Rotor Hub Research Articles

Design and Analysis for Multi-Layered Elastomeric Bearing on Rotor Hub Considering Large Axial Load and Nonlinear Motion

This paper examines the initial sizing theory of elastomeric bearings. In addition, the manufacturing process of the elastomeric bearing was analyzed to define the essential contents necessary to manufacture reliable elastomeric bearings. Mooney-Rivlin parameters were presented based on the test results to predict the characteristics of metal materials and rubber stacked. Mooney-Rivlin parameters were inputted to determine whether the structure was abnormal, by constructing a finite element model for the elastomeric bearing. A modeling technique was established to meet the structural rigidity and strength requirements of full elastomeric bearings, critical components of the helicopter main rotor system. In addition, the flight situation in which the maximum load of an actual helicopter equipped with an elastomeric bearing can be applied was selected, and linear structural analysis and nonlinear analysis were performed to confirm the behavior of the elastomeric bearing. As a result of performing linear static analysis, a negative margin was generated, but when nonlinear analysis was performed again, it was confirmed that there was a sufficient safety margin. It was shown that design reliability for the molding and manufacturing process of elastomeric bearings could be improved in terms of strength.

Steady and Unsteady Numerical Characterization of the Secondary Flow Structures of a Highly Loaded Low-Pressure Compressor Stage

This paper presents the numerical characterization of a highly loaded compressor by means of 3D unsteady RANS simulations. The focus is on critical flow structures and their evolution at different operating points of the machine. First, the numerical setup and mesh quality are presented to support the reliability of the provided results. The comparison against experiments is then described for this purpose. Later, a full description of the unsteady behavior of the machine is provided, giving special attention to the two regions where the most critical features are expected: the rotor hub wall and the casing. Rotor–stator interactions are then investigated and the role of the inlet guide vane (IGV) is finally discussed. Results are analyzed at design and near-stall conditions, with a focus on the behavior close to the stability limit at 100% speed.

Analytical Model Development for Rotors Hovering Above Heaving Surfaces

A potential flow model was developed for rotors hovering in-ground-effect above a heaving surface using a similar methodology to classical, analytical, static ground effect models. Experimental performance measurements for rotors hovering above a surface undergoing single degree-of-freedom heaving motion were used for model validation, and potential mission trends were generated for a representative naval helicopter. Unlike prior empirical models, the current model was able to capture the effect of rotor hub height as well as ground motion parameters. A new thrust ratio was proposed that compared the thrust produced in-ground-effect above a heaving surface to that produced above a static surface. Better agreement was found to occur at the higher collective pitch settings and hub heights, with the model predicting the classical thrust ratio within 5% of the measured value for approximately 75% of the test points. The thrust ratio relative to a static surface was found to produce better agreement, and approximately 80% of the experimental tests points were predicted within 5% of the measured value.

Helicopter performance enhancement by alleviating retreating blade stall using active flow control

In this study, a model was created numerically to study the effect of active flow control on helicopter blade to improve the retreating blade stall. Active flow control is applied to helicopter blades to improve retreating blade stall along with the overall helicopter performance. The novel aspect of this study is using active control in a helicopter with a complex aerodynamic environment and using it in all significant flight modes. This has the advantage of practical application since active control can be used constantly without having to turn it off. The flow control is established by using blowing jets at the leading edge of the airfoil that is used at the tip part of the main rotor blade. 2D and 3D computational fluid dynamics (CFD) Turbulent-unsteady models are developed to investigate the effect of applying blowing jets on different helicopter modes of flight (hover and forward flight). Different excitation parameters and turbulence models are compared to get the maximum possible enhancement of the aerodynamic characteristic of the flow both in 2D and 3D. The results are validated against a NASA helicopter main rotor hub with known geometry and available performance parameters. Both models illustrated a good agreement with the published benchmark problem. The results revealed that the active control blowing jets can enhance the stall characteristics and aerodynamic performance of helicopter blades. The method delays the flow separation and increase the blade lift by about 10% in addition to a 40% increase in the lift to drag ratio. Thus, we can conclude that the present active control technique can enhance the helicopter in forward flight by delaying the retreating blade stall. This research offers insightful information on the usage of blowing jets for rotor flow control.

