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From Albatrosses to Long Range UAV Flight by Dynamic Soaring

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From Albatrosses to Long Range UAV Flight by Dynamic Soaring From Albatrosses to Long Range UAV Flight by Dynamic Soaring Vincent Bonnin A Thesis submitted in partial fulfilment of requirements of the University of The West of England, Bristol, and ISAE, Toulouse, for the degree of Doctor of Philosophy Abstract In the domain of UAVs, endurance and range are key utility factors. However, small-sized UAVs are faced with serious limitations regarding energy storage options. A way to address this challenge is to seek for energy from the surrounding environment. One flight technique, called dynamic soaring, has been perfected by large seabirds like the albatross, which enables them to wander effortlessly in southern oceans. This thesis investigates the feasibility to find inspiration from the biological world in order to address the issue of limited endurance. First of all, an extensive literature background sums-up a range of technical aspects that can be learnt out of the flight of albatrosses. It reviews their morphology, flight performance and sensitivity to wind strength, their flight characteristics and energy expenditure management. Then, a methodology to simulate dynamic soaring flight is built-up by focusing first on adequate models for the vehicle and for the environment. It details the way those models are described quantitatively and qualitatively. As for the vehicle, a point mass model is chosen and applied to fixed-wing gliders of several scales, as well as to an albatross of generic dimensions. The environment is first modelled by classical boundary layer theory on a rather flat surface and then refined by taking into account specificity about the ocean boundary layer, such as varying roughness length and surface waves. Equations of motion are detailed for both points of views, earth-relative and air relative. This yields two different sets of equations of motion, eventually representing equivalent physics. An optimization problem is then set in order to determine, for the vehicle, how to extract energy from its environment. Variations in objective function and in constraints are described before presenting the numerical integration scheme which converts the optimization problem into that of finite-dimension. The solving tools and their specificity are presented, followed by a validation of the overall methodology with a particular study case from the literature. Basic principles of dynamic soaring flight are explicated by using a specific closed-loop study case. Energy-harvesting mechanisms are disclosed locally and next integrated over the whole flight path. A further illustration of dynamic soaring is i provided by relaxing some periodicity constraints and opening the trajectory. The specificity of the ocean boundary layer environment is finally implemented and a refined energy-harvesting strategy is presented. Air relative equations of motion are dimensionless so as to highlight specific dynamic soaring behaviours, in the case of a simplified linear wind profile and eventually by finding an appropriate non-dimensionalization for a logarithmic wind profile. Conditions of similarities between dimensionless solutions are described and some basic DS characteristics are outlined. Finally, various dynamic soaring performance study case are computed. Optimized trajectories are implemented for the selected vehicles and compared on a required wind strength basis. The sensitivity of the required wind strength to the net flight heading as well as to the ground clearance and to the surface roughness length is determined by drawing performance charts. In order to enlarge the scope of favourable dynamic soaring conditions, thrust-augmented trajectories are considered. The range improvements offered by dynamic soaring are compared to the straight line case, for different wind strength and different net flight headings. ii Résumé de la thèse en français Dans le domaine des drones, l'endurance et l’autonomie sont reconnues comme étant des facteurs d'utilité clés. Ces véhicules sans pilotes permettent d’élargir le spectre des missions accessibles aux aéronefs, notamment, pour certaines applications, en réduisant leur taille. Cependant, les drones de faibles dimensions sont confrontés à des limitations importantes concernant les options de stockage d'énergie. Des solutions alternatives de propulsion ont été approchées dans la littérature, comme le recours aux piles à combustible qui permet de tirer parti de l’importante densité énergétique du dihydrogène. Ou encore la modification de la chaine énergétique à bord, en déportant la source au sol et en transmettant au véhicule l’énergie via un laser. Chacune de ces options a ses limites à l’heure actuelle. Une autre façon de répondre à cet enjeu est d’extraire de l'énergie à l'environnement ambiant au véhicule. Cela peut se décliner sous plusieurs formes et des solutions ont été abordées par le passé, comme le vol de pente, le vol de thermique ou encore l’exploitation photovoltaïque