Showing papers on "Helicopter rotor published in 2015"

Unbalance Compensation by Recursive Seeking Unbalance Mass Position in Active Magnetic Bearing-Rotor System

Abstract: In active magnetic bearing-rotor systems, unbalance compensation is used to force the rotor to spin around its geometric axis for better rotary precision. A method, which produces the control signal according to the real-time position of the rotor's unbalance mass, is proposed to actualize the unbalance compensation. Unlike the unbalance force, the unbalance mass' magnitude and position are the inherent property of the rotor and independent of the rotor speed. While the rotor speed is varying, the unbalance mass position needs not to be recalculated continuously and frequently. It will significantly reduce the computational cost. Additionally, the seeking step size in the proposed method is a constant in order to avoid the signal disturbance, so that the control can achieve the further steady-going convergence. The control performance in the condition with different unbalance phases, different seeking step sizes and different noise levels are analyzed in simulations. The experiments indicate that the proposed method achieves the good effectiveness to suppress the rotor vibration. Furthermore, it is of benefit to reducing the computational cost and suitable for application in the varying-speed operation.

Optimization of Damping Compensation for a Flexible Rotor System With Active Magnetic Bearing Considering Gyroscopic Effect

Abstract: Active magnetic bearing (AMB) levitated rotating machineries are always required to operate above the rotor first bending critical speed to achieve a high power density. It is important to ensure safe rotor run-down through critical speeds. This paper presents an optimization of damping compensation for a flexible rotor to make maximal use of the limited electromagnetic force to retain flexible deformation. The design, rotor modal properties, and bending model test for an AMB test rig which typifies a 10-kW centrifugal compressor are described in detail. The optimal compensation phase angle with and without considering the gyroscopic effect is first derived from a theoretical model. The flexible deformations with the different control radius and phase are also analyzed. Then, the phase angle from flexible deformation to electromagnetic force in the existing control system is experimentally identified. A phase-shift filter is designed to compensate the corresponding phase difference around the bending frequency. The damping compensation experiments validate the proposed optimization approach.

Differential infrared thermography for boundary layer transition detection on pitching rotor blade models

Abstract: Differential infrared thermography (DIT) was investigated and applied for the detection of unsteady boundary layer transition locations on a pitching airfoil and on a rotating blade under cyclic pitch DIT is based on image intensity differences between two successively recorded infrared images The images were recorded with a high framing rate infrared camera A pitching NACA0012 airfoil served as the first test object The recorded images were used in order to investigate and to further improve evaluation strategies for periodically moving boundary layer transition lines The measurement results are compared with the results of unsteady CFD simulations based on the DLR-TAU code DIT was then used for the first time for the optical measurement of unsteady transition locations on helicopter rotor blade models under cyclic pitch and rotation Image de-rotation for tracking the blade was employed using a rotating mirror to increase exposure time without causing motion blur The paper describes the challenges that occurred during the recording and evaluation of the data in detail However, the results were found to be encouraging to further improve the method toward the measurement of unsteady boundary layer transition lines on helicopter rotor models in forward flight

Three-dimensional reconstruction of helicopter blade–tip vortices using a multi-camera BOS system

Abstract: Noise and structural vibrations in rotorcraft are strongly influenced by interactions between blade–tip vortices and the structural components of a helicopter. As a result, knowing the three-dimensional location of vortices is highly desirable, especially for the case of full-scale helicopters under realistic flight conditions. In the current study, we present results from a flight test with a full-scale BO 105 in an open-pit mine. A background-oriented schlieren measurement system consisting of ten cameras with a natural background was used to visualize the vortices of the helicopter during maneuvering flight. Vortex filaments could be visualized and extracted up to a vortex age of 360°. Vortex instability effects were found for several flight conditions. For the camera calibration, an iterative approach using points on the helicopter fuselage was applied. Point correspondence between vortex curves in the evaluated images was established by means of epipolar geometry. A three-dimensional reconstruction of the main part of the vortex system was carried out for the first time using stereophotogrammetry. The reconstructed vortex system had good qualitative agreement with the result of an unsteady free-wake panel method simulation. A quantitative evaluation of the 3D vortex system was carried out, demonstrating the potential of the multi-camera background-oriented schlieren measurement technique for the analysis of blade–vortex interaction effects on rotorcraft.

