Dynamic Soaring

Early observations of dynamic soaring in literature occur as early as the 16th century. Coleridge’s poem Rime of the Ancient Mariner is about a sea bird called the albatross that possesses God-like status because it could fly long distances effortlessly [1]. In the 16th century, there was no documented discussion of how albatross were able to produce the energy to perform such a feat without flapping their wings (Figure 1). Today, with an advanced understanding of the aerodynamics and mechanics of bird flight, scientists and engineers have developed concepts that explain how the albatross and other birds are able to harness energy by flying through the boundary layer between slower moving air and faster moving air, which is known as dynamic soaring.

Key to Dynamic Soaring

In a posted correspondence, aerospace engineer Blaine Rawdon explained that dynamic soaring requires air to move in a particular way: a steady strong wind moving along a surface. This results in a variation in wind speed called a wind gradient, where the wind speed varies with altitude. The gradient is strongest near the surface and diminishes gradually with altitude [2]. The air closest to the surface will move slower than the air farther away due to a frictional force between the surface and the wind, an effect that can be observed on breezy days. Branches on tall trees sway in the wind, but at ground level far less wind is felt. This happens because the ground is slowing down the air closer to it. Traveling in and out of different wind speeds in certain patterns has the potential to provide extra energy. Flying in the correct pattern is the key to dynamic soaring.

1. Fly at a height where the wind is traveling faster then it does at ground level
2. Dive down to the ground level increasing your speed (due to gravity)
3. Turn 180 degrees and travel into the wind, flying against the ground level wind speed.
4. Turn 180 degrees and using the speed of the wind; climb back up to the previous altitude gaining speed.

Observations by William Jameson base the albatross’s ability to soar dynamically utilizing of these vertical wind gradients that occur over the ocean, extracting energy from the wind [3]. More recently, Joe Wurts observed several Albatross flying inclined oval circles in a repetitive pattern as they headed out to sea. “What they were doing was to dive downwind until they got quite close to the water, turn around, then climb against the wind” [4].

He gives the example:

“Let’s say the bird is flying at about 40 mph, and the wind is 10 mph. When the bird is going downwind, the airspeed ‘of the Bird’ is 40 mph, but the ground speed is 50 due to the 10 mph tailwind. Now the bird gets close to the ocean surface where the wind is less, let’s say 5 mph. It keeps its groundspeed of 50 mph but since the tailwind is only 5 mph, it has 45 mph airspeed. It turns around, keeping the 45 mph airspeed, and now is heading upwind at 45 mph, with 40 mph groundspeed. It then climbs into the stronger headwind, and keeps the groundspeed of 40 mph, but with the wind of 10 mph, it is now at 50 mph airspeed. It then turns around and repeats the process” (1).Joe Wurts

Thus, from one cycle of the loop, the albatross has gained 10 mph worth of energy. The albatross can continue to do this as long as a gradient exists. One limiting factor is that the bending forces on a bird’s wing increases as its speed increases. This is the reason birds don’t fly over 100 mph; they choose instead to gain speed by performing several loops, gliding and then resting.

Using radio-controlled sailplanes, Joe Wurts and Pat Bowman were able to test dynamic soaring using a very sharp wind gradient created by a mountain. It is the same principal and technique that the albatross uses, but the extremely sharp gradient makes manipulating the dynamic forces easier. The drastic wind gradient creates a boundary layer separating fast moving air and still air only a few feet apart from each other.

By flying their planes in this type of rotated loop, they were able to increase from an initial velocity of 3 mph from a hand toss to over 150mph after several dynamic soaring loops. Bowman and Wurts clocked their sailplane at 156 mph, pulling 7-8 times the force of gravity in the turns [5]. The limiting factor on their top speed was the sailplane fragility; it couldn’t stand up to the increasing forces as it continued to go faster. In theory, if the sailplane could handle the increasing structural loads, its speed due to dynamic soaring is limitless. From their experiments, Bowman and Wurts were able to derive equations to calculate the energy gained for each cycle, and ponder how fast a dynamic soarer could really go.

Dr. Peter Lissaman of the University of Southern California Department of Aerospace Engineering explains that “Dynamic soaring implies flying a closed loop in inertial space.” It is possible to return to the starting position of a loop with the original speed with no loss in energy, having traveled a finite distance. This is somewhat like conventional soaring which uses rising air called thermals to produce an increase in potential energy by physically lifting a bird or sailplane up against gravity. Thermals are a means of static soaring: the air doesn’t move away, but rather up. Local air drafts have an upward velocity that lifts the sailplane. Performing loops in such drafts increases only an object’s potential energy by increasing its altitude. Dynamic soaring differs from conventional soaring in that it does not use rising air to sustain flight, but rather the boundary layer that separates different wind speeds. Energy is extracted from the air simply by flying in and out of air masses moving at different speeds. Useable thermals occur in certain conditions and are not in fixed locations because hot air moves toward colder regions. However, wind gradients occur everywhere around Earth, and thus a great advantage of dynamic soaring is its ability to be widely implemented.

Humans have observed birds engaged in dynamic soaring for centuries, but have not been able to recreate such soaring themselves. Only in the last decade, using radio controlled sailplanes, has man been able to test the theories and formulas proposed by aerodynamicists that explain the mechanics of dynamic soaring. Experiments are currently being conducted to construct mathematical models to better explain the properties of this specialized form of flight. A completed model will allow for autonomous aircraft to fly indefinitely.