Optimal dynamic soaring consists of successive shallow arcs

Abstract

Albatrosses can travel a thousand kilometres daily over the oceans. They extract their propulsive energy from horizontal wind shears with a flight strategy called dynamic soaring. While thermal soaring, exploited by birds of prey and sports gliders, consists of simply remaining in updrafts, extracting energy from horizontal winds necessitates redistributing momentum across the wind shear layer, by means of an intricate and dynamic flight manoeuvre. Dynamic soaring has been described as a sequence of half-turns connecting upwind climbs and downwind dives through the surface shear layer. Here, we investigate the optimal (minimum-wind) flight trajectory, with a combined numerical and analytic methodology. We show that contrary to current thinking, but consistent with GPS recordings of albatrosses, when the shear layer is thin the optimal trajectory is composed of small-angle, large-radius arcs. Essentially, the albatross is a flying sailboat, sequentially acting as sail and keel, and is most efficient when remaining crosswind at all times. Our analysis constitutes a general framework for dynamic soaring and more broadly energy extraction in complex winds. It is geared to improve the characterization of pelagic birds flight dynamics and habitat, and could enable the development of a robotic albatross that could travel with a virtually infinite range.

Soaring styles of extinct giant birds and pterosaurs

“…However, recent studies have argued that the real sea surface is not flat, and wind separations in ocean waves may occur more often than expected (44). To describe wind-separation-like wind profiles, a sigmoidal model has been proposed (19,45). We also employed the sigmoidal wind model with a minor change, [13].…”

“…Previous studies discarded the existence of the sea surface (19) or restricted birds to only flying higher than a given height (1.5 m) from the sea surface (18). However, the height a bird can fly depends on the wing length and the bank angle (e.g., with a shorter wing length and a lower bank angle, a bird can fly at a lower height).…”

Making BREAD: Biomimetic Strategies for Artificial Intelligence Now and in the Future

“…This strategy has been adopted in passive walker robots that utilize orders of magnitude less energy than conventional walking robots (Collins et al, 2005; Bhounsule et al, 2014). Birds of prey and long-range migrating birds take advantage of thermal plumes to reduce energy usage during flight (see Figure 4, panel 1) (Akos et al, 2010; Weimerskirch et al, 2016; Bousquet et al, 2017). Gliders have mimicked this strategy in their flight control systems (Allen and Lin, 2007; Edwards, 2008; Reddy et al, 2018).…”

The UNAV, a Wind-Powered UAV for Ocean Monitoring: Performance, Control and Validation

“…For Ψ = ±40 • , the speed is still over 65% of the maximum speed. c) Comparison with albatrosses and sailboats: The trim study suggests that the flying sailboat could stay aloft in winds as low as 2.8 m/s, about a third of the wind required for albatrosses to perform dynamic soaring [9], [17] (a difference not explained by differences in wing loading alone). Furthermore, computer simulations suggest that upwind dynamic soaring is hardly feasible [17], consistent with the high energy expenditure of albatrosses traveling upwind [18].…”

“…The wandering albatross (Diomedea Exulans), is the size of a small drone at a typical 10 kg and 3 m span. Mostly found in the southern oceans between 30 • and 60 • S, where strong winds prevail, albatrosses fly by extracting their propulsive energy from the wind through a specific flight technique called dynamic soaring [9]. They typically travel 500 miles per day [10].…”