Rotorcraft aerodynamic

Citation

(2002), "Rotorcraft aerodynamic", Aircraft Engineering and Aerospace Technology, Vol. 74 No. 6. https://doi.org/10.1108/aeat.2002.12774fac.001

Rotorcraft aerodynamic

Rotorcraft aerodynamic

Keywords: Rotorcraft, Aircraft, Aerodynamics

At a recent Royal Aeronautical Society/CEAS Aerospace Aerodynamics Research Conference at Cambridge, the work being undertaken relating to rotorcraft occupied a substantial part of the proceedings. Within Targeted Research Action in Aerospace Aerodynamics (TRA3) there are various key technology areas which are being addressed; one, KTA9 relating to the potential for helicopters and tilt rotor aircraft. The papers presented provided an assessment of the problems and possible solutions and gave indications of the extensive investigations into the phenomena inherent in the operation of such aircraft, such as unsteady flow conditions, the prediction and alleviation of dynamic stall, and the understanding of convoluted vortex wake structures. Tilt rotor aerodynamics are even more complex and lack test data and prediction capabilities.

In the first session relevant to these aircraft, the initial contribution came from Agusta and Eurocopter which described the aerodynamic challenges of the tilt rotor. The design of such an aircraft must follow a different strategy than previously, since it merges the characteristics of the helicopter and the aeroplane. With respect to the currently known configurations of tilt rotors, the European advanced tilt rotor ERICA introduces an additional degree of freedom; the tilt of the outboard portion of the wing. Several improvements can be achieved because of this: rotors can be of reduced diameter; a wider conversion corridor and an optimisation of the flight path can be obtained; and a further improvement in handling quality. On behalf of the European Community, several Critical Technology Projects (CTPs) are and will be studied during 2000-2005, the prime object being to study specific aspects of tilt rotor aeromechanics, flight charactertics and technology that are considered to be of the highest importance before designing and testing a flying demonstrator.

The paper also gave an overview of the activities to be performed and the results already obtained in these different CTPs in particular those dealing with aeromechanics and flight mechanics: PHILP; (Rotorcraft Handling, Interactions, and Load Predictions), a project aimed at flight controls and mechanics.

TILTAERO; the main fields to be investigated within this project are the aerodynamic interactions between rotor and wing, rotor and fuselage, in the various modes of flight, helicopter mode, conversion, and fixed-wing mode.

ACT-TILT; this project innovation is mainly to define and validate through pilots-in-the-loop simulation an advanced tilt rotor Flight Control System (FCS) meeting all the stringent requirements for use of the tilt rotor aircraft in a commercial environment.

ADYN; the main goal of this project is to define, design, manufacture and test a half-span model of the advanced tilt rotor configuration for instability (in the conversion corridor) and whirl flutter (at high speed) investigation and codes adaptation/tuning. The TILTATRO scaled model will be re-used as much as possible, to reduce costs.

A paper by ONERA dealt with simplified models for Tilt Rotor Aerodynamic Phenomena in hover and low speed flight; in particular with the nacelles tilted at 90° for the helicopter mode. On tilt rotor in this mode, a strong aerodynamic interaction exists between the rotors and the wing creating a penalising download in terms of hover performance and payload capability. The object of the present work is to propose simplified models for the main phenomena of the rotor/wing interaction to improve the tilt rotor model in the Eurocopter’s helicopter flight mechanics code-Helicopter Overall Simulation Tool (HOST). The work was partly supported by the European Union under the Brite Zuram programme. Proposed models include win download calculation by either estimating the download in hover based on the drag force of the wing under −90° angle of attack for the direct effect (rotor wake impinging on the wing) or a second approach that could be used consists of calculating the aerodynamic forces in the flight mechanics code. This could be done with the induced velocity computed at the wing level and the wing airfoil lift and drag coefficients for a wide range of negative wing’s angles of attack, and for different angles of flap deflection. Another proposed model deals with the effect of the wing on the rotor. This is to “block” the rotor wake and therefore diminish the induced velocities on the rotor blades when the rotor blade is over the wing. This effect could be assimilated at a local ground effect. The rotor blade induced velocity decrease, for an azimuthal sector over the wing, has been computed by using published results for the rotor mean induced, velocity in ground effect at the height of the rotor/wing distance. This effect decreases the rotor required power.

Experimental and theoretical assessment of the Helicopter Ground Vortex Phenomena came from NLR and DLW research facilities. Activities conducted within the Brite Euram HELIFLOW project aim at aerodynamic knowledge, validation of wind tunnel test data and verification of related theoretical tools. One task focuses on helicopter handling during sideways flight (in-ground-effect, i.e. accurate prediction of trim conditions). Tests were conducted with facilities including those in the Large Low Speed DNW wind tunnel. Pre-test calculations were made using NLR’s in-house developed ground vortex prediction tool OUTWASH.

