Dynamic Soaring
Dynamic soaring is a flight technique that extracts energy from a wind gradient (wind shear), letting a bird or glider gain energy by repeatedly climbing into faster air and diving back into slower air. Wandering albatrosses use it to cross oceans almost without flapping.
Dynamic soaring is the exploitation of a gradient in wind velocity — a wind shear layer — to extract energy for sustained flight without flapping or engine power. Over the open ocean, wind blows faster at height than near the surface, and a flyer can repeatedly harvest that difference. The maneuver is a corkscrew-like cycle: the flyer climbs while heading upwind into progressively faster air, turns at the top, then descends heading downwind into slower air. Energy is gained because the technique transfers momentum across the shear layer, redistributing it from the fast upper air to the slow lower air. The classic practitioner is the wandering albatross (Diomedea exulans), which can cover around 5,000 km a week relying on wind energy alone and circle the Southern Ocean with minimal wing-beats. The phenomenon was noted by Leonardo da Vinci and has since been used by radio-controlled gliders and studied for unmanned aerial vehicles. Dynamic soaring differs from ordinary thermal soaring, which relies on rising warm air (thermals) rather than horizontal wind shear. Both let aircraft stay aloft without fuel, but each depends on specific and intermittent atmospheric conditions, which is why neither has yielded a practical always-aloft platform of the kind a Solar-Powered Aircraft aims to be.