Dynamic analysis and validation of a multi-body floating wind turbine using the moving frame method

This research applies the moving frame method (MFM) to the multi-body dynamic analysis of an OC3 phase IV spar buoy with the NREL 5MW turbine. Further, it verifies previous results obtained through numerical comparisons with commercial software. The long-term goal is to lay the foundation for leveraging the MFM to create a self-contained software system for future analyses that can incorporate effects that are more sophisticated, when commercial codes fall short. In this first evidentiary phase, this project treats the floating turbine as a three-bodied system consisting of the platform (platform + tower), nacelle and rotor (hub + blades). Then the paper presents the MFM in a tutorial style—in the context of this problem’s resolution. The paper supplements the multi-body dynamic equations of motion obtained through the MFM with simplified and reduced hydrodynamic, aerodynamic and mooring loads to simulate the translational and rotational response of the floating turbine under various load conditions. The results closely approximate those found in previous work and, in the process, demonstrates MFM’s analytical advantage. Current results capture the coupled responses in all degrees of freedom and gyroscopic effects occurring when the platform pitches with the spinning rotor. The project thus provides an accurate model for the dynamics of the turbine and opens the door to inserting correct advanced hydrodynamics to validate the model further. The work presents simulations for the different load cases through a 3D web page using WebGL and the ThreeJS library. Users may download all software to verify the results. An undergraduate student conducted the work alone, demonstrating the ease of implementation of the MFM.

Fuzzy Neural Network PID Control Used in Individual Blade Control

In order to further reduce the vibration level of helicopters, the active vibration control technology of helicopters has been extensively studied. Among them, individual blade control (IBC) independently applies high-order harmonics to each blade with an actuator, which can improve the aerodynamic environment of the blade and effectively reduce the vibration load of the hub. The rotor structural dynamics model based on the Hamilton energy variation principle and the medium deformation beam theory were established firstly, and the aerodynamic model based on the dynamic inflow model and the Leishman–Beddoes unsteady aerodynamic model were also established. The structural finite element method and the direct numerical integration method were used to calculate the vibration response of the rotor to determine the vibration load of the hub. After these, the steepest descent-golden section combinatorial optimization algorithm was used to find the optimization parameters of IBC. Based on this, the input parameters of fuzzy neural network PID control were determined, and the rotor hub vibration load control simulation was conducted. Under the effect of IBC, the vibration loads of the hub could be reduced by about 60%. The article gives the best control laws of individual harmonic pitch control and their combinations. These results can theoretically be applied to the design of control law to reduce helicopter vibration loads.

An intelligent wind turbine with yaw mechanism using machine learning to reduce high-cost sensors quantity

In this paper, with the assistance of some tools and a machine learning model, a smart wind turbine was formed that eliminates some expensive sensors and reduces sensor complexity. Squirrel cage induction generator (SCIG) and six rotor blades make up the proposed design, and depending on the wind's direction, the turbine itself can rotate the rotor hub to produce energy more effectively. Additionally, two stepper motors are coupled to the yaw mechanism with the aid of the rotor hub, and the entire controlling procedure will depend on the direction of the wind. The rotor hub must continuously revolve in the same direction as the wind to maximize wind energy utilization. Additionally, to correctly predict wind degrees, a machine learning model was deployed. Random forest regression was used to train and predict the wind direction. The model is deployed in Raspberry Pi, where the incoming sensor values are being stored. Using the generated data, machine learning model was trained and it can be concluded that the model can potentially replace some of the expensive sensors to reduce cost. The model can be used for similar weather conditions only based on machine learning model and fewer sensors.

A Computational Examination of Side-by-Side Rotors in Ground Effect

This study investigates the interactional aerodynamics of hovering side-by-side rotors in ground effect. The 5.5-ft diameter, three-bladed fixed-pitched rotors are simulated using computational fluid dynamics at a targeted 5 lb/ft2 disk loading. Simulations are performed using the commercial Navier–Stokes solver, AcuSolve ®, with a delayed detached eddy simulation model. Side-by-side rotors are simulated at two heights above the ground (H/D = 0.5 and H/D = 1), and with two hub–hub separation distances (3R and 2.5R). The performance of side-by-side rotors in ground effect (IGE) is compared to isolated rotors out of ground effect. Between the side-by-side rotors IGE, a highly turbulent mixing region is identified where the wakes of each rotor collide. The flow fountains upwards, as well as exits outwards (along a direction normal to a plane connecting the two rotor hubs) with fountaining between the rotors reaching up to 0.75D above the ground. As blades at H/D = 0.5 traverse the highly turbulent flow, strong vibratory loading is induced and a thrust loss is observed over the outboard sections between the rotors that is large enough to negate any nominal ground effect benefits inboard. Side-by-side rotors at H/D = 0.5 with 2.5R hub–hub spacing produce peak-to-peak thrust oscillations up to 16% of the steady thrust. Rotors placed higher, at H/D = 1 are positioned above the turbulent mixing flow and produce significantly lower vibratory loads. The spacing between rotors at H/D = 0.5 and 3R hub–hub separation allows strong vortical structures to develop between the rotors which move from side to side over multiple revolutions. When the vorticity moves closer to one of the rotors, it produces a greater lift deficit over the outboard region and a stronger vibratory loading. For rotors closer together, at H/D = 0.5 and 2.5R separation, the vortical structures between rotors are constrained to a more concentrated area and show less side-to-side drift.

