rotor-shaft system is studied in detail. We observe the change in critical power input for passage through resonance by changing the unbalance positions on the disks. We also investigate the case where two close resonance frequencies interfere with the jump phenomenon associated with the Sommerfeld effect. Prediction of critical power input in complex rotor system is highly essential. Else the system may be destroyed due to large vibration amplitudes generated during operations. In this article, drive and rotor dynamics are modeled together by using multi-energy domain bond graph approach and theoretically obtained steady-state characteristics are validated through transient simulations.

Investigating the Aeroelastic Behaviors of Rotor Blades with Nonlinear Energy Sinks

Abstract: The aim of this paper is to investigate the effectiveness of the nonlinear energy sink on the aeroelastic behaviors of rotor blades. For this purpose, a single hingeless helicopter rotor blade in a...

Nonlinear Free Vibration of Spinning Viscoelastic Pipes Conveying Fluid

Abstract: Drill strings are one of the most significant rotor components employed in oil and gas exploitation. In this paper, an improved dynamical model of drill-string-like pipes conveying fluid is develop...

New crack breathing mechanism under the influence of unbalance force

Abstract: In this paper, a new analytical model (unbalanced one), which considers the coupling effects of unbalance force, rotor weight, and rotor physical and dimensional properties, is developed to study the actual breathing mechanisms of the transverse fatigue crack in a cracked rotor system. The results are also compared with those of the existing balanced model, where only rotor weight is considered. It has been identified that a crack in the unbalanced model breathes differently from the one in the balanced model. A crack’s breathing mechanism in the unbalanced model depends strongly on its location along shaft length. At some special locations, a crack in the unbalanced model may remain fully closed or open during the shaft rotation, which will never occur in a balanced model. It may also behave completely like the one in the balanced shaft. Depending on the crack location, unbalance force magnitude and orientation, the unbalanced shaft may be stiffer or more flexible than the balanced counterpart. It is also demonstrated that the unbalanced model will progressively approach balanced one as unbalance force decreases. Further, different crack breathing mechanisms between two models lead to a large difference along shaft length in the second area moment of inertia, which forms the elements of local stiffness matrix at crack location. It is expected that more accurate prediction of the vibration response of a cracked rotor can be achieved when the effect of unbalance force and rotor properties on the crack breathing has been taken into account.

Generation of Mars Helicopter Rotor Model for Comprehensive Analyses

Abstract: The present research is aimed at providing a performance model for the Mars Helicopter (MH), to understand the complexity of the flow, and identify future regions of improvement. The low density of the Martian atmosphere and the relatively small MH rotor, result in very low chord-based Reynolds number flows. The low density and Reynolds numbers reduce the lifting force and lifting efficiency, respectively. The high drag coefficients in subcritical flow, especially for thicker sections, are attributed to laminar separation from the rear of the airfoil. In the absence of test data, efforts have been made to explore these effects using prior very low Reynolds number research efforts. The rotor chord-based Reynolds number range is observed to be subcritical, which makes boundary layer transition unlikely to occur. The state of the two-dimensional rotor boundary layer in hover is approximated by calculating the instability point, laminar separation point, and the transition location to provide understanding of the flow state in the high Mach-low Reynolds number regime. The results are used to investigate the need for turbulence modeling in Computational Fluid Dynamics (CFD) calculations afterwards. The goal is to generate a performance model for the MH rotor for a free wake analysis, because the computational budget for a complete Navier-Stokes solution for a rotating body-fitted rotor is substantial. In this study, a Reynolds-Averaged Navier-Stokes (RANS) based approach is used to generate the airfoil deck using C81Gen with stitched experimental data for very high angles of attack. A full Grid Resolution Study is performed and over 4,500 cases are completed to create the full airfoil deck. The laminar separation locations are predicted within the accuracy of the approximate method when compared with the CFD calculations. The model is presented through airfoil data tables (c81 files) that are used by comprehensive rotor analysis codes such as CAMRADII, or the mid-fidelity CFD solver RotCFD. Finally, the rotor performance is compared with experimental data from the 25ft Space Simulator at the NASA Jet Propulsion Laboratory (JPL) and shows good correlation for the rotor Figure of Merit over the available thrust range.