Dynamic soaring
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Dynamic soaring is a flying technique used to gain kinetic energy without effort by repeatedly crossing the boundary between air masses of significantly different horizontal velocity. Such zones of high wind gradient are generally found close to obstacles and close to the ground, so the technique is mainly used by birds, but glider pilots have very occasionally been able to soar dynamically in meteorological wind shears at higher altitudes.
When seabirds are seen repeatedly diving into the valleys of ocean waves, and then wheeling back up into the air above them, dynamic soaring is what they are doing. Albatrosses are particularly adept at exploiting the technique and they use it to travel many thousands of miles with hardly any expense of 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 indefinitely without having to put in any effort. In effect it is harvesting energy from the wind gradient.
In his 1978 book Streckensegelflug (Cross-Country Soaring), Helmut Reichmann describes a flight made by Ingo Renner in a Libelle sailplane 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 model soaring awakened to the idea of dynamic soaring (a "discovery" largely credited to RC soaring luminary Joe Wurts). With the aid of current composite materials, model ships have grown stronger and their ability to utilize dynamic soaring have increased dramatically. As of March 2005, the current speed record for a 3-meter wingspan model is 262 miles per hour, set by Kyle Paulson.
[Kyle Paulson's 262 MPH World Record (WMV video) (http://slopeaddiction.com/user/KyleHigh262mph.wmv)] [How the Albatross Dyanmically Soars (http://www.wfu.edu/albatross/atwork/dynamic_soaring.htm)] [Dynamic Soaring (DS) -- the hottest development in R/C soaring in many years! (http://www.geocities.com/soaringbythebay/dsoar.htm)]