Optimisation of hybrid high-modulus/high-strength carbon fibre reinforced plastic composite drive shafts

International audienceThis study deals with the optimisation of hybrid composite drive shafts operating at subcritical or supercritical speeds, using a genetic algorithm. A formulation for the flexural vibrations of a composite drive shaft mounted on viscoelastic supports including shear effects is developed. In particular, an analytic stability criterion is developed to ensure the integrity of the system in the supercritical regime. Then it is shown that the torsional strength can be computed with the maximum stress criterion. A shell method is developed for computing drive shaft torsional buckling. The optimisation of a helicopter tail rotor driveline is then performed. In particular, original hybrid shafts consisting of high-modulus and high-strength carbon fibre reinforced epoxy plies were studied. The solutions obtained using the method presented here made it possible to greatly decrease the number of shafts and the weight of the driveline under subcritical conditions, and even more under supercritical conditions. This study yielded some general rules for designing an optimum composite shaft without any need for optimisation algorithms

Dynamics of a supercritical composite shaft mounted on viscoelastic supports

International audienceThe damping in a carbon fiber reinforced plastic (CFRP) laminate is greater than that which occurs in most metallic materials. In the supercritical regime, the damping can trigger unstable whirl oscillations, which can have catastrophic effects. The vibrations occurring in a supercritical composite drive shaft are investigated here in order to predict instabilities of this kind. A simply supported carbon/epoxy composite tube mounted on viscoelastic supports is studied, using an approximation of the Rayleigh-Timoshenko equation. The damping process is assumed to be hysteretic. The composite behavior is described in terms of modulus and loss factor, taking homogenized values. The critical speeds are obtained in several analytical forms in order to determine the effects of factors such as the rotatory inertia, the gyroscopic forces, the transverse shear and the supports stiffness. Assuming that the hysteretic damping can be expressed in terms of the equivalent viscous model, the threshold speed is obtained in the form of an analytical criterion. The influence of the various factors involved is quantified at the first critical speed of a subcritical composite shaft previously described in the literature. The influence of the coupling mechanisms on the unsymmetrical composite laminate and the end fittings is also investigated using a finite element model. None of these parameters were found to have a decisive influence in this case. Those having the greatest effects were the transverse shear and the supports stiffness. The effects of the composite stacking sequence, the shaft length and the supports stiffness on the threshold speed were then investigated. In particular, drive shafts consisting only of ±45° or ±30° plies can be said to be generally unstable in the supercritical regime due to their very high loss factors

Dynamic instability of supercritical driveshafts mounted on dissipative supports ― effect of viscous and hysteretic internal damping

International audienceThe case of a rotating shaft with internal damping mounted either on elastic dissipative bearings or on infinitely rigid bearings with viscoelastic suspensions is investigated in order to obtain the stability region. A Euler–Bernoulli shaft model is adopted, in which the transverse shear effects are neglected and the effects of translational and rotatory inertia, gyroscopic moments, and internal viscous or hysteretic damping are taken into account. The hysteretic damping is incorporated with an equivalent viscous damping coefficient. Free motion analysis yields critical speeds and threshold speeds for each damping model in analytical form. In the case of elastic dissipative bearings, the present results are compared with the results of previous studies on finite element models. In the case of infinitely rigid bearings with viscoelastic suspensions, it is established that viscoelastic supports increase the stability of long shafts, thus compensating for the loss of efficiency which occurs with classical bearings. The instability criteria also show that the effect of the coupling which occured between rigid modes introducing external damping and shaft modes are almost more important than damping factor. Lastly, comparisons between viscous and hysteretic damping conditions lead to the conclusion that an appropriate material damping model is essential to be able to assess these instabilities

Design of high rotation frequency composite tubes

International audienceThis work relates to the sizing of subcritical and supercritical laminated composite drive shafts. The hollows drive shafts are designed to transmit the torsional load and to minimize the dynamic effects due to rotation. To meet this need, these structures must combined strength, rigidity and lightness. New high modulus carbon fibres can be adapted to a progress in this field. This work presents sizing tools for optimisation of supercritical drive shafts mounted on viscoelastic suspensions. Two items are discussed: dynamic analysis (response to forces excitation; free motion; instability), strength (buckling of thin tubes; torsional strength). Supercritical drive shafts examples made of high modulus fibres or high-strength fibres are presented

Effets des non linéarités géométriques sur les vibrations d'un Jeffcott rotor surcritique – mise en évidence d’un cycle limite

On s’intéresse à la stabilité d'un rotor à grande vitesse de rotation. Cette vitesse est limitée par des modes instables issus de l’amortissement tournant. L’ajout des non linéarités géométriques fait apparaître une bifurcation de Hopf supercritique. Les modes instables sont repoussés vers un cycle limite stable de rayon faible. Ce résultat permet de limiter la dangerosité de ces modes en bloquant la translation des paliers et permettra l’observation et la mesure expérimentale de ces modes

