Dynamic soaring

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Dynamic soaring is a gliding technique used to gain kinetic energy by repeatedly crossing the boundary between air masses of significantly different horizontal velocity. Such zones of high wind gradient or low level wind shear are generally found close to obstacles and close to the surface, so the technique is mainly used by birds, but glider pilots have occasionally been able to soar dynamically in meteorological wind shears at higher altitudes.

Some seabirds dynamically soar by repeatedly diving into the valleys of ocean waves, and then wheeling back up into the air. Albatross are particularly adept at exploiting the technique and they use it to travel many thousands of miles using hardly any energy. 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:

"…a bird without working his wings cannot, either in still air or in a uniform horizontal wind, maintain his level indefinitely. For a short time such maintenance is possible at the expense of an initial relative velocity, but this must soon be exhausted. Whenever therefore a bird pursues his course for some time without working his wings, we must conclude either

that the course is not horizontal,
that the wind is not horizontal, or
that the wind is not uniform.

It is probable that the truth is usually represented by (1) or (2); but the question I wish to raise is whether the cause suggested by (3) may not sometimes come into operation."

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 manoeuvre 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). With the aid of improved composite materials, model sailplanes grew stronger and their ability to utilize dynamic soaring increased dramatically. Dynamic soaring is often used by radio controlled glider pilots on the back side of ridges under appropriate conditions. The principles are the same as those used by birds, but the velocity gradients between the front and back sides of a hill can be much greater, allowing more power extraction. Current world records for radio control gliders employing dynamic soaring are in the 600 km/hr (370 mph) range.

[edit] References

^ Boslough, Mark B.E. (2002-06). "Autonomous Dynamic Soaring Platform for Distributed Mobile Sensor Arrays". . Sandia National Laboratories, Albuquerque, New Mexico. SAND2002-1896

Dynamic Soaring of Thermals is also being done by exploiting the difference in rising and sinking air. This is known as "Pumping a Thermal" and can be done with RC gliders.