Showing papers on "Helicopter rotor published in 2006"

A framework for CFD analysis of helicopter rotors in hover and forward flight

Abstract: A framework is described and demonstrated for CFD analysis of helicopter rotors in hover and forward flight. Starting from the Navier–Stokes equations, the paper describes the periodic rotor blade motions required to trim the rotor in forward flight (blade flapping, blade lead-lag and blade pitching) as well as the required mesh deformation. Throughout, the rotor blades are assumed to be rigid and the rotor to be fully articulated with separate hinges for each blade. The employed method allows for rotors with different numbers of blades and with various rotor hub layouts to be analysed. This method is then combined with a novel grid deformation strategy which preserves the quality of multi-block structured, body-fitted grids around the blades. The coupling of the CFD method with a rotor trimming approach is also described and implemented. The complete framework is validated for hovering and forward flying rotors and comparisons are made against available experimental data. Finally, suggestions for further development are put forward. For all cases, results were in good agreement with experiments and rapid convergence has been obtained. Comparisons between the present grid deformation method and transfinite interpolation were made highlighting the advantages of the current approach. Copyright © 2006 John Wiley & Sons, Ltd.

Rotor Airloads Prediction Using Loose Aerodynamic/Structural Coupling

Abstract: A computational fluid dynamics (CFD) code and rotorcraft computational structural dynamics (CSD) code are coupled to calculate helicopter rotor airloads across a range of flight conditions. An iterative loose (weak) coupling methodology is used to couple the CFD and CSD codes on a per revolution, periodic basis. The CFD code uses a high fidelity, Navier‐Stokes, overset grid methodology with first principles-based wake capturing. Modifications are made to the CFD code for the aeroelastic analysis. For a UH-60A Blackhawk helicopter, three challenging level flight conditions are computed: 1) high speed, μ = 0.37, with advancing blade negative lift, 2) low speed, μ = 0.15, with blade‐vortex interaction, and 3) high thrust with dynamic stall, μ = 0.24. Results are compared with UH-60A Airloads Program flight test data. For all cases the loose coupling methodology is shown to be stable, convergent, and robust with full coupling of normal force, pitching moment, and chord force. In comparison with flight test data, normal force and pitching moment phase and magnitude are in good agreement. The shapes of the airloads curves are well captured. Overall, the results are a noteworthy improvement over lifting line aerodynamics used in rotorcraft comprehensive codes.

Variable speed transmission for a rotary wing aircraft

Abstract: A transmission gearbox for a rotary-wing aircraft includes a main gearbox and a variable speed gearbox in meshing engagement with the main rotor gearbox. The variable speed gearbox permits at least two different RPMs for the main rotor system without disengaging the engine(s) or changing engine RPMs. The variable speed gearbox includes a clutch, preferably a multi-plate clutch, and a freewheel unit for each engine. A gear path drives the main gearbox in a “high rotor speed mode” when the clutch is engaged to drive the main rotor system at high rotor rpm for hover flight profile. A reduced gear path drives the main gearbox in a “low rotor speed mode” when the clutch is disengaged and power is transferred through the freewheel unit, to drive the main rotor system at lower rotor rpm for high speed flight. The variable speed gearbox may be configured for a tail drive system that operates at a continuous speed, a tail drive system that changes speed with the main rotor shaft or for no tail drive system.

Analysis methodology for 3C-PIV data of rotary wing vortices

Abstract: 3C-PIV data from tip vortices of either fixed-wing or rotating wing experiments are challenging from an analysis point of view Model motion, vortex wander, spurious vectors, periodic and aperiodic effects, turbulence, and other disturbing effects are all present in the data In most cases the vortices are not measured perpendicular to their axis as well Engineers need time-averaged properties from the vortex in the vortex axis system for a proper modelization within simulation codes This article describes the methods needed to deal with all the mentioned problem areas, including the conditional averaging and rotation into the vortex axis system The methods are validated by using numerically generated vortex vector fields, and finally applied to experimental data from a hover condition of a model rotor