de l’énergie solaire. Ce sujet se rallie à la même démarche générale d’exploitation de l’énergie ambiante, mais se base sur des principes d’extraction différents. Par ailleurs, ces problématiques s’inscrivent dans un contexte de bio-mimétisme, où la nature est considérée comme une source d’inspiration, d’autant plus alors que les drones peuvent atteindre des dimensions comparables à celles des oiseaux. Une technique de vol a été perfectionnée par de grands oiseaux de mer comme l'albatros, ce qui leur permet de parcourir les océans de l’hémisphère sud, sur des milliers de kilomètres, sans effort. En effet, les travaux de biologistes ont permis d’identifier certaines caractéristiques du vol de l’albatros. Parmi les plus significatives figurent leur vol non battu et les distances considérables qu’ils parcourent, ce qui sous- entend une gestion énergétique intéressante. Afin de bénéficier d’une source d’informations aussi large que possible concernant le vol de l’albatros, une vaste revue bibliographique est faite des contributions biologistes offrant un éclairage sur les aspects physiques de l’albatros et de son vol. A ce titre, cette bibliographie s’attache à se distancer des aspects comportementaux, ou du moins à les distinguer, pour se concentrer sur les informations physiques du vol. Ainsi, on peut apprendre que des spécimens équipés de balises GPS ont parcouru des distances de voyages considérables, allant même jusqu’à des circonvolutions dans les latitudes sud. Ces performances sont associées à des vitesses de iii voyage pouvant dépasser 900 kilomètres par jour. Cette vaste couverture des mers s’explique, pour l’aspect comportemental, par un besoin de l’oiseau de maximiser la probabilité de rencontrer des proies qui ont une distribution très sporadique en surface. Ces performances soulèvent d’autant plus d’interrogations que la morphologie des albatros démontre une incapacité à battre durablement des ailes. En outre, toutes les observations font de fait état d’un vol strictement plané, pour lequel l’albatros est en revanche particulièrement adapté. En effet, ses ailes ont un grand allongement et peuvent être maintenue déployées sans que l’oiseau force, par l’intermédiaire d’un tendon. Dès la fin du XXème siècle, il a été conjecturé que ces oiseaux tiraient parti du vent. De fait, la répartition de leurs colonies sur les iles isolées des mers du sud correspond effectivement aux zones qui voient les vents les plus fréquents et les plus puissants. Cette dépendance au vent est même marquée par des oiseaux bloqués sur l’eau lorsque le vent vient à tomber, alors qu’ils passent la majorité de leur temps en l’air. D’autres informations sont utiles pour enrichir la compréhension de ce rapport au vent. Leur direction de vol par rapport au vent est orientée dans le même sens que le vent, mais avec une composante de travers fortement prononcée, de telle sorte que l’oiseau vole aux alentours de 45 ° par rapport au sens du vent. Enfin, à plus faible échelle, leur vol fait apparaitre des trajectoires sinueuses et cycliques, avec des virages successifs en montée et en descente, entre la surface et des altitudes aux alentours de 15-20 mètres maximum. Il apparait que les albatros tirent profit des gradients de vents qui se forment à la surface, dues aux interactions de couches limites atmosphériques. L’ensemble de ces considérations, associées au fait qu’un drone de faibles dimensions peut, à priori, reproduire les dimensions et performances planés de l’albatros, font de cette technique un sujet de recherche prometteur dans le domaine des drones. En effet, la source d’énergie requiert uniquement la présence de vent et est uniformément présente sur de vastes zones géographiques, au contraire des thermiques par exemples, qui sont limitées à des zones réduites et clairsemées. La technique de vol démontrée par les albatros est quelquefois appelée vol de gradient en français, mais on lui préfèrera l’expression anglo-saxonne dynamic soaring (DS), plus couramment usitée. Elle traduit en effet la dynamique nécessaire à l’extraction d’énergie, là où d’autres techniques, comme le vol de thermique ou de pente, voit l’extraction se faire sous forme constante, via un régime de vol statique. Lors du vol par DS, au contraire, les échanges d’énergie sont périodiques, avec des gains et des pertes, au travers de manœuvres dynamiques de vol. La question principale de ce travail de recherche consiste à savoir s’il est faisable, et dans quelle mesure, d’exploiter le DS afin de répondre aux enjeux de vol longue distance pour des drones. Ce travail de recherche a une dimension iv clairement multidisciplinaire et on conceptualisera trois domaines de recherche afin de structurer notre raisonnement. Il s’agit du véhicule, de l’environnement et des trajectoires. Différents objectifs de recherche sont déterminés, relatifs à chacun de ces concepts et à la façon dont ils s’influencent. Afin de circonscrire le sujet et ses développements, une bibliographie de la littérature est faite, parcourant les différentes approches d’extraction d’énergie au vent qui ont pu être étudiées.
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