Bifurcation analysis for 2:1 and 3:1 super-harmonic resonances of an aircraft cracked rotor system due to maneuver load

Abstract: This paper focuses on the local bifurcation characteristics of an aircraft cracked rotor system mainly for the 2:1 and 3:1 super-harmonic resonances induced by the maneuver load. The motion equations of the system are formulated with the consideration of the nonlinear stiffness of the Duffing type and the breathing of a transverse crack on the shaft, as well as the maneuver load induced by the climbing and diving flight of the aircraft. By using the multiple scales method, the motion equations are analytically solved to obtain the bifurcation equations for 2:1 and 3:1 super-harmonic resonances, respectively. Furthermore, the two-state variable singularity method is employed to analyze the local bifurcation characteristics of the system affected by crack coefficient and maneuver load. For each case, two curves of hysteresis set dividing $$K-G$$ parameter plane into three regions are demonstrated. Accordingly, bifurcation modes for different parameter combinations from the three regions and the two curves are obtained. The approach in this paper will provide an effective and convenient way to analyze the bifurcation characteristics of dynamical systems. The results in this paper will contribute to a better understanding of the effect of the maneuver load on the response and bifurcation characteristics of aircraft cracked rotor systems.

Stability and steady-state response analysis of a single rub-impact rotor system

Abstract: Using a lumped mass model of a single rub-impact rotor system considering the gyroscopic effect, the stability and steady-state response of the rotor system are investigated in this paper. The contact between the rotor and the stator is described by the simple Coulomb friction and piecewise linear spring models. An algorithm combining harmonic balance method with pseudo arc-length continuation is adopted to calculate the steady-state vibration response of a nonlinear system. Meanwhile, Hill’s method is used to analyze the stability of the system. The nonlinear dynamic characteristics of the system are investigated when the gap size, stator stiffness and unbalance are regarded as the control parameters. The results show that the gap size determines the location of the rub-impact; besides, the smaller gap can improve the stability of the system. The unsteady motion can be found as the stator stiffness increases. Moreover, the unbalance directly affects vibration amplitude, which becomes greater with the increasing imbalance.

Dynamic characteristics of a disk–drum–shaft rotor system with rub-impact

Abstract: The dynamic model of a disk–drum–shaft rotor system with rub-impact is established and its dynamic characteristics are analyzed. According to an elastic impact model and Coulomb’s friction law, two rub-impact force models are firstly developed which corresponding to the disk–stator contact and the drum–stator contact, respectively. Taking into account, the coupling of the whole rotor whirl and the local drum vibration, the dynamic model of a disk–drum–shaft rotor system with the disk–stator and the drum–stator rubbing is established by employing Sanders’ shell theory and Lagrange equation. The numeric results reveal that the continuous increase in rotating speed, disk mass eccentricity, and stator radial stiffness induces alternation of periodic-one motion of the disk without disk–stator contact, periodic-one motion with disk–stator full annular rubbing and the disk–stator partial rubbing region in which the quasi-periodic and periodic motions of the disk appear alternately. In addition, the motion of the drum is synchronous with that of the disk and has three situations, i.e., un-deformation, deformation with one circumferential wave, and periodic and quasi-periodic vibration with drum–stator rubbing. Due to the effects of the drum, the rotating speed corresponding to the start of the disk–stator full annular rubbing in the disk–drum–shaft rotor system is smaller than that in the disk–shaft rotor system, and there is only one disk–stator partial rubbing region which is near the critical speed of rotor system. What is more, compared with parameters of the disk, those of the drum can only affect the motion of the rotor system slightly.