CFD/CSD Prediction of Rotor Vibratory Loads in High-Speed Flight

Abstract: A computational fluid dynamics (CFD) model is coupled with a computational structural dynamics (CSD) model to improve prediction of helicopter rotor vibratory loads in high-speed flight. The two key problems of articulated rotor aeromechanics in high-speed flight-advancing blade lift phase, and underprediction of pitch link load-are satisfactorily resolved for the UH-60A rotor. The physics of aerodynamics and structural dynamics is first isolated from the coupled aeroelastic problem. The structural and aerodynamic models are validated separately using the UH-60A Airloads Program data. The key improvement provided by CFD over a lifting-line aerodynamic model is explained. The fundamental mechanisms behind rotor vibration at high speed are identified as: 1) large elastic twist deformations and 2) inboard wake interaction. The large twist deformations are driven by transonic pitching moments at the outboard stations. CFD captures 3-dimensional unsteady pitching moments at the outboard stations accurately. CFD/CSD coupling improves elastic twist deformations via accurate pitching moments and captures the vibratory lift harmonics correctly. At the outboard stations (86.5% radius out), the vibratory lift is dominated by elastic twist. At the inboard stations (67.5% and 77.5% radius), a refined wake model is necessary in addition to accurate twist. The peak-to-peak pitch link load and lower harmonic waveform are accurately captured. Discrepancies for higher harmonic torsion loads remain unresolved even with measured airloads. The predicted flap-bending moments show a phase shift of about 10 deg over the entire rotor azimuth. This error stems from 1, 2, and 3/rev lift. The 1/rev lift is unaffected by CFD/CSD coupling. The 2 and 3/rev lift are significantly improved but do not fully resolve the 2 and 3/rev bending moment error.

Simulation of unsteady rotor-fuselage aerodynamic interaction using unstructured adaptive meshes

Abstract: A three-dimensional parallel Euler flow solver has been developed for the simulation of unsteady rotor-fuselage interaction aerodynamics on unstructured meshes. In order to handle the relative motion between the rotor and the fuselage, the flow field was divided into two zones, a moving zone rotating with the blades and a stationary zone containing the fuselage. A sliding mesh algorithm was developed for the convection of the flow variables across the cutting boundary between the two zones. A quasi-unsteady mesh adaptation technique was adopted to enhance the spatial accuracy of the solution and to better resolve the wake. A low Mach number pre-conditioning method was implemented to relieve the numerical difficulty associated with the low-speed forward flight. Validations were made by simulating the flows around the Georgia Tech configuration and the ROBIN fuselage. It was shown that the present method is efficient and robust for the prediction of complicated unsteady rotor-fuselage aerodynamic interaction phenomena.

Numerical and Experimental Investigation on the Flow Induced Oscillations of a Single-Blade Pump Impeller

Abstract: This contribution is addressed to the periodically unsteady flow forces of a single-blade sewage water pump, which affect the impeller and produce radial deflections of the pump shaft. The hydrodynamic excitation forces were calculated from the time dependent flow field, which was computed by numerical simulation of the three-dimensional, viscous, time-dependent flow in the pump. A commercial computer code was used to determine the time accurate Reynolds averaged Navier-Stokes equations. The transient radial flow forces at all time steps for a complete impeller revolution affect the rotor of the single-blade pump and stimulate it to strong oscillations. To determine the influence of the vibration stimulation forces on the dynamic behavior of the pump rotor, an investigation of the rotor's structural dynamics was accomplished. A dynamic time analysis for the pump rotor provided the dynamic answer from the structural model of the rotor under the influence of the flow forces. The hydrodynamic forces, which were calculated before, were defined as external forces and applied as the load on the rotor. The resulting impeller deflections were calculated by a transient analysis of the pump rotor system using the commercial finite element method software PROMECHANICA. To verify the results obtained by standard numerical methods, the radial deflections of the impeller of a commercial sewage water pump, which has been investigated numerical in advance, were measured for the horizontal and for the vertical coordinate direction by proximity sensors. The measured data were compared to the computed amounts for a wide range of pump operation. The results show a good agreement for a strong part of an impeller revolution for all investigated operating points. The simultaneous measurement of vibration accelerations at the outer side of the pump casing showed the effects of the time-dependent flow, which produce hydrodynamic forces acting at the impeller of the pump and stimulating it to strong oscillations.

