Helicopter Rotor Aerodynamics Research Articles

Assessment of the Vortex Feature-Based Vorticity Confinement Method Applied to Rotor Aerodynamics and Aeroacoustics

The accurate prediction of helicopter rotor aerodynamics and aeroacoustics using Computational Fluid Dynamics (CFD) techniques still remains a challenge, as the over-dissipation of numerical schemes results in a higher diffusive rate of rotor wake and vortices than what can be expected from the fluid governing equations. To alleviate this issue, a vortex feature-based vorticity confinement (FVC2-L2) method that combines the locally normalized λ2 vortex detection method with the standard second vorticity confinement (VC2) scheme is presented to counterbalance the truncation error introduced by the numerical discretization of the convective term while avoiding the over-confinement inside the boundary layer. The FVC2-L2 scheme is adopted for helicopter rotor aerodynamic and aeroacoustic predictions through its implementation in the multi-block structured grid CFD solver ROSITA and coupling with the aeroacoustic code ROCAAP based on the permeable surface Ffowcs Williams–Hawkings (PS-FWH) equation. This approach is assessed in helicopter rotor flows via three databases. Firstly, the well-documented HART-II rotor in the baseline condition is used to evaluate the capability of the presented VC scheme in blade–vortex interaction (BVI) phenomena prediction. Subsequently, the UH-1H non-lifting hovering rotor and the AH-1/OLS low-speed descending flight rotor are adopted for assessment of such a method in aeroacoustics. The benefits of the FVC2-L2 scheme in terms of aerodynamics prediction, wake preservation, and noise signal prediction are well demonstrated by comparison with the experimental data and the results obtained without VC schemes. Particularly, the FVC2-L2 scheme mainly improves the highly unsteady airloads prediction, and results in an improvement of BVI noise prediction by more than 5 dB with respect to the case without VC schemes for AH-1/OLS rotor case. Additionally, some shortcomings of the approach are noticed in engineering applications. On the basis of a simplified convective vortex, some provisional guidelines on the required εo value in terms of number of cells per vortex diameter are provided: an εo value ranging from 0.01 to 0.04 for grids which may represent the vortex core diameter with 6 to 12 cells.

CALCULATED RESEARCH OF INFLUENCE OF HELICOPTER MAIN ROTORS GEOMETRY ON THE EFFICIENCY IN HOVER MODE BASED ON THE NONLINEAR VORTEX MODEL

The efficiency of the helicopter main rotor in the hover mode is very important, because this mode essentially determines the performance characteristics of the helicopter. A feature of the helicopter rotor aerodynamics is a significant inductive blade influence that highly defines its aerodynamic characteristics. The problem of the influence of the blade twist and spatial geometric layout of the main rotor on its aerodynamic characteristics in the hover mode for a fixed value of the rotor solidity has been considered in this article. As a criterion of efficiency of the rotor in the hover mode relative efficiency (FoM – Figure of Merit) is used. The results are obtained by numerical simulation based on the nonlinear vortex blade model of the rotor, developed at the Helicopter Design Chair of the MAI. The model allows taking into account a complicated spatial shape of the free vortex path of the rotor blades that determines their inductive interaction. As the example of a four-blade main rotor with rectangular blades in plan, the influence of the value of the blades twist on the efficiency in the hover mode is studied. For different values of the rotor thrust, the values and ranges of the blade twist angles are determined, providing the maximum positive effect of the efficiency increase in hovering. For a fixed value of the blade twist, the rotor solidity, and the same operating conditions, the effect of various schemes and configurations of rotor on its efficiency in hover mode is studied. A single rotor with a different number of blades (from 2 to 6), an X-shaped rotor, coaxial rotor and rotor with crossed blades type "synchropter" are considered. The values of the efficiency increase in hovering depending on the rotor layout in comparison with the two-blade rotor are obtained. The comparative analysis of inductive velocities and streamlines for the "synchropter" rotor scheme, coaxial rotor scheme and its equivalent single rotor scheme is presented. The obtained results can be useful at the stage of preliminary design of vertically taking-off aircraft when selecting the parameters of their main rotor system.

