Showing papers on "Helicopter rotor published in 2012"

Dynamic Stall Control Using Deployable Leading-Edge Vortex Generators

Abstract: A new concept of active dynamic stall control is proposed, designed and experimentally tested on a OA209 airfoil model. The active control principle is based on leading-edge vortex generation in order to alleviate the dynamic stall vortex formed and convected at the leading-edge of an airfoil operating at a helicopter blade in fast forward flight. The active device aims to beused only during retreating blade side for dynamic stallflight conditions in order to avoid drag penalties on the advancing blade side. The designed actuator is a row of deployable vortex generators (DVGs) located at the leading-edge of the airfoil that fit the airfoil shape when retracted. Deployment is possible for different heights as well as different phases and frequencies with respect to the airfoil oscillation. The paper addresses the validation of the effectiveness of the devices to delay static stall and alleviate dynamic stall penalties. Results show a delay in static stall angle of attack of 3 deg and a reduction of negative pitchingmoment peak up to 60% for dynamic stall. The analysis of the experimental database indicates that different compromises between lift and pitchingmoment can be achieved depending on the phase actuation of the DVGs.

Parametric instability of a rotor-bearing system with two breathing transverse cracks

Abstract: When the rotor rotates at a constant speed, the transverse crack opens and closes alternatively, due to gravity, and thus a “breathing effect” occurs. This variance in shaft stiffness is time-periodic, and hence a parametrically excited system is expected. The parametric excitation from the time-varying stiffness causes instability and severe vibration under certain operating conditions. Current research mostly focused on the rotor with single transverse crack. There are few studies on the multi-cracked rotor system. In fact, the interaction between the multiple parametric excitations with various phasing and amplitude, which are induced by the multiple breathing transverse cracks, would make the instability behavior of the system differ distinctly from that of the single cracked rotor system. Moreover, how the instability regions change with various crack breathing mechanisms should also be investigated. Thus, the parametric instability of a rotor-bearing system with two breathing transverse cracks is studied in the paper. First, the finite element equations of motion are established for the cracked rotor system. Two types of crack breathing mechanisms, of which one is more accurate (new) and the other is empirical (old), are adopted in the finite element formulation. Then, a generalized Bolotin's method is introduced for determining the boundaries of the primary and secondary instability regions. Based upon these, instability analysis for a practical used rotor-bearing system with single and two cracks are conducted, respectively. The instability regions induced by the single transverse crack with new and old breathing mechanisms are compared with each other. For the two-cracked rotor system, the variations of the unstable boundaries with crack depths, orientation angles and positions are observed and discussed in detail. It is shown from the results that the dynamic instability of the two-cracked rotor-bearing system indeed have some unique features that differ from that of the single cracked rotor system.

Measurement of helicopter rotor blade deformation using digital image correlation

Abstract: An experimental study on the application of the digital image correlation (DIC) technique to measure the deformation of rotating helicopter blades is described. Measurements on two different rotors of diameter 24 and 39 in., with different rotor hubs, were carried out to explore applicability of the technique over a range of scales. Commercial DIC software was synchronized with the frequency of rotation such that rotor blade images could be obtained at a constant rotor azimuth. Bending and torsion mode shapes were extracted from the data with deformation as high as 0.4 in. measured with an accuracy of 0.0038 in. This technique is very advantageous because it is noncontact, cost effective, accurate and simple to implement while yielding full-field, three-dimensional data with a high spatial resolution.

A surrogate-based approach to reduced-order dynamic stall modeling

Abstract: The surrogate-based recurrence framework (SBRF) approach to reduced-order nonlinear unsteady aerodynamic modeling associated with pitching/plunging airfoils subject to xed or time-varying freestream Mach numbers is described. Using full-order solutions generated by the OVERFLOW CFD code, the SBRF reduced-order modeling approach is shown to eectively mimic full-order solutions of unsteady lift, moment, and drag under dynamic stall conditions, but at a fraction of the computational cost. In addition to accounting for realistic helicopter rotor blade dynamics, it is shown that the SBRF can model advancing rotor blade stall due to shock induced separation, as well as retreating blade stall associated with excessive angles of attack. Therefore, the SBRF reduced-order modeling approach is ideally suited for a variety of aeroelasticity and active/passive design optimization studies that require high delity aerodynamic response solutions with minimal computational expense.