Experimental and Numerical Study of Cyclogiro Aerodynamics

Abstract: A combined experimental and numerical study of a cyclogiro rotor operating at Reynolds numbers about 40,000 has been conducted. The study reveals complex flow field with complex (unsteady) interactions between the blades and the wakes of other blades. In spite of this complexity, time-averaged integral forces acting on the rotor can be predicted by a simple momentum theory, properly corrected for thrust-producing area of the rotor and for large Magnus effect. The effective thrust producing area was estimated to be about half of its projected area, suggesting that the effectiveness of a cyclogiro rotor may be comparable with that of a heavy-loaded helicopter rotor.

Rotor drive and control system for a high speed rotary wing aircraft

Abstract: A drive system for a high speed rotary-wing aircraft includes a combiner gearbox in meshing engagement with a main gearbox. The combiner gearbox is driven by one or more engines such that a main rotor system and a translational thrust system are driven thereby. The engine drives the combiner gearbox and thus the main gearbox through an overrunning clutch. The drive system permits the main rotor system RPM to be controlled by offloading power to the translational thrust system during a high speed flight profile.

Surrogate based optimization of helicopter rotor blades for vibration reduction in forward flight

Abstract: The effectiveness of surrogate modeling of helicopter vibrations, and the use of the surrogates for minimization of helicopter rotor vibrations are studied. The accuracies of kriging, radial basis function interpolation, and polynomial regression surrogates are compared. In addition, the surrogates are used to generate an objective function which is employed in an optimization study. The design variables consist of the cross-sectional dimensions of the structural member of the blade and non-structural masses. The optimized blade is compared with a baseline rotor blade which resembles an MBB BO-105 blade. Results indicate that: (a) kriging surrogates best approximate vibratory hub loads over the entire design space and (b) the surrogates can be used effectively in helicopter rotor vibration reduction studies.

Aeroelastic response of helicopter rotors using a 3D unsteady aerodynamic solver

Abstract: The prediction of blade deflections and vibratory hub loads concerning helicopter main rotors in forward flight is the objective of this work. They are determined by using an aeroelastic model derived through the coupling between a nonlinear blade structural model and a boundary integral equation solver for three-dimensional, unsteady, potential aerodynamics. The Galerkin method is used for the spatial integration, whereas the periodic blade response is determined by a harmonic balance approach. This aeroelastic model yields a unified approach for aeroelastic response and blade pressure prediction that may be used for aeroacoustic purposes, with the possibility of including effects from both blade-vortex interaction and multiple-body aerodynamic interaction. Quasi-steady aerodynamic models with wake-inflow from the three-dimensional aerodynamic solver are also applied, in order to perform a comparative study. Numerical results show the capability of the aeroelastic tool to evaluate blade response and vibratory hub loads for a helicopter main rotor in level flight conditions, and examine the sensitivity of the predictions on the aerodynamics model used.

Control of rotorcraft retreating blade stall using air-jet vortex generators

Abstract: A series of low-speed wind tunnel tests was carried out on an oscillating airfoil fitted with two rows of air-jet vortex generators (AJVGs) The airfoil used had an Royal Aircraft Establishment 9645 section, and the two spanwise arrays of AJVGs were located at x/c = 012 and 062 The devices and their distributions were chosen to assess their ability to modify/control dynamic stall, the goal being to enhance the aerodynamic performance of helicopter rotors on the retreating blade side of the disk The model was pitched about the quarter chord with a reduced frequency k of 01 in a sinusoidal motion defined by a = 15 + 10sin ωt deg The measured data indicate that, for continuous blowing from the front row of AJVGs with a momentum blowing coefficient C μ greater than 0008, modifications to the stalling process are encouraging In particular, the pitching moment behavior exhibits delayed stall and there is a marked reduction in the normal force hysteresis

Target vector optimization of composite box beam using real-coded genetic algorithm: a decomposition approach