Numerical Simulation and Investigation of Transonic & Symmetrical Airfoil for Helicopter Main Rotor Blade Application

Helicopter rotor aerodynamics is prognosticated to be one of the most perplexing and enigmatic affliction encountered by both researchers and aviators throughout the ages. The bewilderment of the flow field around the main rotor blade ceaselessly remains unrequited in tangible flight environments. Appalling calamities repeatedly befall owing to these unforeseen and equivocal instances. In order to extricate these impediments, one must go back to the brass tacks and apprehend the cause. However, it is every so often exceedingly disconcerting to obtain experimental data due to intricacy, perplexity and substantial price tag. Subsequently, computational simulation is progressively becoming more of a preferred choice in recent times. Bearing this in mind, this study intended to simulate and visualize the air flow configuration of the main rotor blade using symmetrical and transonic airfoils to demarcate their physiognomies and behavior. Results have revealed that the transonic airfoil has a higher lift coefficient (Cl) than the symmetrical airfoil. Contrariwise, if used in an actual rotorcraft, the transonic airfoil can cause stern apprehension in terms of stability and control.

Simulating unsteady aerodynamics of helicopter rotor with panel/viscous vortex particle method

The unsteady aerodynamics of a helicopter rotor has been a central issue in the field of rotorcraft aerodynamics. This is because the rotor generates a complex time-dependent pattern of vorticity in its wake, which has significant effects on its performance, stability, loading, and vibration. Conventional free-wake methods used in most of the current comprehensive rotorcraft analysis codes are limited by the potential flow assumption and empirical formulations, such as vortex core size. Based on a numerical solution of the unsteady fluid-dynamic equations governing transportation and diffusion of vorticity, a viscous vortex particle method is coupled with an unsteady panel method to predict the unsteady aerodynamics of helicopter rotor blades with fewer empirical formulations in viscous flow. The coupled method is implemented through the trailing-edge Kutta condition, Neumann boundary condition, and by converting shed-wake doublet panels to wake vorticity. A TreeCode method is also employed to reduce computational cost for practical analysis. Helicopter rotors including the scaled model, Caradonna–Tung, and AH-1G rotors are simulated in hover and forward flight to validate the accuracy of the present approach. The unsteady dynamics of the rotor wake, such as wake contraction, tip-vortex pairing, and vortex roll-up, are well simulated. The predicted inflow distribution is more accurate than that in the conventional free-wake method, and the predicted pressure coefficient distribution and unsteady aerodynamic loads of rotor blades agree well with measured data and computational fluid-dynamics results.

Numerical simulations of unsteady aerodynamics of helicopter rotor in manoeuvring flight conditions

This article presents a new time-marching fully unsteady method for simulating helicopter rotors in manoeuvring flight conditions. The unsteady wake model is based on Mudry's theory which rigorously derives the evolution equation for the wake considered as a vortex sheet from the fluid dynamic equations. The numerical treatment of the sheet with a high-order panel representation allows an accurate simulation of the wake without using numerical regularisation techniques. An unsteady lifting line model, based on Matched Asymptotic Expansion, is used to represent the blade and to extend the concept of induced velocities to unsteady configurations. A first application to the Bo 105 rotor in manoeuvring flight shows that the method is capable to handle such kind of complex configuration.

Navier-Stokes Analysis of Helicopter Rotor Aerodynamics in Hover and Forward Flight

Reynolds-averaged compressible Navier ‐Stokes computations are presented for the hovering 7A model rotor and a low aspect ratio NACA0012 rotor in nonlifting forward e ight. For enhanced hover performance prediction, aeroelastic effects are taken into account by tightly coupling the e ow solver with a e nite element model of the blade based on Timoshenko beam theory. Good agreement between computed and measured rotor performance is achieved for three collective pitch settings on structured periodic grids featuring noncongruent cell faces along the periodicity planes. Results of a hybrid Navier ‐Stokes/Euler chimera hover computation are presented, in addition to the periodic grid analysis demonstrating the superiority of the overset grid approach with respect to tip vortex conservation. The unsteady Navier ‐Stokes computations for the nonlifting forward e ight test case show more sensitivity relative to time step size than comparable Euler analyses. Overall correlation of computed and experimental near-tip pressure distributions over blade azimuth is considered to be fair, and the strong transonic effects on the advancing blade are adequately captured by the numerical analysis.

