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 498 mph (801 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.
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.
- "...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.
Radio controlled glider
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 March 2012, the highest reported speed for radio control dynamic soaring was 498 mph (801 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 February 27, 2013.
- 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. Sandia National Laboratories, Albuquerque, New Mexico. SAND2002-1896.
- Don’t Blink: 392 MPH Glider Tears Through the Air