CHANCE: A FRENCH-GERMAN HELICOPTER CFD-PROJECT

The paper gives an overview of the CHANCE research project (partly supported by the French DPAC and DGA and the German BMWA) which was started in 1998 between the German and French Aerospace Research Centres DLR and ONERA, the University of Stuttgart and the two National Helicopter Manufacturers, Eurocopter and Eurocopter Deutschland. The objective of the project was to develop and validate CFD tools for computing the aerodynamics of the complete helicopter, accounting for the blade elasticity by coupling with blade dynamics. The validation activity of the flow solvers was achieved through intermediate stages of increasing geometry and flow modelling complexity, starting from an isolated rotor in hover, and concluding with the time-accurate simulation of a complete helicopter configuration in forward-flight. All along the research program the updated versions of the CFD codes were systematically delivered to Industry. This approach was chosen to speed up the transfer of capabilities to industry and check early enough that the products meet the expectations for applicability in the industrial environment of Eurocopter

GARTEUR Helicopter Cooperative Research

This paper starts with an overview about the general structure of the Group for Aeronautical Research and Technology in EURope (GARTEUR). The focus is on the activities related to rotorcraft which are managed in the GARTEUR Helicopter Group of Responsables (HC GoR). The research activities are carried out in so-called Action Groups. Out of the 5 Action Groups which ended within the last four years results generated in the Helicopter Action Groups HC(AG14) “Methods for Refinement of Structural Dynamic Finite Element Models”, HC(AG15) “Improvement of SPH methods for application to helicopter ditching” and HC(AG16) “Rigid Body and Aeroelastic Rotorcraft-Pilot Coupling” are briefly summarized

Evolution of Neural Networks for Helicopter Control: Why Modularity Matters

The problem of the automatic development of controllers for vehicles for which the exact characteristics are not known is considered in the context of miniature helicopter flocking. A methodology is proposed in which neural network based controllers are evolved in a simulation using a dynamic model qualitatively similar to the physical helicopter. Several network architectures and evolutionary sequences are investigated, and two approaches are found that can evolve very competitive controllers. The division of the neural network into modules and of the task into incremental steps seems to be a precondition for success, and we analyse why this might be so

Analytical modeling of rotor-structure coupling using modal decomposition for the structure and the blades.

This paper presents a linear semi-analytical model that is able to predict complex rotor-structure coupling phenomena and their stability. It was primarily designed so as to gain a better physical understanding of this kind of aeroelastic instabilities, triggering at higher frequencies than air and ground resonance, and involving several blade and structure modes. The analytical approach has a two-fold advantage since fast parametric studies can be carried out and a term-by-term analysis of the helicopter stability equations can be performed. In order to represent the elasticity of the structure and the blades, a modal decomposition method is introduced. The modal basis for the structure can either be obtained by a Finite Element Method or rigid degrees of freedom can be inputted. For the blades, a preliminary finite element routine is run, allowing for varying characteristics along the span. Blade offsets are introduced, and an unsteady aerodynamic model is implemented. The modal basis of the coupled system is then computed and a partial validation is done with HOST (Helicopter Overall Simulation Tool), a comprehensive aeroelastic code. Except for the built-in twist and the non-circulatory terms which are taken in a different manner in HOST and the presented model, the linearization results are similar. Future work using this model includes investigation of the helicopter stability thanks to parametric studies

Helicopter tail rotor orthogonal blade vortex interaction

The aerodynamic operating environment of the helicopter is particularly complex and, to some extent, dominated by the vortices trailed from the main and tail rotors. These vortices not only determine the form of the induced flow field but also interact with each other and with elements of the physical structure of the flight vehicle. Such interactions can have implications in terms of structural vibration, noise generation and flight performance. In this paper, the interaction of main rotor vortices with the helicopter tail rotor is considered and, in particular, the limiting case of the orthogonal interaction. The significance of the topic is introduced by highlighting the operational issues for helicopters arising from tail rotor interactions. The basic phenomenon is then described before experimental studies of the interaction are presented. Progress in numerical modelling is then considered and, finally, the prospects for future research in the area are discussed

Influence of blade aerodynamic model on the prediction of helicopter high-frequency airloads