The objectives of the experimental activities were to: demonstrate the suitability of various measurement techniques on complex unsteady ground vortex structures; investigate the flow conditions resulting from the changes in test conditions like tunnel speed and model height above the ground; and provide accurate velocity field measurements in order to improve and validate theoretical models for the prediction of the rotor outwash flow feature.

Analysis and software

In this session the first paper came from the University of Glasgow and dealt with the comparison between two implicit time-stepping schemes in an Euler code for helicopters. The EROS project is developing a code for the analysis of helicopter flow rotor flowfields and the load pressure distribution along the blades. This code started from the industrial requirement for a tool in Europe adapted for the study of helicopters. Possible errors in various previous methods of computation have been remedied by Glasgow University’s use of the UNFACtored method. Regarding the aerodynamic results, this is in good agreement with previous test cases results with the advantage of getting a faster convergence for a coarse grid. The effect of far-field boundary conditions on tip vortex path prediction in hovering was given by Departmento do Ingegneria Aerospatiale Politecnico di Milano and concerned the accurate predictions of the behaviour of the trailing vortex generated at the blade tip of helicopter rotors, which are important for rotor design. Hovering and descent flight the interaction between a blade and the tip vortex shed by the preceding blade is particularly intense and the blade vortex interaction (BVI) mechanism influences the blade loads and also the impulse noise generated by the rotor itself. The capability for European helicopter industries to perform multi-blade rotor calculations with wake capturing was one of the main objectives that motivated the EROS project, co-funded by the European Commission within the Brite Euram programme.

The ONERA/ECF 7AA model rotor was used in the DNW facility during the HELISHAPE research campaign to assess the effects of the model. It is a four-bladed rotor of 2.l m radius, 0.14 m chord with linear twist distribution and rectangular planform. The effect of the “Froude” boundary conditions on the blade pressure distribution is not very relevant. On only about 90 per cent of the blade radius the suction side pressure distribution is in better agreement with the experimental results. A strong influence of the far-field conditions is seen instead on the overall flow field. It is evident how the use of the “Froude” boundary conditions allows for a better vortex definition and an improved trajectory prediction.

The University of Bristol gave a paper on Application and Extension of the GEROS Grid Generation System. The software suite in the EROS project consists of many modules that together constitute a flow-solver and grid generator. GEROS is the name of the EROS grid generator. The EROS consortium consists of various countries’ research institutes, universities and industrial partners, the project running from April to September 1999, being co-ordinated by CIRA (Italy).

The system is currently being improved and extended to include the following options: automatic generation of background meshes; new grid spacing parameters being developed since the EROS code is being extended to a Navier-Stokes solver; generation of acoustic surfaces; generation of common slip-surface; adaptation by embedding cartesian meshes; and investigation of more robust taggers.

Steep descent, simulation and optimisation

The initial paper in this session was by ONERA, France and was concerned with helicopter flight test in steep descent. For military rotorcraft particularly, more aggressive manoeuvres will be possible while civil applications will permit a reduction in noise pollution. Aerodynamic understanding of handling qualities in these conditions is not sufficient and the helicopter flight envelope in steep descent is limited by the dangerous region of vortex ring state. This was involved in many helicopter accidents between 1982 and 1997 and probably also in the marines V-22 tilt rotor crash.

Implementation of an empirical induced velocity model working in all flight configurations in Eurocopter flight mechanics code HOST improved its capability to simulate descent at high slope angles, including the vortex ring state. Flight tests were performed with the CEV Dauphin 6075, and provided the power versus rate of descent curves for different forward velocities in vertical descent. The vortex ring state remains still not understood and tests analysed helicopter behaviour. Vortex ring state limits were obtained during the CEV flight tests. The limits obtained with the vortex ring state prediction criteria reported in 2001 when plotted showed a good correlation. The vortex ring state, most dangerous feature is the sudden V_z fall. The pilot’s instinctive reaction is then to increase the collective level in order to stabilize the rate of descent. Flight test and HOST simulations with the proposed model confirmed that the rate of descent is insensitive to collective increases. The fastest way to leave the dangerous region is rather to increase the forward velocity. Nevertheless, the altitude loss will be important.

A numerical simulation of rotary wing flowfields on parallel computers was by the MiddleEast Technical University of Ankara, Turkey, which described an in-house, parallel Euler solver developed to compute inviscid flows around rotary wing configurations. Parallel processing with distributed memory is utilized to reduce computational time and memory requirements. Computations are performed for subsonic and transonic flows over a two-bladed hovering helicopter rotor. Non-lifting and lifting rotor solutions are obtained separately and the blade vortex interaction phenomena is investigated. Preliminary results show that the vortex wake experiences a non-physical dissipation due to the numerical algorithm. This can partly be overcome using local mesh clustering.