Abstract: This paper aims to obtain the optimal composite box-beam design for a helicopter rotor blade. The cross-sectional dimensions and the ply angles of the box beam are considered as design variables. The objective is to optimize the box beam to attain a target vector of stiffness values and maximum elastic coupling. The target vector is the optimal stiffness values of helicopter rotor blade obtained from a previous aeroelastic optimization study. The elastic couplings introduced by the box beam have beneficial effects on the aeroelastic stability of helicopter. The optimization problem is addressed by decomposing the optimization into two levels, a global level and a local level. The box-beam cross-sectional dimensions are optimized at the global level. The local-level optimization is a subproblem which finds optimal ply angles for each cross-sectional dimension considered in the global level. Real-coded genetic algorithm (RCGA) is used as the optimization tool in both the levels of optimization. Hybrid operators are developed for the RCGA, thereby enhancing the efficiency of the algorithm. Min–max method is used to scalarize the multiobjective functions used in this study. Optimal geometry and ply angles are obtained for composite box-beam designs with ply angle discretization of \(10\), \(15\), and \(45^o\).

Aeroelastic Modeling Effect in Rotor BVI Noise Prediction

Abstract: The acoustic field generated by a helicopter main rotor experiencing blade-vortex interaction (BVI) during a descent flight path is examined. The prediction procedure starts from the determination of the aeroelastic steady periodic solution. Then, a boundary integral formulation for the velocity potential suited for configurations where stong wake/blade impingement occurs is applied. It is fully three-dimensional, can be applied to blades with arbitrary shape and motion and performs the calculation of both wake shape and blade pressure field. Finally, the noise field generated by the helicopter rotor is evaluated through an aeroacoustic tool based on the Ffowcs Williams and Hawkings equation. The numerical investigation discusses the sensitivity of BVI noise prediction on the aeroelastic models applied for the calculation of blade steady periodic deformations. The eects of the dierent blade deformations given by the aeroelastic solvers considered are examined both in terms of local acoustic signatures and in terms of noise radiation characteristics.

New hybrid method for predicting the flowfields of helicopter rotors

Abstract: Based on a Navier-Stokes/full potential/free wake solver, a new hybrid method was developed for efficient predictions of the three-dimensional viscous flowfield of a helicopter rotor under both hover and forward flight conditions. The developed flow solver was composed of three modules: 1) a compressible Navier-Stokes analysis to model the viscous flow and near wake about the blade, 2) a compressible potential flow analysis to model the inviscid isentropic potential flow region far away from the rotor, and 3) a free wake model to account for tip vortex effects once the tip vortex leaves the viscous flow region and enters the potential flow region. In this hybrid method, a moving embedded grid methodology was adopted that accounts for rigid blade motions in rotation, flapping, and pitching. A dual-time method was employed to fulfill the calculation of the unsteady flowfields of helicopter rotors, and a third-order upwind scheme (MUSCL) and flux-difference splitting scheme without introducing artificial viscosity were used to calculate the flux. To search suitable donor elements in embedded grids to pass information between the viscous flow and potential flow zones, a new searching scheme was implemented. The sectional pressure distributions of a UH-60A helicopter rotor and an AH-1G model rotor in hover and forward flight, with and without the wake model, were calculated, and the developed hybrid model was validated by comparing with available experimental data. The simulated steady and unsteady lifting results of a model rotor with three different blade-tip planforms demonstrate the benefits of the curvilinear swept tip and constant swept tip in suppressing supercritical flows.

Micro-PIV and ELDV wind tunnel investigations of the laminar separation bubble above a helicopter blade tip

Abstract: Studies of the boundary layer profile and of the characteristics of the flow velocity distribution close to the leading edge of a helicopter blade profile were conducted using embedded laser Doppler velocimetry (ELDV) and stereo particle image velocimetry (PIV). The relatively small scales of flow structures leading to dynamic stall motivated an additional 2C-PIV study in which the flow field has been measured with a relatively high spatial resolution. The feasibility of PIV measurements utilizing a mirror telescope in a wind tunnel has been demonstrated successfully. The spatial resolution of approximately 50 µm allowed a judgment on the choice of turbulence models and damping coefficients for the improvement of CFD predictions.