Brown’s vorticity transport model has been used to investigate the influence of the blade aerodynamic model on the accuracy with which the high-frequency airloads associated with helicopter blade–vortex interactions can be predicted. The model yields an accurate representation of the wake structure yet allows significant flexibility in the way that the blade loading can be represented. A simple lifting-line model and a somewhat more sophisticated liftingchord model, based on unsteady thin aerofoil theory, are compared. A marked improvement in the accuracy of the predicted high-frequency airloads of the higher harmonic control aeroacoustic rotor is obtained when the liftingchord model is used instead of the lifting-line approach, and the quality of the prediction is affected less by the computational resolution of the wake. The lifting-line model overpredicts the amplitude of the lift response to blade–vortex interactions as the computational grid is refined, exposing the fundamental deficiencies in this approach when modeling the aerodynamic response of the blade to interactions with vortices that are much smaller than its chord. The airloads that are predicted using the lifting-chord model are relatively insensitive to the resolution of the computation, and there are fundamental reasons to believe that properly converged numerical solutions may be attainable using this approach

March 1971 wind tunnel tests of the Dorand DH 2011 jet flap rotor, volume 1

The results of wind tunnel tests, second series of tests performed in the NASA Ames 40 x 80 foot wind tunnel, of the DH 2011 jet-flap rotor are presented and analyzed. The tests have been focused on multicyclic effects and the capability of this rotor to reduce the vibratory loads and stresses in the blades. The reductions of the vibrations and stresses at tip speed ratio of 0.4 have attained 50%. The theory shows further reductions possible, reaching 80%. The results show that the performance characteristics after the modifications introduced since 1965 remained unchanged. The domain of investigation has been enlarged to include the tip speed ratios of 0.6 and 0.7. To analyze the complex aeroelastic phenomena a new analytical technique has been utilized to represent the mathematical model of the rotor. This technique, based on transfer matrices and transfer functions, appears very simple and it is believed that this analysis is applicable to many kinds of investigations involving large numbers of variables

High resolution computation of the aerodynamics and acoustics of blade vortex interaction

In the present work, high resolution CFD simulations have been performed on an idealised problem of the interaction of an independently generated vortex with a rotor blade, including a case where the vortex directly impacts on the blade. The resulting blade pressures and acoustics are comprehensively compared against experimental measurements. Two different modelling approaches are used: the first is to impose the vortex as a perturbation to the velocity field, and the second is to fully resolve the vortex formation, evolution and its interaction with the blade. For a case in which the vortex passes near the blade surface, the the fully resolved approach is confirmed to accurately preserve the vortex structure. The far field acoustic predictions offered by the fully resolved approach are seen to be very accurate and definite improvements are observed in the computed blade pressures and acoustics over the imposed vortex approach and other similar works in the literature. For a case in which the vortex axis passes through the blade, the shape and width of the acoustic pulse in the far field is accurately represented by the fully resolved approach, while the magnitude is slightly underpredicted. The improvement in prediction offered by the fully resolved approach is because this method allows for a more realistic representation of phenomena, such as dynamic change in vortex structure and trajectory due to the blade passage, that become important when the vortex miss-distance becomes small

Aeroacoustic research programs at the Army Aviation Research and Technology Activity

The Army rotorcraft aeroacoustic programs are reviewed, highlighting the theoretical and experimental progress made by Army researchers in the physical understanding of helicopter impulsive noise. The two impulsive noise sources addressed over this past decade are high-speed impulsive noise and blade-vortex interaction noise, both of which have had and will continue to have an increasing influence on Army rotorcraft design and operations. The advancements discussed are in the areas of in-flight data acquisition techniques, small-scale-model tests in wind tunnels, holographic interferometry/tomographic techniques, and the expanding capabilities of computational fluid dynamics in rotorcraft acoustic problems. Current theoretical prediction methods are compared with experimental data, and parameters that govern model scaling are established. The very successful cooperative efforts between the Army, NASA, and industry are also addresse

Whirl and Stall Flutter Simulation Using CFD

This paper presents recent research on numerical methods for whirl and stall flutter using computational fluid dynamics. The method involves coupling of the HMB3 CFD solver of the University of Glasgow and a NASTRAN derived structural model. Based upon a literature survey, a significant amount of research has been conducted on the numerical investigation of tiltrotors, with a focus on the XV-15 and V-22 aircraft. Within this paper, the coupling procedure is presented along with a steady CFD computation to highlight the accuracy of the high-fidelity method. In addition to this, a simple method is used to investigate the whirl flutter boundary of a standard propeller and the XV-15 blade