Mesh Deformation Method for Rotor Flows

Abstract: Helicopter blades experience large in-flight deformations that affect the aerodynamics of the rotor. Consequently, computational fluid dynamics (CFD) methods applied to helicopter flows must have appropriate algorithms to account for the blade deformation without deterioration in the CFD mesh quality. In this work, a hybrid mesh deformation method, suitable for use with helicopter blades, is proposed. The method begins by accounting for the blade deformation using a modal structural model. The interpolation between the finite element and CFD meshes for the blade surface is based on the constant-volume tetrahedron method and is combined with transfinite interpolation as well as the spring analogy method. The final algorithm is efficient and resulted in deformed meshes with good qualities. A range of rotor cases were considered, and the changes in volume of the CFD cells were less than 30% of their original values. The cell skewness was also kept at acceptable levels. The mesh deformation method was coupled with the helicopter multiblock CFD solver, and computations were undertaken for rigid and deformed blades. It was found that the structural deformation affected the blade loads even for hovering rotor cases, although it had a more pronounced effect in forward flight. The mesh method was efficient and accounted for less than 1% of the total central processing unit time.

The flow around a model helicopter main rotor in ground effect

Abstract: Wind tunnel measurements of the wake below and ahead of a model helicopter main rotor in simulated forward flight in ground effect are presented. The wind tunnel used was equipped with a rolling road, and the ground speed was matched to the wind tunnel speed for a representative simulation in the wind tunnel of forward flight over the ground. Particle image velocimetry was used to investigate the structure of the wake, and it was observed that the moving ground had a remarkable effect on the flow; the wake is closer to the rotor and its size is reduced compared with the stationary ground case. The detailed distribution of vorticity within the wake is affected by the moving ground, and the mechanism for this is discussed in the paper.

Performance of a Turboshaft Engine for Helicopter Applications Operating at Variable Shaft Speed

Abstract: An off-design steady state model of a generic turboshaft engine has been implemented to assess the influence of variable free power turbine (FPT) rotational speed on overall engine performance, with particular emphasis on helicopter applications. To this purpose, three off-design flight conditions were simulated and engine performance obtained with different FTP rotational speeds were compared. In this way, the impact on engine performance of a particular speed requested from the main helicopter rotor could be evaluated. Furthermore, an optimization routine was developed to find the optimal FPT speed which minimizes the engine specific fuel consumption (SFC) for each off-design steady state condition. The usual running line obtained with constant design FPT speed is compared with the optimized one. The results of the simulations are presented and discussed in detail. As a final simulation, the main rotor speed Ω required to minimize the engine fuel mass flow was estimated taking into account the different requirements of the main rotor and the turboshaft engine.Copyright © 2012 by ASME

Method for displaying images and/or other information on aircraft blades

Abstract: A method for displaying information via a light source on rotating helicopter blades of an aircraft such as a helicopter is provided The light source may be a laser light source and the light therefrom may be directed onto the aircraft blade by one or more motion controlled mirrors and a computer controller A sensor is capable of determining the position of helicopter blades during rotation, such that light may be projected upon only the moving blade at precisely timed intervals, so as to form perceivable graphics and/or messages thereon In addition, the method may be utilized for displaying graphics and/or messages on rotating propellers of fixed wing aircraft

Multi-Objective Optimization of Helicopter Airfoils Using Surrogate-Assisted Memetic Algorithms

Abstract: solver. First, the peculiar features of the algorithm are described with particular attention to its advantages when compared with more traditional evolutionary or gradient-based algorithms. Finally, the results of the optimizations carried out using different operating conditions are presented; starting from the optimal Pareto fronts, several solutions are selected and compared in terms of shapes and performance.