Parallel simulation of unsteady hovering rotor wakes

Abstract: Numerical simulation using low diffusion schemes, for example free-vortex or vorticity transport methods, and theoretical stability analyses have shown the wakes of rotors in hover to be unsteady. This has also been observed in experiments, although the instabilities are not always repeatable. Hovering rotor wake stability is considered here using a finite-volume compressible CFD code. An implicit unsteady, multiblock, multigrid, upwind solver, and structured multiblock grid generator are presented, and applied to lifting rotors in hover. To allow the use of very fine meshes and, hence, better representation of the flow physics, a parallel version of the code has been developed, and parallel performance using upto 1024 CPUs is presented. A four-bladed rotor is considered, and it is demonstrated that once the grid density is sufficient to capture enough turns of the tip vortices, hover exhibits oscillatory behaviour of the wake, even using a steady formulation. An unsteady simulation is then performed, and also shows an unsteady wake. Detailed analysis of the time-accurate wake history shows that three dominant unsteady modes are captured, for this four-bladed case, with frequencies of one, four, and eight times the rotational frequency. A comparison with theoretical stability analysis is also presented. Copyright © 2006 John Wiley & Sons, Ltd.

Rotor system with pitch flap coupling

Abstract: An articulated rotor system with a servo-flap rotor control system locates the focal point of a blade retention spherical elastomeric bearing inboard of a joint in the servo-flap pitch control tube which separates the flap hinge and supports the servo-flap pitch control tube on a single pivot bearing within a blade retention spindle. Rotor blade flapping produces relative movements between the servo-flap pitch control tube and the blade retention spindle which is converted through a servo-flap drive linkage into servo-flap pitch motions to provide flap/pitch coupling which reduces steady and transient blade flapping.

Air-cooled brake rotor system

Abstract: An air-cooled brake rotor system defining an inter-rotor disk slot through which spacers in the form of turbine or fan vanes propel air when the brake rotor system is turned. An inner disk brake rotor plate is attached to a hub which in turn may be attached to an axle on a vehicle such as an automobile. Pins having ends of opposite threading are used to connect the inner rotor to the outer rotor. The outer rotor is spaced apart from the inner rotor by means of spacers which are shaped to propel air through the slots defined between the two rotors. The spacers are also configured so that compression of the two rotors by a caliper system always serves to have a spacer beneath the area engaged by the brake pad to provide mechanical support for the rotor system. Each rotor generally has defined in it venting holes and inscribed debris-channeling slots in an arcuate, volute, or turbinate manner. An enhanced braking system experiencing lower heat retention is thereby attained.

Validation of a Time Domain Formulation for Propeller Noise Prediction

Abstract: The present paper is oriented at the definition of a series of validation test cases for the far-field subsonic formulation of Farassat in the time domain, solving the Lighthill analogy for aeroacoustics in the Ffowcs William-Hawkings formulation for far-field noise predictions. The focus will be on the definition, validation and verification of four test cases for far-field acoustic propagation. The test cases are: (i) a rotating rotor with a pre-defined body force, (ii) a translating and rotating small disk case with a moving observer, (iii) a conventional helicopter rotor case, (iv) and the consistency test known as the Isom thickness noise. The acoustic pressure signature, sound pressure level and directivity have been calculated for each test case. Graphical outputs and comparisons of results with those of analytical solutions are presented.

Effect of Piezoelectric Hysteresis on Helicopter Vibration Control Using Trailing-Edge Flaps

Abstract: This study investigates the effect of piezoelectric actuator hysteresis on helicopter vibration control using trailingedge flaps. An aeroelastic analysis is used represent the helicopter with trailing-edge flaps. A compressible unsteady aerodynamic model is used to predict the incremental airloads due to trailing-edge flap motion. The material and mechanical hysteresis in the piezoelectric actuator is modeled using the classical Preisach model. Experimental data from the literature are used to estimate the weighting function through geometric interpretation and numerical implementation. Results are obtained from vibration control studies performed using a trailing-edge flap with 1) ideal linear actuator and 2) real actuator with hysteresis modeled. Multicyclic control input gives 90 and 81% reduction in hub vibration at high-speed flight $(\mu = 0:30)$ for the ideal and real actuator, respectively. In low-speed flight $(\mu = 0:15)$, the hub vibration is reduced by 99 and 86% for ideal and real actuator, respectively. Results indicate that the presence of actuator hysteresis nonlinearity leads to deterioration in controller performance and lower vibration reductions in both high- and low-speed forward flight.