Multi-fault diagnosis of rotor system based on differential-based empirical mode decomposition:

Abstract: Multi-fault diagnosis of rotor systems is a topical issue in the research of rotor dynamics. Empirical mode decomposition (EMD) has been introduced in fault diagnosis of rotor systems, which has proved to be usable and efficient in single fault diagnosis of a rotor system. However, in multi-fault diagnosis of a rotor system, because of the existence of a high-frequency fault signal, EMD is not as powerful as the diagnosis of a single fault in dealing with the decomposition of multi-faults. In this paper, differential-based empirical mode decomposition (DEMD) is introduced into multi-fault diagnosis of a rotor system. The analysis consequences show that the combination of any two faults among crack, rub-impact and pedestal looseness in multi-fault diagnosis can be decomposed successfully and efficiently under the application of the DEMD method, which is also more accurate and precise than the traditional EMD method.

Controllability Analysis for Multirotor Helicopter Rotor Degradation and Failure

Abstract: fi = lift of ith rotor, N g = acceleration of gravity, kg · m∕s h = altitude of helicopter, m Jx, Jy, Jz = moment of inertia around roll, pitch, and yaw axes of helicopter frame, kg · m Ki = maximum lift of ith rotor, N kμ = ratio between reactive torque and lift of rotors L,M, N = airframe roll, pitch, and yaw torque of helicopter, N · m m = number of rotors ma = mass of helicopter, kg p, q, r = roll, pitch, and yaw angular velocities of helicopter, rad∕s ri = distance from center of ith rotor to center of mass, m T = total thrust of helicopter, N vh = vertical velocity of helicopter, m∕s ηi = efficiency parameter of ith rotor φ, θ, ψ = roll, pitch, and yaw angles of helicopter, rad

Flight Dynamics Investigation of Compound Helicopter Configurations

Abstract: Compounding has often been proposed as a method to increase the maximum speed of the helicopter. There are two common types of compounding known as wing and thrust compounding. Wing compounding offloads the rotor at high speeds, delaying the onset of retreating blade stall, and hence increasing the maximum achievable speed, whereas with thrust compounding, axial thrust provides additional propulsive force. There has been a resurgence of interest in the configuration due to the emergence of new requirements for speeds greater than those of conventional helicopters. The aim of this paper is to investigate the dynamic stability characteristics of compound helicopters and compare the results with a conventional helicopter. The paper discusses the modeling of two compound helicopters, which are named the coaxial compound and hybrid compound helicopters. Their respective trim results are contrasted with a conventional helicopter model. Furthermore, using a numerical differentiation technique, the dynamic stabil...

Super-harmonic responses analysis for a cracked rotor system considering inertial excitation

Abstract: In this paper, an investigation on the nonlinear vibration, especially on the super-harmonic resonances, in a cracked rotor system is carried out to provide a novel idea for the detection of crack faults in rotor systems. The motion equations of the system are formulated with the consideration of the additional excitation from an inertial environment as well as the forced excitation of the rotor unbalance. By using the harmonic balance method, the analytical solutions of the equations with four orders of harmonic exponents are obtained to analyze the nonlinear response of the system. Then through numerical calculations, the vibration responses affected by system parameters including the inertial excitation, the forced excitation, the crack and damping factors are investigated in detail. The results show that the occurrence of the super-harmonic resonances of the rotor system is due to the interaction between crack breathing and the inertial excitation. Correspondingly, the super-harmonic responses are significantly affected by the inertial excitation and the crack stiffness (or depth). The rotor unbalance, however, does not make apparent effects on the super-harmonic responses. Consequently, the super-harmonic resonances peaks can be viewed as an identification signal of the crack fault due to the application of the inertial excitation. By utilizing the inertial excitation, the super-harmonic response signals in rotor systems with early crack faults can be amplified and detected more easily.