Extensions of prescribed wake modelling for helicopter rotor BVI noise investigations

Abstract: The prediction of blade–vortex interaction noise is highly sensitive to the blade–vortex missdistance and thus requires the knowledge of the vortex position relative to the blade interacting with it. The generally accepted solution to the problem is to apply either free-wake codes (vortex lattice methods) or computational fluid dynamics (CFD) codes, both of which are very time consuming, especially CFD. Prescribed wake codes are computationally faster in comparison, since they are based on steady rotor operational conditions only, but they are considered to be not accurate enough due to the prescription of the geometry based on a few general parameters. First, they lack any wake geometry response due to variations in rotor loading distributions at constant thrust caused by, e.g., non-uniformity of the aerodynamic environment in forward flight, elastic blade motion, or by any means of active control. Second, wake deflections due to the presence of the fuselage are ignored. This paper provides an approximate solution to both problems, based on simple momentum theory considerations for the first problem, and based on the displacement flow of the fuselage for the second. This provides extensions for any existing prescribed wake model to account for rotor loading distribution and fuselage effects on the prescribed wake geometry to a first order of accuracy, sufficient for the investigations of the sensitivity analysis of noise radiation due to variations of blade design or due to applications of active control.

Hovering rotor computations using an aeroelastic blade model

Abstract: In this paper, helicopter rotor blades are analysed in hover using computational fluid dynamics (CFD) coupled with a structural model. The method relies on a mesh deformation algorithm that allows for exchange of forces and deformations between a beam-based finite-element model and the fluid flow volume mesh. The method is demonstrated against experimental data, and the aerodynamic predictions appear to improve when the aeroelastic model is used. For all employed cases the flexibility of the method allows the CFD mesh deformation to be spread over the computational domain in a controlled fashion. The influence of the aeroelastic deformations on the blade loads was limited yet evident on the rotor performance. The lack of adequate test cases and experiments for validation of CFD/CSD methods is also highlighted.

Preliminary Study on a Wide-Speed-Range Helicopter Rotor/Turboshaft System

Abstract: this paper, a preliminary performance assessment of the coupling between a wide-speed-range helicopter rotor and the connected turboshaft engine was carried out. For this purpose, a well-known test case has been chosen (UH-60A helicopter equipped with a GE-T700 engine) to study the implications of such coupling from both the helicopter and theengineviewpoints.Asimple helicopterrotorcalculationmodelisusedtodeterminetheoptimumrotorspeedand required rotor power in various flight conditions. Next, these data are used for the engine-performance analysis, which was carried out using a zero-dimensional, block-based validated engine model to determine the fuel weight necessary to accomplish a given simple mission (i.e., to fly in trimmed forward flight for a given time). The results obtained are compared with the baseline helicopter/engine configuration, and the implications of using variablespeed technology are clearly highlighted.

Nonlinear dynamic analysis of a rotor system with fixed-tilting-pad self-acting gas-lubricated bearings support

Abstract: Based on the nonlinear theory, the unbalanced responses of a fixed-tilting-pad gas-lubricated journal bearing-rigid rotor system are investigated. A time-dependent mathematical model is established to describe the pressure distribution of gas-lubricated journal bearing with nonlinearity. The rigid rotor supported by a fixed-tilting-pad self-acting gas-lubricated journal bearing is modeled. The differential transformation method has been employed to solve the time-dependent gas-lubricated Reynolds equation, and the dynamic motion equation has been solved by the direct integral method. The unbalanced responses of the rotor system supported by fixed-tilting-pad gas-lubricated journal bearings are analyzed by the orbit diagram, Poincare map, time series, and spectrum diagram. The numerical results reveal periodic, period-3, and quasiperiodic motions of nonlinear behaviors of the system. Finally, the effects of pivot ratio and preload coefficient on the nonlinear dynamic characteristics of the fixed-tilting-pad self-acting gas-lubricated journal bearing-rotor system are investigated.

Clutch system for rotary-wing aircraft with secondary thrust system

Abstract: An aircraft includes a powerplant system operable to power a main rotor system and a secondary thrust system, the secondary thrust system is selectively driven through operation of a clutch system, and a clutch system synchronization time corresponds to a response time of the powerplant system.

Electromechanical actuator for helicopter rotor damper application

Abstract: Development trends in aeronautics involve a better employment of electric motors also in safety critical hazardous applications till now covered by mechanical systems. The electromechanical actuators are gaining a growing interest thanks to their force and power density capability, and the high dynamical performance by electronic control, allowing the design of very compact high efficiency drives with satisfactory characteristics from the reliability point of view. The present paper highlights the actuation solution adopted in the development of a rotor damper system for helicopter application, based on permanent magnet motors. Design details will be presented focused on the integration of the electrical machine in the specific application. Reports of analyses and tests carried out on the motor prototypes are included to confirm the capabilities and the performances of the proposed solution.