Use of single crystal and soft piezoceramics for alleviation of flow separation induced vibration in a smart helicopter rotor

Abstract: Dynamic stall and flow separation induced vibration alleviation is investigated using single crystal and soft piezoceramic-induced shear actuation. PZT-5H performs well to reduce vibration at cruise speed (about 50%) but fails to achieve a substantial vibration reduction (about 20%) at high speed forward flight because dynamic stall and flow separation require a larger stroke for vibration reduction. $PZN-8\%PT{\langle}111{\rangle}$-induced shear actuation is found to generate a larger stroke and eventually a higher vibration reduction (about 60–70%) at both cruise and high speed flight. It is observed that the controller performs satisfactorily up to a noise level of 20% in the sensed data. Optimum placement of actuators along the span, as well as the eventualities of actuator failure and degradation are also addressed.

Aerodynamic Shape Optimization of Hovering Rotor Blades in Transonic Flow Using Unstructured Meshes

Abstract: An aerodynamic shape optimization technique has been developed for helicopter rotor blades in hover based on a continuous adjoint method on unstructured meshes. For this purpose, the Euler flow solver and the adjoint sensitivity analysis were formulated on the rotating frame of reference. To handle the repeated evaluation of the flow solution and the sensitivity analysis efficiently, the flow and adjoint solvers were parallelized using a domain decomposition strategy. A solution-adaptive mesh refinement technique was adopted to better resolve the tip vortex. Applications were made to the aerodynamic shape optimization of the Caradonna-Tung rotor blade and the UH-60 rotor blade. The results showed that the present method is effective in determining optimum aerodynamic configurations of rotor blades in hover, which require minimum inviscid torque while maintaining the desired thrust level at transonic flight conditions.

Swashplateless Helicopter Rotor System with Trailing-Edge Flaps for Flight and Vibration Controls

Abstract: The objective of this study is to demonstrate the concept of active trailing-edge flaps as primary flight control and vibration reduction devices for a typical full-scale helicopter. A comprehensive rotorcraft analysis based on UMARC was developed to analyze the swashplateless rotor. A parametric study of various key design variables involved in the trailing-edge flap design was carried out. An optimal design of a trailing-edge flap system that provides effective control authority within the complete range of advance ratios as well as minimum actuation requirements was achieved. Trailing-edge flaps demonstrated the capability of performing both primary flight control and active vibration control functions. At a high forward speed (advance ratio of 0.32), the 4/rev vertical force and roll and pitch moments at hub are successfully eliminated (by 90%), and the 4/rev in-plane hub forces are reduced by more than 40%. The half peak-to-peak value of the trailing-edge flap deflection for primary flight control is 7.1 deg, and an additional 4.7 deg is required for active vibration control.

Aeroelastic optimization of a helicopter rotor using orthogonal array-based metamodels

Abstract: Aeroelastic optimization of a four-bladed, soft-inplane hingeless rotor is performed to reduce hub loads and blade root loads with blade stiffness design variables. An aeroelastic analysis based on the finite element method is used. Aerodynamic modeling includes a time domain unsteady aerodynamic model and a free wake model. Metamodels (models of models) of the aeroelastic analysis are investigated in a systematic manner including various experimental designs such as factorial designs, central composite designs (CCD), gradient-enhanced CCD, and orthogonal arrays (OA). Linear, quadratic, and cubic polynomial response surfaces are obtained, and graphical, statistical, and optimization results obtained are used to compare the different designs. It is found that the CCD and OA are able to capture the basic trends of the analysis using sequential second-order polynomial response surfaces and are further investigated for use in optimization. However, the OA, which is a fractional factorial design, requires significantly fewer analysis runs than the CCD. Numerical results obtained for single-objective and multiobjective optimization problems show a 16‐22% reduction in vibratory hub loads for a nonuniform blade with six elastic stiffness design variables. The multiobjective optimization problem based on the min‐max method reduces the vibratory hub loads by about 16% and the 1/rev and 2/rev blade root loads, which are the principal cause of dynamic stresses, by about 18 and 31%, respectively. The OA-based metamodels provide an efficient and interactive approach to perform preliminary design studies using comprehensive simulation codes and is suitable for use in an industrial setting.