Вплив форми крила на характеристики при використанні інтегрованої роторної силової установки літака

The subject of the study is the optimization of aerodynamic characteristics by improving the wing profile shape for the fan-wing configuration. The aim of this work is to find a more universal wing shape suitable for various aircraft. The task is to compare the characteristics of three wings with the same rotor engine installed in the nose section, including the base wing used in most previous studies by scientists. Sources suggested that this profile is not well-suited for rotor engines with a large aspect ratio. Experimental methods were used to measure the dynamic characteristics of three wing profiles: a flat curved profile with a cut-out in the rear wing section, a flat lower surface profile, an S-shaped profile with a lower surface bend in the rotor area, and a two-bend contour with a cut-out in the rear wing section on top and bottom. The results showed an improvement in characteristics compared with wing-rotor engine schemes used in previous cases. Specifically, an increase in the flow velocity at the wing exit of 3...8 % depending on the power plant operating mode was found, which led to an increase in lift up to 20 % depending on the flight mode, and an increase in thrust up to 16 %. Conclusions. The proposed S-shaped profile with a lower surface bend in the area of the rotor hub, and a profile with two bends in the contour with a cut in the rear part of the wing from the top and bottom contours have the most optimal and balanced characteristics. In the future, it is planned to manufacture these profiles using composite materials to reduce weight and install them on the aircraft prototype, replacing the rotor structure with a more flexible one using more flexible materials such as PETG plastic for the synchronization of the installed rotor motors, which is necessary to increase reliability and backup the rotor drive. Additionally, several sections with independent engines are planned to be made on each wing console to counteract rotor rupture in the event of engine failure. Due to significant advantages over the standard profile shape and minor structural complexity, the manufacture of more complex profile shapes can find wide applications in aircraft designed according to the fan-wing scheme.

A method for reducing parasite drag of medium transport helicopter

At present, more and more stringent emission requirements are being imposed on helicopters. Carbon dioxide emissions are a function of fuel consumption, and fuel consumption is directly dependent on the power required for a helicopter cruise flight. The power, required for horizontal flight of a helicopter directly depends on the design of the fuselage and other non-lifting elements, and accordingly, on parasite drag. Therefore, the desire to reduce parasite drag is expected and understandable. Helicopter parasite drag reduction was hailed as one of the key topics of the Seventh Clean Sky Framework Conference. For sixty years of research on this topic, a tremendous amount of experience has been accumulated. In particular, it was found that the main contributors to the total parasite drag of the helicopter were the main rotor hub and swashplate. The subject matter of this article is the study of the parasite drag of the hub and swashplate of a medium passager/transport helicopter Mi-8/17. This study aimed to develop methods to reduce the parasite drag of the Mi-8/17 helicopter. Mi-8/17 is the most mass-produced twin-engine helicopter in the world. Currently, more than 4,5 thousand units are in operation around the world. Therefore, the task of reducing parasite drag for this helicopter is very relevant. Modern methods of computation fluid dynamics (CFD) are used. Modern methods of computational fluid dynamics with an acceptable degree of accuracy make it possible to consider the interference phenomena of the components of the hub, swashplate, and helicopter fuselage, which is not always possible, to obtain correctly during expensive wind tunnel tests. During the work, the following results were obtained. An analysis was made of the contribution of each element of the design of the hub and the main rotor swashplate to the total parasite drag. A method for reducing the parasite drag of a helicopter using a mast fairing is proposed. Several variants of the mast fairing were considered, and the most effective one was chosen. The effect, as well as its degree of influence, of the airflow blown by the fan cooling the air-oil radiators, on the general parasite drag of the helicopter was revealed. The results of the studies, given in this article have been implemented in the production.

Numerical study of the main rotor hub and swashplate parasite drag estimation of a medium transport helicopter

This work is devoted to the determination of the parasite drag of the components of the main rotor hub and the swashplate of a medium transport helicopter Mi-8 type. As well as interference of components between themselves and the fuselage. The calculations were carried out using modern methods of computational fluid dynamics (CFD). The work was carried out taking into account the decision of The European Commission 7th Framework Programme "Clear Sky" Joint Technology Initiative (JTI).