Theoretical and experimental study on the dynamic response of multi-disk rotor system with flexible coupling misalignment

Abstract: The dynamic response of a multi-disk rotor system with coupling misalignment is investigated theoretically and experimentally, considering the nonlinear oil film force. The rotor is simplified to a lumped mass model and the governing equations are derived considering the gyroscopic effect. The reacting forces and moments caused by misalignment are treated as excitations to the rotor system. The unbalanced responses of the system with/without misalignment are calculated using a numerical integration method and comparisons made. Spectrum cascades are utilized to obtain the overall view of the response characteristics during the starting up of the rotor. The modified Bode plot is used to trend the amplitude variation of different frequency components. The study indicates that coupling misalignment can cause 2X, 3X, 4X, and other multiple frequency responses. The amplitude of the 2X vibration could be larger than that of the 1X one, depending on the misalignment level. Also, the amplitude of 2X vibration decr...

Nonlinear dynamics of a rotor-ball bearing system with Alford force

Abstract: The nonlinear dynamic behaviors of an unbalanced rotor system supported on ball bearings with Alford force are investigated. In the rotor model, the rotor unbalance that varied with rotating speed,...

Surrogate based design optimisation of composite aerofoil cross-section for helicopter vibration reduction

Abstract: Design optimisation of a helicopter rotor blade is performed. The objective is to reduce helicopter vibration and constraints are put on frequencies and aeroelastic stability. The ply angles of the D-spar and skin of the composite rotor blade with NACA 0015 aerofoil section are considered as design variables. Polynomial response surfaces and space filling experimental designs are used to generate surrogate models of the objective function with respect to cross-section properties. The stacking sequence corresponding to the optimal cross-section is found using a real-coded genetic algorithm. Ply angle discretisation of 1 degrees, 15 degrees, 30 degrees and 45 degrees are used. The mean value of the objective function is used to find the optimal blade designs and the resulting designs are tested for variance. The optimal designs show a vibration reduction of 26% to 33% from the baseline design. A substantial reduction in vibration and an aeroelastically stable blade is obtained even after accounting for composite material uncertainty.

A wind tunnel investigation of ship airwake/rotor downwash coupling using design of experiments methodologies

Abstract: A wind tunnel investigation was performed to examine the interaction between ship airwakes and helicopter rotor downwash. This interaction is known to limit helicopter flight envelopes and increase overall pilot workload. A simplified 1/50th scale naval frigate model and appropriately scaled rotor model were chosen for this fundamental investigation. Particle image velocimetry (PIV) surveys were conducted to investigate the ship airwake/rotor downwash flowfield and to devise a velocity based coupling analysis technique to quantify the level of coupling between the rotor and airwake flowfields. The developed coupling technique examines the component-wise velocity discrepancies between the experimentally observed flowfield and the flowfield generated by superposition. Significant aerodynamic coupling was found below a rotor-over-deck height of Z/D=1.2 for an advance ratio of 0.075 and zero wind-over-deck angle. Detailed rotor thrust surveys were conducted as a first step in correlating thrust changes to coupled regions.

Multiblade Reduced-Order Aerodynamics for State-Space Aeroelastic Modeling of Rotors

Abstract: Reduced-order aerodynamic models are tools that may be conveniently applied in a wide range of research and design applications in the aeronautical and mechanical fields. This paper presents a methodology for the identification of a reduced-order model (ROM) describing the linearized unsteady aerodynamics of helicopter rotors in arbitrary steady flight, which is particularly suited for the derivation of the state-space perturbation aeroelastic operators and is hence useful for stability analysis and aeroservoelastic applications. It is defined in terms of multiblade coordinates and yields a (finite state) constant-coefficient, linear, differential form relating them to the corresponding multiblade aerodynamic loads. This approach requires the prediction of a set of harmonic perturbation responses by an aerodynamic solver. The accuracy of the identified ROM in describing unsteady aerodynamics phenomena is strictly connected to that of the aerodynamic solver. Complex aerodynamic effects (like wake roll-up and wake―blade interactions) are included in the ROM if they are taken into account in evaluating the harmonic responses. Numerical results concerning a flap-lag helicopter rotor in forward flight are presented. These examine the accuracy the aerodynamic ROM introduced both in terms of aerodynamic loads predictions and in terms of aeroelastic stability analysis. An aeroservoelastic application is also included in order to demonstrate the suitability of the ROM proposed for the design of controllers.