Experimental Investigation of Micro Air Vehicle Scale Helicopter Rotor in Hover

Abstract: This paper describes a fundamental experimental study, which involved systematic performance and flowfield measurements (PIV) to understand and optimize the hover performance of a MAV-scale helicopter rotor operating at Reynolds numbers lower than 30,000. The rotor parameters that were varied include blade airfoil profile, blade chord, number of blades, blade twist, planform taper and winglets at blade tip. Blade airfoil section had a significant impact on the hover efficiency and among the large number of airfoil sections tested, the ones with the lower thickness to chord ratios and moderate camber (4.5% to 6.5%) produced the highest rotor hover figure of merit. Increasing the solidity of the rotor by increasing the number blades (with constant blade chord) had minimal effect on efficiency; whereas, increasing the solidity by increasing blade chord for a 2-bladed rotor, significantly improved hover efficiency. Moderate blade twist (-10° to -20°) and large planform taper (larger than 0.5) marginally impro...

Aeroacoustic analysis of a high-speed open rotor

Abstract: Owing to their inherent fuel efficiency, there is renewed interest in developing open rotor propulsion systems that are both efficient and quiet. The major contributor to the overall noise of an open rotor system is the propulsor noise, which is produced as a result of the interaction of the airstream with the counter-rotating blades. Prediction of the propulsor noise is, therefore, a necessary ingredient in any approach for designing low-noise open rotor systems that can meet community noise regulations and have acceptable cabin noise levels. To that end, there has been a resurgence of activities in the aeroacoustic modeling of open rotors in recent years. While direct numerical simulations are gaining traction, the bulk of existing prediction capability resides in hybrid approaches in which the aerodynamics of the open rotor system is computed via CFD and is used as input in some appropriate “linear” acoustic model for computing the open rotor noise. At NASA the focus has been on assessing the utility o...

Independent speed and attitude control for a rotary wing aircraft

Abstract: One aspect is a flight control system for independent speed and attitude control of a rotary wing aircraft that includes a main rotor system and a translational thrust system. The flight control system includes a flight control computer configured to interface with the main rotor system and the translational thrust system. The flight control computer includes processing circuitry configured to execute control logic. A pitch attitude reference generator provides a pitch attitude reference to a main rotor controller to command the main rotor system based on pilot input. A longitudinal reference generator produces a longitudinal reference as a longitudinal position or longitudinal velocity based on pilot input. An attitude-to-propulsor crossfeed converts the pitch attitude reference into a propulsor trim adjustment. A propeller pitch controller combines the longitudinal reference and the propulsor trim adjustment into a propeller command, and provides the propeller command to the translational thrust system.

Transient Hub Loads and Blade Deformation of a Mach-Scale Coaxial Rotor in Hover

Abstract: A Mach-scale, rigid, counter-rotating coaxial rotor system, 80 inches in diameter and operating at a tip speed of 190 m/s was tested in hover in two configurations: two-bladed single rotor and two bladed counter-rotating coaxial rotor. Individual upper and lower rotor steady and vibratory hub loads were collected at four operating conditions for each rotor configuration. An in-place, uncoupled dynamic calibration was performed on both upper and lower rotor load cells using an electrodynamic shaker to correct unsteady thrust load measurements. The lower coaxial rotor exhibits significant four-per revolution vibrational loads with an average amplitude 11% of the mean thrust Full span three-dimensional blade deformations were measured for a single lower rotor blade at 11 azimuth locations between 66◦ and 126◦ with 6◦ resolution via the digital image correlation (DIC) optical method. From these measurements high resolution elastic axis flap-bending displacements, as well as blade sectional pitch angles were extracted and examined. The lower coaxial rotor was found to operate at higher pitch angles while also exhibiting increased variation in tip bending displacement when compared to the isolated rotor at similar blade loadings.

