Dynamic soaring is a flying technique used to gain energy by repeatedly crossing the boundary between air masses of significantly different velocity. Such zones of high wind gradient are generally found close to obstacles and close to the surface, so the technique is mainly of use to birds and operators of radio-controlled gliders, but glider pilots have occasionally been able to soar dynamically in meteorological wind shears at higher altitudes. The highest speeds reported are by radio controlled gliders at over 400 mph (643 km/h).
Dynamic soaring is sometimes confused with controllable slope soaring which uses a similar but different technique for achieving elevation.
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While different flight patterns can be employed in dynamic soaring, the simplest example to explain the energy extraction mechanism is a closed loop across the boundary layer between two airmasses in relative movement. The gain in speed can be explained in terms of airspeed or groundspeed:
The energy is extracted by reducing the velocity difference between the two airmasses during the 180°-turns which accelerate air in opposite directions.
Some seabirds dynamically soar by repeatedly diving into the valleys of ocean waves, and then wheeling back up into the air. Albatrosses are particularly adept at exploiting the technique and they use it to travel many thousands of miles using very little energy from flapping. When the bird pulls up into the wind out of the still air in the lee of a wave, it suddenly becomes exposed to a head wind, so the speed of the air over its wings increases. It then turns in the other direction and, with the wind behind it, dives back into the shelter of a wave. This also results in an increase in its air-speed. So by repeating this "wheeling" pattern, the bird can continue flying almost indefinitely without having to put in much effort besides steering. In effect it is harvesting energy from the wind gradient.
Lord Rayleigh first described dynamic soaring in 1883 in the British journal Nature:
The first case-described above by Rayleigh is simple gliding flight, the second is static soaring (using thermals, lee waves or slope soaring), and the last is dynamic soaring.[1]
In his 1975 book Streckensegelflug (published in English in 1978 as Cross-Country Soaring by the Soaring Society of America), Helmut Reichmann describes a flight made by Ingo Renner in a Glasflügel H-301 Libelle glider over Tocumwal in Australia on 24 October 1974. On that day there was no wind at the surface, but above an inversion at 300 metres there was a strong wind of about 70 km/h (40 knots). Renner took a tow up to about 350 m from where he dived steeply downwind until he entered the still air; he then pulled a sharp 180-degree turn (with very high g) and climbed steeply back up again. On passing though the inversion he re-encountered the 70 km/h wind, this time as a head-wind. The additional air-speed that this provided enabled him to recover his original height. By repeating this maneuver he successfully maintained his height for around 20 minutes without the existence of ascending air, although he was drifting rapidly downwind. In later flights in a Pik 20 sailplane, he refined the technique so that he was able to eliminate the downwind drift and even make headway into the wind.
In the late 1990s, radio-controlled gliding awoke to the idea of dynamic soaring (a "discovery" largely credited to RC soaring luminary Joe Wurts). Radio controlled glider pilots perform dynamic soaring using the leeward side of ground features such as ridges. On ground features whose shape combine with the prevailing wind direction to produce a stagnant or reverse flow on the leeward side, the velocity gradients can be much greater than those used by birds or full scale sailplanes. These higher velocity gradients allow for correspondingly greater energy extraction, although at the cost of high airframe loads imparted at the boundary between the prevailing wind and the stagnant air. Due to the strength and stiffness needed to withstand these high loads, dynamic soaring models are commonly built using composite materials. As of February 2011, the highest reported speed for radio control dynamic soaring was 468mph (753 km/h). There is no official sanctioning organization that certifies speeds, so records are listed unofficially based on readings from radar guns, although analysis from video footage and other sources is also used. [2]