Accurate calculation of rotary wing flowfield requires correct prediction of the vortical wake, in particular, the two following features: (1) contraction of the wake as it enters in the axial direction below the rotor, and (2) interaction of the tip vortex of one rotor blade with the advancing blade. The developed code was run for various flow conditions and blade configurations and some results obtained. Studies have continued to improve the numerical algorithm and obtain more reliable results.

From EUROCOPTER Deutschland came the Optimisation of Air Intake Flow with CFD which dealt with the Computational Fluid Dynamics now being used to optimise the shape of helicopter components such as air intakes, fuselage shell, empennage, etc. computing the aerodynamic loads necessary for structural analysis; and predicting the rotor performance in both hover and forward flight conditions. The paper presents an internal flow CFD optimisation study of the air intake geometry of the engine mounted on the EC 135 helicopter. The numerical prediction is conducted by means of a commercial unstructured code.

The aim of this design optimisation study is to reduce the velocity peak values, to get a more uniform static pressure distribution, and to reduce the pressure losses inside the engine air intake. Two modified geometries have been studied. Comparing them shows a slightly better behaviour of the second. Summed up over the whole entrance section area, the gain in pressure losses is as high as 88 per cent for the second modification and 64.6 per cent for the first modification.

Navier-Stokes analysis

Contributed from joint French and German sources was this description of the application of the Navier-Stokes Codes developed in the framework of the joint German/French CFD Research Programme CHANCE. Considerable effort is being put into the development and validation of advanced CFD methods with ONERA, DLR and EUROCOPTER, the last named validating them against test cases of industrial interest. These CFD codes are also integrated in rotor and fuselage design systems at Eurocopter. The activities described relate to the validating of the CFD solvers prediction capabilities about an isolated rotor in hovering flight, and isolated fuselages in forward flight. The main difficulty in performance prediction in hover is related to the complex vortical field shed by the blades during rotation. The vortices emitted by the rotor blades are the driving force for the downwash across the rotor disc and the wake contraction, which have a strong influence on the rotor loading and the induced power consumption. Using the FLOWer code, a performance study has been conducted for the ATR-4, four-bladed rotor mounted on the EC145 helicopter. Two elementary effects which significantly impact performance are being addressed separately: effect of blade torsional flexibility; and effect of laminar- turbulent transition. Further computations are foreseen.

Regarding the fuselage flow in forward flight, the main difficulty is associated with the intricated geometry of some rather blunt protruding elements such as the landing gear sponson, the engine fairing, or the IRS fairing. The NH90 helicopter has been chosen as a validation test case of industrial interest for the CFD elsA software, due to its complex but realistic shape. Further refinements to the grid in limited zones may be necessary.

From DLR Germany came a Navier-Stokes analysis of the Helicopter RotorFuselage Interference in Forward Flight. Computing rotary wing flows is associated with extremely complicated flow physics. That is, the combined rotational and pitching motion of helicopter blades results in a strong aerodynamic interaction between each blade and the wakes of the preceding blades thus creating a complex vortex system. Moreover, essentially different flow regimes are formed on the blade by large variations in velocity according to the flying conditions and radial and azimuthal positions resulting in inherently different flow structures. For example, in forward flight, the flow may become supersonic at the tips of advancing blades while it remains subsonic near their roots. On the retreating blades, separation regions may be created by large pitch angles and possible flow reversal on parts of the blade. The problem is even more challenging when the fuselage is taken into account. In the extreme case of vertical ascent, the rotor creates strong downwash which impinges the fuselage, resulting in a substantially complex flow pattern.

There were two main approaches to simulate rotors numerically. The first resolves the motion of the blades accurately in time. Numerical grids are generated round the blades and are allowed to rotate intersecting a background grid by means of an overlapping grid technique (Chimera). Many rotor revolutions are usually required for complete evolution of the flow field. Fine grid resolutions over a large extent of the background grid is needed to predict advancing rotor flows by Chimera. Alternatively, the momentum jump and mass flow across the rotor are introduced to the flow as special boundary conditions. These discontinuities are calculated by blade element or momentum theories allowing variations in the radial and azimuthal directions on the disk.

To minimise the grid generation efforts overlapping grid- methods can be used. The actuator disk model is particularly attractive as it removes the need for unsteady simulation of the rotor and represents the effects of the rotor by a simple, computationally inexpensive boundary condition. Only limited fuselage/actuator disk results are at present available, which are sufficient to show the validity of the actuator disk approach to compute helicopter flows. Both actuator disk and Chimera techniques will be applied to study the rotor-fuselage interaction in the forward flight case. The validity and reliability of both rotor simulation approaches to predict helicopter flows will be assessed by direct comparison of the numerical, results with experimental measurements.