Model Helicopter Rotor Aerodynamics and Acoustics As Measured in Two Anechoic Wind Tunnels

The qualities that are important for helicopter impulsive noise wind-tunnel testing are evaluated by comparing aerodynamic and acoustic data gathered on a single-scaled model main rotor that was tested in two open-jet wind anechoicwindtunnelsofmarkedlydifferentsize:theCEPRA19tunnelinFranceandtheDNWinTheNetherlands. The rotor was instrumented with miniature blade-mounted pressuretransducers and was tested on thesamerotor test stand located within the e ow of both open-jet wind tunnels. In-the-e ow rotor acoustic signatures and selected on-blade dynamic pressure signatures are presented for each facility and discussed in relation to the differences in tunnel geometry, tunnel e ow quality, and the aeroacoustic characteristics of each facility. This comparison shows that lowbackground noiselevels,a large in-the-e ow anechoicmeasurement space, and lowine ow turbulenceto the rotorarenecessarytoyieldhighqualityaerodynamicandacousticdata.Thebestmatchofacousticdatabetweenthe two facilitiesoccurswhen thetip-path planeoftherotorischosen tomatch closely dynamicblade-surface-mounted pressures. Achieving a good match between scaled wind-tunnel acoustics and in-e ight measurements are shown to be more dife cult: Agreement is reasonably good at low advance ratios but marginal at higher advance ratios.

Aerodynamics of helicopter rotor in hover: the lifting-vortex line method applied to dihedral tip blades

There are various methods for predicting the aerodynamic performance of helicopter rotors. In this study, the vortex method, which schematizes the blades and the rotational flow areas on the basis of a distribution of vortices, has been used to calculate the air flow around a rotor in hover. This method is an application to helicopter rotors of the Prandtl's lifting-vortex line theory for aircraft wings. It assumes perfect incompressible fluid with non-rotational flow outside the vortex sheets. The induced velocities are determined using Biot and Savart's law, and the circulation distribution by Kutta-Joukowski's law. Data on initial wake geometry and aerodynamic characteristics, the latter being obtained by bidimensional wind tunnel tests, are necessary for implementing this method. On this theoretical basis, a numerical computational program has been developed to determine rotor performance at hover with stabilisation of the vortex sheet. The rigid blades with a flapping hinge are represented by a lifting-vortex line, angled or rectilinear, located in the leading quarter of the chord; the wake is discretized into a finite number of vortex lines. Comparisons of computational and experimental test results are presented on the overall magnitudes: figure of merit and pitch angle. The tests were carried out at Aerospatiale using rotors fitted with blades with either rectangular or dihedral tip planform. In the latter case, the lifting-vortex line, no more rectilinear, takes into account the actual shape of the blades. The results obtained in this study show a good correlation between computations and tests, for both blades with classical geometry or with non-rectilinear quarter line and non-rectangular planform.

Recent applications of particle image velocimetry in aerodynamic research

Particle image velocimetry (PIV) is increasingly used to investigate unsteady velocity fields instantaneously. For the first time the PIV technique allows the recording of a complete velocity field in a plane of the flow within a few microseconds. The PIV technique thereby provides information about unsteady flow fields which is difficult to obtain with other experimental techniques. The short acquisition times and fast availability of data reduce the operational time, and hence cost, in large scale wind tunnels and test facilities. At DLR a variety of PIV systems for use in industrial wind tunnels has been developed in the past decade. The flexibility of these portable systems is illustrated by presenting several results of recent PIV applications. More recently the original photographic means of PIV image recording has been partially replaced by high resolution electronic imaging which can provide PIV data nearly on-line. Images recorded by either system use the same multiple-pass, cross-correlation analysis software, whose algorithms are briefly described. Several examples of actual applications are given: the flow issuing from a jet nozzle was imaged by a specially developed high-speed video camera at close proximity. A high resolution dual-frame digital camera was applied in the study of helicopter rotor aerodynamics and wake vortex measurements of an airplane model. Further, large image sequences exceeding 100 PIV recordings provided detailed information on the structure of a turbulent boundary layer.

Measurements of the dynamic stall vortex convection speed

AbstractThis paper considers the dynamic stall vortex of importance in helicopter rotor aerodynamics and discusses previous measurements of its convection speed. It emerges that an anomaly exists between the available data sets, i.e. that some workers find that the convection speed is dependent upon the aerofoil motion, while others find that this is not the case. Measurements of the convection speed from data gathered at Glasgow University for a variety of aerofoil shapes and motion types are then presented, which support the conclusion that the dynamic stall vortex convection speed is independent of aerofoil type and motion type to a first order.