A BEM theory adaption for inclusion of hub ratio effects in HAWT rotor design and analysis

A large hub ratio in a horizontal axis wind turbine (HAWT) rotor can provide aerodynamic and structural design benefits, but the flow effects are not accounted for in the classical Blade Element Momentum Method (BEMM). Research into the effect of HAWT rotor hub ratio, necessitated development of an adaption to the BEMM, so that rotors with large hub ratios, could be designed to be aerodynamically efficient, and so that the flow effects could be included in performance prediction. Results from the BEMM with large-hub adaption were compared with results from using the classical BEMM and with results from CFD simulation. Two sets of rotors were analysed, one set in a viscous flow regime and the other in a turbulent flow regime. The BEMM with large-hub adaption, was found to provide rotor designs with better performance and provided a more accurate prediction of relative rotor power than the classical BEMM.

Turbine Endwall Contouring Through Advanced Optimization Techniques

AbstractNon-axisymmetric endwall profiling offers features to simultaneously mitigate aerodynamic losses and hot gas ingestion in axial turbines. This paper presents an optimization methodology to generate a contoured surface integrated with real geometrical effects such as blade fillets and a rim seal channel with the aim of achieving higher efficiencies while reducing hot gas ingestion. The contoured rotor platform is constructed using a B-spline surface clamped in the axial direction. In the azimuthal direction, the surface is unclamped to allow geometrical continuity across the periodic boundaries. The endwall parameterization is used to optimize a rotor hub platform of a high-pressure turbine stage. A differential evolution optimizer is used to rank individuals in terms of efficiency. The single-objective optimization is set to maximize the aerodynamic efficiency and it is defined such that it accounts for the flow non-uniformity through a mixed-out averaging procedure. Engine representative conditions typical of a two-stage high-pressure turbine are used as boundary conditions. Geometrical and aerodynamic constraints are set to guarantee a fair comparison among individuals and to meet engine requirements. Two surface parameterizations, which use a different number of design variables but share the same construction strategy, are presented to show the trade-off between the number of degrees-of-freedom and the aerodynamic improvement. Different purge flow conditions are considered to assess the robustness of the optimization results at off-design conditions for relevant geometries. The aim of this paper is to show the advanced shape flexibility of the implemented parameterization for contoured platforms featuring technological effects such as blade fillet and rim seal channel. The work provides design guidelines to setup engine-realistic constraints for endwall contour optimization of turbine stages.

Effect of Blade Tip Configurations on the Performance and Vibration of a Lift-Offset Coaxial Rotor

This present study investigates the effect of blade tip configurations, such as the sweepback angle and anhedral angle, on the performance and hub vibratory loads for the lift-offset coaxial rotor of a 30,000-pound-class high-speed long-range utility helicopter. The rotorcraft comprehensive analysis code, CAMRAD II, is utilized to conduct the performance and hub vibratory load analyses for the present lift-offset coaxial rotor. The total rotor thrust, torque, and individual rotor’s hub pitch moment and hub roll moment are considered the trim targets. The general properties for the lift-offset coaxial rotor are designed from the X2TD, S-97 Raider, and SB > 1 Defiant, which are lift-offset compound helicopters. The rotor performance and hub vibratory loads are studied with the various blade tip configurations including the sweepback angle and anhedral angle. The rotor power when the rotor blade tip considers only the sweepback angle (20°) is lower than the baseline rotor model by 41.25% at 170 knots. The maximum rotor effective lift-to-drag ratio (L/De) for the lift-offset coaxial rotor using only the sweepback angle and the rotor with both sweepback (20°) and anhedral angles (10°) at 170 knots increase by 10.82% and 5.02%, respectively, compared with the baseline rotor model without both sweepback and anhedral angles. The vibration index (VI) for the rotor with only the sweepback angle is higher than that for the baseline rotor model without both sweepback and anhedral angles by 37.14%. Furthermore, when the rotor blade tip has the anhedral angle, the magnitude of the Blade Vortex Interaction (BVI) decreases compared with the rotor without the sweepback and anhedral angles.

