Dynamic soaring is a flying technique used to gainenergy by repeatedly crossing the boundary between air masses of differentvelocity. Such zones ofwind gradient are generally found close to obstacles and close to the surface, so the technique is mainly of use tobirds and operators ofradio-controlled gliders, butglider pilots are sometimes able tosoar dynamically in meteorologicalwind shears at higher altitudes.
Dynamic soaring is sometimes confused withslope soaring which is a technique for achieving elevation.
While different flight patterns can be employed in dynamic soaring, the simplest is a closed loop across the shear layer between two airmasses in relative movement, e.g. stationary air in a valley, and a layer of wind above the valley. The gain in speed can be explained in terms of airspeed and groundspeed:
The energy is extracted by using the velocity difference between the two airmasses to lift the flying object to a higher altitude (or to reverse the descent respectively) after the transfer between the airmasses.
In practice, there is aturbulent mixing layer between the moving and stationary air mass. In addition,drag forces are continually slowing the plane. Since higher speed gives rise to higher drag forces, there is a maximum speed that can be attained. Typically around 10 times the windspeed for efficient glider designs.
When seabirds perform dynamic soaring, thewind gradients are much less pronounced, so the energy extraction is comparably smaller. Instead of flying in circles as glider pilots do, birds commonly execute a series of half circles in opposite directions, in a zigzag pattern. An initial climb though the gradient while facing into the wind causes it to gain airspeed. It then makes an 180° turn and dives back through the same gradient but in the downwind direction, which again causes it to gain airspeed. It then makes an 180° turn at low altitude, in the other direction, to face back up into the wind... and the cycle repeats. By repeating the manoeuvre over and over it can make progress laterally to the wind while maintaining its airspeed, which enables it to travel in a cross-wind direction indefinitely.
As drag is slowing the bird, dynamic soaring is a tradeoff between speed lost to drag, and speed gained by moving through the wind gradient. At some point, climbing higher carries no additional benefit, as the wind gradient lessens with altitude.
Albatrosses are particularly adept at exploiting these techniques and can travel thousands of miles using very little energy.Gulls andterns also exhibit this behaviour in flight. Birds that soar dynamically have a skeletal structure that allows them to lock their wings when they are soaring, to reduce muscle tension and effort.
Lord Rayleigh first described dynamic soaring in 1883 in the British journalNature:[1]
The first case described above by Rayleigh is simple gliding flight, the second is static soaring (usingthermals,lee waves orslope soaring), and the last is dynamic soaring.[2]
In his 1975 bookStreckensegelflug (published in English in 1978 asCross-Country Soaring by theSoaring Society of America),Helmut Reichmann describes a flight made byIngo Renner in aGlasflügel H-301 Libelleglider overTocumwal in Australia on 24 October 1974. On that day there was no wind at the surface, but above aninversion at 300 meters there was a strong wind of about 70 km/h (40knots). 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 180-degree turn (with highg) and climbed back up again. On passing through 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 aPik 20 sailplane, he refined the technique so that he was able to eliminate the downwind drift and even make headway into the wind.
The dynamic soaring technique is adapted in unmanned aerial vehicles for enhancing their performance under a thrust-off condition. This improves the endurance and range of the aircraft in austere conditions.[clarification needed]
Dynamic soaring can be used as a means to exceed the solar wind speed, by exploiting differences in this speed near the sun, the Earth, and/or the heliopause.[3][4]
In the late 1990s,radio-controlled gliding awoke to the idea of dynamic soaring (a "discovery" largely credited to RC soaring luminary Joe Wurts).[5] Radio-controlled glider pilots perform dynamic soaring using the leeward side of ground features such as ridges, saddles, or even rows of trees. If the ridge faces the wind, and has a steep back (leeward) side, it can cause flow separation off the top of the hill, resulting in a layer of fast air moving over the top of a volume of stagnant or reverse-flow air behind the hill. The velocity gradient, orwind shear, can be much greater than those used by birds or full scale sailplanes. The higher gradient allows for correspondingly greater energy extraction, resulting in much higher speeds for the aircraft. Models repeatedly cross the shear layer by flying in a circular path, penetrating a fast-moving headwind after flying up the back side, turning to fly with the wind, diving down through the shear layer into the stagnant air, and turning again to fly back up the back side of the hill. The loads caused by rapid turning at high speed (the fastest models can pull over 100Gs) require significant structural reinforcement in thefuselage and wing. Because of this, dynamic soaring models are commonly built usingcomposite materials.
As of February 21, 2023, the highest reportedground speed for radio control dynamic soaring was 908kph or 564 mph (490 kn).[6] 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. Lately, some models have begun carrying on-board telemetry and other instruments to record such things as acceleration, air speed, etc.