Vortex dynamics resulting from the interaction between two NACA 23 012 airfoils

International audienceAn experimental study of the interaction between two airfoils, corresponding to a T-tail aircraft configuration, is implemented in a wind tunnel for a range of medium Reynolds numbers. Measurement of aerodynamic forces is carried out by strain gauges and the velocity field is obtained by particle image velocimetry. The study focuses on large angles-of-attack configurations, corresponding to detached flows on the airfoils and the presence of a downstream vortex shedding. Phase averages of the velocity fields are made building the flow time development. A lift modulation depending on geometric parameters and resulting from different pathlines of the shedding vortices, is discussed

Influence de l’endommagement matriciel sur la résistance en compression sens fibre pour des composites stratifiés

International audienceL'étude expérimentale proposée ici vise à caractériser le comportement d'un composite tissé verre/epoxy et d'étudier l'influence de l'endommagement matriciel sur ses propriétés à rupture en compression. Tout d'abord, des résultats sur l'influence de l'endommagement matriciel sont rappelés. Ensuite, le comportement en compression du matériau non endommagé est étudié en réalisant des essais de flexion quatre points et de compression normalisés (ASTM-D695). Dans un second temps, une campagne d'essais à haute température permet de montrer l'effet de la dégradation thermique de la matrice sur la résistance sens fibre. Enfin, l'effet de l'endommagement sur la rupture en compression est étudié. Abstract The present research focuses on the identification of the compressive behavior of a woven glass/epoxy composite and the influence of matrix damage on its compressive strength. First results concerning the influence of matrix damage on composite failure properties are reminded. In the following part the behavior of the glass/epoxy composite is identified via four-points flexion tests and standard compressive tests (ASTM-D695). Then, the results of high temperature flexion tests are presented to highlight the influence of matrix damage on compressive strength in fiber direction. Finally, effect of transverse damage on compressive strength is studied

Simulation of the aeroelastic behavior of a possibly detached flow airfoil by a discrete vortex method

International audienceThe aeroelastic behavior of an airfoil results in a complex coupling between the elastic response of the structure and the dynamics of the flow. It can lead to the failure of a lifting surface which consequences could be catastrophic. Experiments and high-order computations contribute to the understanding of this phenomenon, but fast low-order methods are needed for engineering tasks. In the present work, we implement a loose fluid-structure coupling between a discrete-time vortex method, using a leading edge shedding criterion, and the structure motion equations. For each time step, the aerodynamic coefficients are first calculated before the computation of the motion of the structure. Flutter velocity is obtained with the same precision as unsteady standard method. The advantage of the method proposed is the ability to catch the limit cycle for velocities larger than flutter speed due to dynamic stall of the airfoil

Optimisation conceptuelle d'un drone HALE à énergie solaire en structure composite

Les drones haute altitude et longue endurance à énergie solaire peuvent être une solution pour de nombreuses missions. La difficulté dans la réalisation de tels drones est due aux très hautes altitudes visées et à la faible puissance électrique disponible pour alimenter les moteurs. L'optimisation réalisée ici consiste à maximiser la charge utile pour une masse totale fixée. Elle nécessite l'expression des postes de masse constituants le drone. En particulier, la masse de la voilure est minimisée par l'utilisation de matériaux composites et en tolérant une grande souplesse. L'optimisation montre l'existence du drone dans un plan vitesse de croisière/coefficient de portance. Celle-ci fait apparaître une solution optimale présentant une charge utile de l'ordre de 10% de la masse totale

Maîtriser la conception des drones solaires à voilure souple : vers l’avènement des pseudo-satellites à hautes altitudes (HAPS)

International audienceInitié dans les années 1950 avec l'invention des cellules photovoltaïques, le concept d'un drone solaire de haute altitude à endurance quasi-illimitée fait toujours l'objet d'une recherche active en raison de verrous technologiques persistants. La faible puissance propulsive extraite de la source solaire impose une architecture très particulière d'un tel aéronef avec notamment une voilure très allongée et très souple particulièrement vulnérable à des interactions destructrices. Ces dernières relèvent du domaine de l'aéroélasticité, discipline à l'interface entre l'aérodynamique et la mécanique des structures. Le présent article illustre ainsi l'approche nécessairement transdisciplinaire de ce défi technologique et présente des solutions innovantes issues de la recherche. Celles-ci devraient à terme permettre l'exploitation commerciale de ce concept capable de combler dans le domaine de l'observation et des télécommunications un creux capacitaire entre les actuels drones conventionnels de haute altitude et les solutions satellitaires. ABSTRACT. Initiated in the 1950s with the invention of photovoltaic cells, the almost unlimited endurance high altitude solar drone concept is still under heavy development because of persistent technological barriers. The low propelling power extracted from solar sources involves a specific design of an airframe consisting of a very lightweight and flexible airframe particularly vulnerable to destructive interactions. The latter fall within the scope of aeroelasticity, a discipline between aerodynamic and structural mechanics. This article illustrates the transdisciplinary approach required to achieve such a challenging goal and presents innovative solutions based on research work. These should enable commercial exploitation of such a concept and then fill the gap between conventional HALE drones and satellites in the field of observation and telecommunication. MOTS-CLÉS. Drone HALE, HAPS, énergie solaire, aéroélasticité, flutter, endurance