Actuator fault tolerant offshore wind turbine load mitigation control

Offshore wind turbine (OWT) rotors have large diameters with flexible blade structures which are subject to asymmetrical loads caused by blade flapping and turbulent or unsteady wind flow. Rotor imbalance inevitably leads to enhanced fatigue of blade rotor hub and tower structures. Hence, to enhance the life of the OWT and maintain good power conversion the unbalanced loading requires a reliable mitigation strategy, typically using a combination of Individual Pitch Control (IPC) and Collective Pitch Control (CPC). Increased pitch motion resulting from IPC activity can increase the possibility of pitch actuator faults and the resulting load imbalance results in loss of power and enhanced fatigue. This has accelerated the emergence of new research areas combining IPC with the fault tolerant control (FTC)-based fault compensation, a so-called FTC and IPC “co-design” system. A related research challenge is the clear need to enhance the robustness of the FTC IPC “co-design” to some dynamic uncertainty and unwanted disturbance. In this work a Bayesian optimization-based pitch controller using Proportional–Integral (PI) control is proposed to improve pitch control robustness. This is achieved using a systematic search for optimal controller coefficients by evaluating a Gaussian process model between the designed objective function and the coefficients. The pitch actuator faults are estimated and compensated using a robust unknown input observer (UIO)-based FTC scheme. The robustness and effectiveness of this “co-design” scheme are verified using Monte Carlo simulations applied to the 5MW NREL FAST WT benchmark system. The results show clearly (a) the effectiveness of the load mitigation control for a wide range of wind loading conditions, (b) the effect of actuator faults on the load mitigation performance and (c) the recovery to normal load mitigation, subject to FTC action.

Computational Characterization of Unsteadiness and Turbulence in Rotor Hub Wakes

Reynolds stresses and turbulent kinetic energy are studied in the wakes of several helicopter rotor hub geometries in forward flight. Computational fluid dynamics simulations are performed using NASA's OVERFLOW 2.2n Reynoldsaveraged Navier–Stokes solver. The simulations impose flow conditions based on previous and current experimental and numerical studies. Discrete Fourier transforms are used to examine velocity harmonics for several frequencies and compared against available experimental results. Components of the Reynolds stress tensor are computed and examined. Production and transport of the turbulent kinetic energy are examined through the rotor hub wakes at six streamwise coordinates. It was found that the scissor arms, previously found to have a significant effect on rotor hub force harmonics, also had a significant effect on the magnitudes of Reynolds stresses and turbulent kinetic energy. Integrated values of turbulent kinetic energy flux indicate that drag-reducing designs have a direct effect on turbulent kinetic energy levels in the wake.

Three Rotor Hub Flow Prediction Workshops (2016–2020): What Did We Learn and What's Next?

The 'Rotor Hub Flow Prediction Workshops' have been productive collaborations between experimental and computational efforts in the important area of high-Reynolds number model testing of rotor hubs and associated complex interactional aerodynamics in the long-age wake as relevant to current and future rotorcraft. As such the hub flow workshops have joined the ranks of past successful collaborations such as the UH-60 Airloads and HART-II workshops. This paper begins by describing the basic physics of rotor hub flows and gives a brief summary of recent water-tunnel test campaigns. Following, the evolution of the hub flow workshops is summarized, with emphasis on the productive interactions between experimentalists and computational participants. A compilation of computational blind comparison results against measured data for all three workshops thus far is presented. Challenges associated with uncertainties in both experiments and computations and their effect on quantitative comparisons are discussed. In particular, emphasis is given to the "lessons learned" on both sides and an outlook on remaining challenges and next research steps in the area of rotor hub flows is provided.

Hub Flow Near-Wake Validation Using CREATETM-AV Helios and UMD Mercury Framework

Experimental near-wake flow field measurements from two Penn State University water tunnel tests of a defeatured helicopter hub are compared with two unsteady computational fluid dynamics (CFD) analyses: CREATETM-AV Helios and the University of Maryland Mercury. Both CFD frameworks employ an unstructured/Cartesian multimesh paradigm and turbulent Spalart–Allmaras detached eddy simulation (SA-DES) modeling. Experimental velocimetry measurements of mean wake velocities, harmonic content, and Reynolds stresses provide valuable validation data for CFD. Overall, the two CFD solvers were in good agreement with each other and qualitatively captured the mean and harmonic content of the wake structures with accuracy. Flow feature dissimilarities between the advancing and retreating sides were differentiated and indicated dominant regions of harmonic flow disturbances biased towards the retreating side, in good agreement with experimental observations. Quantitatively, some variation in velocity deficits and downwash were noted, either in profile character, magnitude, and/or location. Encouragingly, there was little tendency of excessive dissipation in the CFD near wake, and the harmonic content actually tended towards a common overprediction. Reynolds number effects were minimal, and grid density effects were studied but inconclusive. These efforts were part of the Third PSU Rotor Hub Flow Workshop in 2020.

Mechanism and Wind Tunnel Experiment of Active Controlled Flap on Vibration Load Control of Rotor Hub

Mechanism and Wind Tunnel Experiment of Active Controlled Flap on Vibration Load Control of Rotor Hub

Performance appraisal of a small wind turbine under the use of three rotor hub configurations

Performance appraisal of a small wind turbine under the use of three rotor hub configurations