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 513 mph (826 km/h).
Dynamic soaring is sometimes confused with controllable slope soaring which uses a similar but different technique for achieving elevation.
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 glider gains airspeed twice during the loop, when it pierces the boundary layer at an acute angle. Since the 180° turns retain most of the airspeed the glider completes the loop within the initial airmass at a higher airspeed.
- The gain in groundspeed occurs when the glider performs a 180° downwind turn within the moving airmass. Since the opposite 180° turn is done within the stationary airmass the groundspeed gain is not reversed.
The energy is extracted by reducing the velocity difference between the two airmasses during the 180° turns which accelerate air in opposite directions.
The following animation is a simplification of what actually happens. In practice, there is a turbulent mixing layer between the moving and stationary air masses. In addition, drag forces are continually slowing the plane once it has crossed the shear layer, so the airspeed gained by crossing the boundary is not all retained through the 180° turns. Higher speed gives rise to higher drag forces, so there is a maximum speed that can be attained, typically around 10 times the windspeed for efficient glider designs.
When seabirds perform dynamic soaring, the wind gradients are much less pronounced, so the energy extraction is comparably smaller. And instead of doing circles as glider pilots do, the birds will execute a series of half circles in opposite directions. For instance, a bird may begin by climbing though the gradient while facing into the increasing wind to gain airspeed, and then make a 180° turn (say) clockwise. This would be followed by a dive back down through the gradient, again increasing its airspeed as it moves into slower air at lower altitude. The cycle would then be completed by making an anti-clockwise turn at low altitude to face back into the wind. This has the effect of transporting the bird laterally to the wind, so it can travel cross wind indefinitely by continuing to execute this maneuver.
There is some speed lost during the climb, as the bird trades speed for altitude, and some speed gained by diving as the reverse happens. Throughout all of this, drag tends to slow the bird, so dynamic soaring is a delicate tradeoff between speed lost to drag, and speed gained by climbing further upward in the wind gradient. At some point, climbing higher carries no additional benefit, as the wind gradient lessens with altitude.
Some seabirds have been observed performing dynamic soaring over flat land, so the technique does not inherently depend on ocean waves. But the presence of waves can have two benefits. First, the velocity gradient can be enhanced by the presence of waves, just as it is enhanced by hills and other terrain features on land. Second, the bird can mix dynamic soaring with slope soaring to extract more energy.
Albatrosses are particularly adept at exploiting these techniques and can travel many thousands of miles using very little energy from flapping. Albatrosses and other birds that soar dynamically also have a skeletal structure that allows them to lock their wings when they are soaring, so 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.
- "...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."
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 and saddles. 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, or wind 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. Because of the speeds involved, significant structural reinforcement in the fuselage and wing is important. Because of this, dynamic soaring models are commonly built using composite materials.
As of January 2016, the highest reported ground speed for radio control dynamic soaring was 513 mph (813 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. Lately, some models have begun carrying on-board telemetry and other instruments to record such things as acceleration, air speed, etc.
- "Fastest 20 Pilots on RCSpeeds". RCSpeeds.com. Retrieved November 28, 2014.
- Lord Rayleigh (5 April 1883) "The soaring of birds," Nature, vol. 27, no. 701, pages 534-535.
- Boslough, Mark B.E. (2002-06). "Autonomous Dynamic Soaring Platform for Distributed Mobile Sensor Arrays" (PDF). Sandia National Laboratories, Albuquerque, New Mexico. SAND2002-1896. Check date values in:
- Don’t Blink: 392 MPH Glider Tears Through the Air