Gradient flight

The dynamic soaring (or Dynamic soaring) is a flight technique used to make the energy passing through repeatedly the boundary between two air masses having different speeds. Such areas of significant wind speed gradient are either near the ground or in an area protected by an obstacle such as at the back of a hill. Therefore this technique is used mainly by birds or radio-controlled gliders. However, glider pilots like Ingo Renner have been able to use it. The maximum speed reported by radio-controlled glider pilots is 500 miles per hour or about 800 km / h .

Gradient flight is sometimes confused with slope flight . In the latter case, the pilot flies slightly upstream of the hill and uses vertical wind deflection.

Basic principle

Several types of trajectories can be used in gradient flight. The simplest mechanism is a vertical loop through the two air masses in relative motion. The gain in speed during a loop is explained by the fact that when the glider passes from one air mass to another, its ground speed remains practically constant because the Earth reference is practically Galilean . The increase in speed can be explained either in terms of air speed or ground speed. The speed gain is approximately 2 times the speed difference between the upper and lower layers. The ideal path is shown in the animation below and is described in detail in the drop-down box.

Birds

Galapagos albatross , Phoebastria irrorata

Some seabirds hover dynamically by diving into the waves’ hollows and rising above the wave ridge . The albatross use this common technique to cover distances of thousands of kilometers in the South Seaswithout notable wing beat. When the bird returns to the wind above the ridge of the swell, it finds itself in the presence of a large headwind and therefore its air speed becomes important. By turning 180 degrees and diving, he finds himself again safe from the wave, his speed increases once again. So, starting this cycle again, the bird can fly almost indefinitely without much effort except to make turns. Indeed, the bird extracts energy from the gradient of the wind speed.

Lord Rayleigh was the first to describe the gradient flight in 1883 in the British scientific journal Nature 1“… a bird without working his wings can not, either in a horizontal wind, maintain his level indefinitely. Such a time may be necessary to the expense of an initial relative velocity, but this must be exhausted.

  1. that the race is not horizontal,
  2. that the wind is not horizontal, gold
  3. that the wind is not uniform.
It is likely that the truth is usually represented by (1) or (2); but the question I wish to raise is the cause suggests by (3) may not sometimes come into operation. “

French Translation: “a bird without using its wings, can not, in an immobile mass of air or inside a mass of uniform velocity air, maintain flight level indefinitely. For a short time, flight altitude conservation can only be done at the expense of the initial flight speed which will be gradually reduced to zero. Therefore, if a bird continues for some time without flapping its wings, we must conclude that either:

  1. its trajectory is not horizontal,
  2. the wind is not horizontal, or
  3. the wind speed is not uniform.
It is likely that the truth lies in assertions (1) and (2). However, I wonder if assertion (3) is not valid in some cases. “

The first case described by Lord Rayleight is a simple glide, the second case is either a flight slope , a thermal flight or a wave flight . The last case is a gradient flight 2 .

Gliding

In the book Streckensegelflug published in English under the title Cross-Country Soaring by the Soaring Society of America and in French under the title of The glider race, Helmut Reichmann describes a flight made by Ingo Renner aboard a glider Glasflügel H- 301 Libelle above Tocumwal in Australia in 1974. That day, the wind was calm on the ground and above a temperature inversion at 1000 feet (300 meters), the wind was blowing at 40 knots ( 70 km / h). Renner towed up to 1,200 feet (350 meters) from which he plunged downwind until he met the still air mass; then he made a 180 degree turn with a large wing load and then went up quickly. While crossing the inversion layer , he found himself again facing a headwind of 40 knots. The air speed gained allowed him to maintain his altitude. By repeating the maneuver, he was able to maintain his altitude for 20 minutes in the absence of any ancestry although he drifted quickly pushed by the wind. Subsequently, he refined his technique aboard a Pik 20 glider and was able to eliminate his rear drift and even move against the wind. asserts that gradient flight is possible with gradients as small as 1m / s / 100m.

Radio-controlled glider

Gradient flight with an R / C glider near Idaho Falls , ID, USA. The wind blows from right to left.

By the end of the 1990s, users of radio-controlled gliders had the idea of ​​practicing gradient flying as a result of Joe Wurts’ genius idea. Pilots of radio controlled gliders perform gradient flights downwind of obstacles on the ground such as ridges, cliffs, etc. These obstacles can either stop the wind completely or even reverse it locally. The wind speed gradient can be much larger than the gradient used by birds or normal sized gliders . Therefore the amount of energy extracted can be increased, notwithstanding the higher wing loads experienced at the boundary of the windward zone and the protected area. Because of these high loads, the ordered gliders are built incomposite materials .

The record was set in 2015 by pilot Spencer Lisenby with a top speed of 826 km / h ( 513 mph ) measured by portable radar. The glider was a Kinetic 130 (3.3m wingspan ). The previous record holder in 2014 at 505 mph (Bruce Tebo) had also used a Kinetic 130 weighted 12kg, in a wind of about 90 km / h and gusts to 105 km / h 4 , 5 .

A rule of thumb says that the maximum speed that can reach a radio-controlled glider is 10 times the speed of wind 6 .

References

  1. ↑ ( in ) Lord Rayleight, ” The soaring of birds ” , Nature , vol.  27, o 701,p.  534-535
  2. ↑ ( in ) Boslough, Mark BE , ” Autonomous Dynamic Soaring Platform for Distributed Mobile Sensor Arrays ”  [ archive ] , Sandia National Laboratories, Albuquerque, New Mexico, (accessed June 16, 2016 ) , p.  9
  3. ↑ ( in ) Slater AE To soar like an albatross , Flight International, ( read online  [ archive ] )
  4. ↑ ( in ) Youtube  [ archive ]
  5. ↑ List of Records  [ archive ]
  6. ↑ ( in ) Philip Richardson, ” Upwind of dynamic soaring albatrosses and UAVs ” , Progress in Oceanography , Elsevier , vol.  130 ( DOI  10.1016 / j.pocean.2014.11.002 )

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