Experimental investigation on misalignment fault detection in induction motors using current and vibration signature analysis

Abstract: In the rotating machinery one of the commonly known reasons of vibration is misalignment of rotating shaft. In this present work experimental investigation is carried out on three phase induction motor to detect rotor misalignment. Proximity and current probes are used to monitor the vibration and current signal respectively. Fast Fourier Transform is used for signal processing. A full spectrum analysis is presented for both current signal and vibration signal to reveal the fault-specific whirl signatures. The results clearly indicate the potential and feasibility of thediscussed approach for the rotor misalignment diagnosis in a shaft/rotor system coupled with a three phase induction motor.

Rotor rub impact fault detection method based on nonlinear compression conversion and rotor rub impact fault detection system based on nonlinear compression conversion

Abstract: The invention discloses a rotor rub impact fault detection method based on nonlinear compression conversion and a rotor rub impact fault detection system based on nonlinear compression conversion, and the method and the system are used for detecting the rub impact fault of a rotor system in a rotating machine. The method comprises the following steps of performing nonlinear compression conversion on a vibration signal of the rotor system so as to obtain time-frequency representation of the vibration signal; calculating the instantaneous frequency of the vibration signal in combination with the time-frequency representation; further calculating the frequency spectrum of a vibration part of the instantaneous frequency; if fr is the rotating frequency of the rotor system, the maximal spectrum peak of the frequency spectrum is within a range of [0.99fr,1.01fr] and the amplitude value is greater than 2% of the rotating frequency, judging that the rotor system has a rub impact fault; otherwise, judging that the rotor system does not has a rub impact fault. The system is implemented on the basis of the method and is capable of facilitating application of the method. By virtue of the method and the system, whether the rub impact fault exists or not can be accurately judged, and the result is rapid and reliable; the method and the system are simple and easy and are applicable to online health monitoring of a rotor.

Mathematical Model of a Monocopter Based on Unsteady Blade-Element Momentum Theory

Abstract: This paper presents a simplified nonlinear dynamic mathematical model of a monocopter. A monocopter is an all-rotating unmanned aerial vehicle with a design inspired by a samara, which is the seed from a maple tree. The aim of the model is to describe the essential dynamics of a monocopter in various regimes of flight. The model is based on the unsteady blade-element momentum theory and combines methodologies that are found both in helicopter and wind turbine theories. A qualitative validation of the proposed model shows that the obtained simulation results are in good agreement with the empirical findings and the simulation results of a more advanced monocopter model. The results also agree with the predictions based on helicopter theory and the stability study of a samara seed. The paper demonstrates that simpler methods (such as the unsteady blade-element momentum theory) could be applied to develop efficient and computationally undemanding monocopter models, which are suitable for further research in ...

Effect of Duct-Rotor Aerodynamic Interactions on Blade Design for Hover and Axial Flight

Abstract: Computational analysis of several ducted rotor systems is performed to determine the effect of rotor inflow on thrust generation in hover and axial flight. An outboard-biased rotor inflow has been shown to increase the performance of a hovering ducted rotor system. The duct-rotor aerodynamic interactions and their resulting effects on rotor inflow are studied to provide insights on duct-rotor design. Multiple duct-rotor configurations are studied using high-fidelity CFD in hover and axial flight for various twist distributions and increasing rotor collective values. Performance and efficiency gains are shown for multiple duct-rotor configurations in hover. Rotor inflow calculations show an outboard bias that allows rotors to be operated more efficiently for a range of collective angles. Benefits of outboard-biased rotor inflow in axial flight for a fixed collective were minimal. This analysis shows the extent to which performance and efficiency benefits are achieved for a ducted rotor based on the behavior of rotor inflow.

Acoustic signature variation of aircraft utilizing a clutch

Abstract: An aircraft is provided including an airframe, an extending tail, and a counter rotating coaxial main rotor assembly including an upper rotor assembly and a lower rotor assembly. A translational thrust system including a propeller is positioned at the extending tail. The translational thrust system provides translational thrust to the airframe. A drive system is configured to operate both the main rotor system and the translational thrust system. The drive system includes a clutch configured to selectively couple or decouple the propeller from the drive system.