System and method of determining rotor loads and motion

Abstract: A system for reconstructing sensor data in a rotor system that comprises a rotating component of the rotor system, a plurality of sensors in the rotating component to sense at least one of loads and motion characteristics in the rotating component and to generate sensor data, and an analysis unit to generate reconstructed sensor data from the sensor data using numerical analysis for low-rank matrices.

Dynamic analysis of modified composite helicopter blade

Abstract: In the present study, modal analysis has been performed on modified Gazelle helicopter blade. The construction of the blade is fully composite with the honeycomb core. The approach to determining structure mode shapes and natural frequencies is presented. Modified blade consists of core material, 3D unidirectional composite spar and thin carbon composite facesheets as blade skin. To determine the stiffness of the honeycomb core, the equivalent mass approach was used. Several methods of eigenvalue extraction have been investigated in order to find optimal method which can be used in dynamic analysis of composite structures containing honeycomb cores. Among all extraction methods investigated, it was found that combined Lanczos method is most effective in terms of accuracy and CPU time for eigenvalue extraction in composite structures with honeycomb core having large number of degrees of freedom. Strain energies for first four mode shapes of modified helicopter blade have been calculated using numerical approach and results are presented.

Unsteady Reynolds-Averaged Navier-Stokes-Based Hybrid Methodologies for Rotor-Fuselage Interaction

Abstract: L IFTING bodies produce wakes that interact with other bodies immersed in the same fluid. In particular for rotorcraft, the problem becomes significantly more complicated since the rotor wake remains near the vehicle in hover, descent, and low-speed forward flight. The proximity of the wake alters the inflow distribution at the rotor andmodifies the helicopter thrust.Moreover, since the main rotor wake may impinge on the fuselage, such interactions are an important consideration in modern rotorcraft design. For example, empennage impingement may result in undesirable handling qualities such as low-speed pitchup and tail buffet. Moreover, the wake can also generate unsteady impulsive loads on the fuselage, resulting in vibrations, thus negatively impacting the crew and passenger flight experience. Given the complexity of rotorcraft interactional aerodynamics problems, it is common for tail and empennage designs to be modified significantly after first flight [1]. Development of many aerospace technologies, not limited to helicopter rotor–fuselage applications, requires accurate resolution of both nearand far-field flow phenomena. Numerical prediction of wakes involve a tradeoff between accuracy, turnaround time, and computational expense [2]. Current grid-based computational fluid dynamics (CFD) codes can theoretically model the entire flowfield, but resolution and preservation of wake features become difficult since typical grid sizes used in industrial simulations are susceptible to numerical dissipation. The artificial diffusion of vorticity that results can be mitigated using grid adaptation techniques and higherorder methods [2–4], but this may not be practical for all applications since computational cost increases significantly. For this reason, computationally efficient hybrid methods may be more attractive, especially during design and for flight-test support. Traditional Lagrangian free-wake methods are inexpensive but become less accurate when vortex elements in the wake become distorted and tangled due to interactions with other vortices and solid bodies (i.e., rotor blades and the fuselage) [5]. These interactions typically occur in the rotor near field, which motivates coupling to a CFD solver to resolve the highly viscous and possibly compressible flow near the rotor. In such an approach, the CFD code does not have to resolve the entire wake region; thus, the size of the CFD domain can be greatly reduced and computational efficiency maximized. Additional challenges associated with surface interactions arise when modeling problems such as rotor–fuselage interactions. The ability of two approaches to hybridize a Reynolds-averaged Navier–Stokes (RANS) CFD solver and a free-wake method for the rotor–fuselage interaction problem is investigated. Predictions are compared with experimental data, as well as prior numerical predictions made with individual code simulations.