Notes on dynamic soaring
December 13 2005 edition
steve at aeroexperiments.org
A widespread, steady, updraft or downdraft or headwind or tailwind or crosswind has no effect whatsoever on the force vectors generated by an aircraft flying at a given airspeed in a given direction. However, transient gusts or gradients or shears do affect the force vectors generated by an aircraft flying in a given airspeed in a given direction. "Dynamic soaring" is the exploitation of gusts or gradients to extract energy from the atmosphere. In some cases, an aircraft or bird using "dynamic soaring" techniques can stay aloft indefinitely in unpowered flight even while flying in an airmass that is entirely devoid of updrafts.
When there is a strong wind gradient, performing a "zoom climb" into the wind allows an aircraft or bird to gain altitude without losing airspeed. Similarly, when there is a strong wind gradient, performing a dive away from the wind allows an aircraft or bird to gain more airspeed than would be normally associated with a given loss of altitude, or to glide further at a constant airspeed than would normally be associated with a given loss of altitude.
This is the basis of the form of "dynamic soaring" that is carried out by RC (model) glider pilots while flying on the lee side of a hill: the aircraft performs a "zoom climb" from the more sheltered air below the ridge top into the faster-moving air above the ridgetop, and then turns tail to the wind before diving back down into the more sheltered air below the ridgeline. These climbs and dives are is merged into a continuous high-speed looping oval flight path and speeds well over 100 mph are sustained continuously, all in an airmass whose overall net motion is only very slightly upward, or may even be neutral or downward.
The albatross uses a milder form of these maneuvers to engage in long "cross-country" flights in the strong wind gradient that exists low over the ocean surface. By doing a zoom climb into the wind, and then gliding back toward the ocean surface on a crosswind heading or a semi-downwind heading, the bird can cover distance with no net loss of energy.
An efficient shape and a high wing loading are important pre-requisites for the efficient use of these techniques.
Of course, all soaring seabirds also use "slope lift" or orographic lift, generated by the relative motion between wave crests and the atmosphere (i.e. by the action of wind on waves, or by the motion of the waves themselves even when there is no wind.)
Even lightly-loaded birds like Northern Harriers can be seen to extract some energy from the wind gradient using dynamic soaring techniques. As long as the bird is flying within a strong wind gradient (usually very close to the earth's surface), and alternates between an ascending flight path pointed into the wind, and a descending flight path pointed away from the wind, the bird will gain some amount of free energy from the wind gradient. Northern Harriers and other raptors that hunt low over the ground can often be seen to execute a small "zoom" climb when turning upwind and a small dive when turning downwind.
Another form of "dynamic soaring" extracts energy from transient side-gusts or up-drafts or down-drafts by changing the direction and/or the magnitude of the aircraft's lift vector (G-loading). The theory behind some of these techniques is rather complex, and fascinating.
For example, the instant after an aircraft enters an updraft, before the aircraft has settled into to equilibrium with the rising air, the relative wind temporarily contains more of an upward component than usual, meaning that all the aerodynamic vectors (lift, drag, etc.) are tilted in such a way that the drag vector is less penalizing than usual (i.e. contains less of a horizontal component than usual, and more of an upward component than usual). This is the time to "load up" the wing, i.e. to pull a bunch of G's, because the accompanying rise in drag is not as penalizing as it normally would be. Then as the aircraft exists the updraft, the relative wind temporarily contains less of an upward component than usual, meaning that all the vectors (lift, drag) etc are tilted in such a way that the drag vector is more penalizing than normal (little or no upward component). This is the time to unload the wing to near 0 G's.
A more sophisticated explanation would focus on "lift gradients" or "sink gradients"--like the "wind gradients" discussed above--rather than simply saying that it takes an aircraft a few moments to come into equilibrium with a change in the direction of the external airflow (wind, lift, sink, etc).
Just as energy can be extracted from a transition from still air to lift, so too can energy be extracted from a transition from sink to still air. In theory, it would be possible to stay aloft using the energy from pockets of sinking air.
Here are some links relating to "dynamic soaring" (these links also appear on the main "links" page of this website):
Dynamic soaring "type 1": RC model gliders dynamic soaring on the lee side of a hill:
"Dynamic Soaring--the challenge" by Klaus Weiss--a practical article on the lee-side-of-the-hill form of dynamic soaring practiced by RC glider pilots
"Dynamic Soaring (DS) -- the hottest development in R/C soaring in many years!"--another practical article on the lee-side-of-the-hill form of dynamic soaring practiced by RC glider pilots
"How and why does dynamic soaring work?" by Mark Drela--a brief technical article on the lee-side-of-the-hill form of dynamic soaring practiced by RC glider pilots
"Dynamic Soaring on Back Side of Hill"--an illustration for the above article
"Dynamic Soaring"--another brief article, with some technical notes, on the lee-side-of-the-hill form of dynamic soaring practiced by RC glider pilots
"Dynamic Soaring Observations" from NorCal slope soaring--practical tips on dynamic-soaring RC models on the lee side of the hill
"Dynamic Soaring"--More practical tips on dynamic-soaring RC models on the lee side of the hill, along with downloadable video clips
Sample clip from "Lift ticket"--video illustrating RC gliders dynamic-soaring on the back side of the hill
Video clip from North Country Flying Machines illustrating an RC glider dynamic-soaring on the back side of the hill
"Dynamic Soaring video clips"--many more video clips from North County Flying Machines of RC gliders soaring on the back side of the hill (also includes some other types of soaring, not always clear from titles what is being depicted)
First page from "Preliminary Dynamic Soaring Research using a Radio Control Glider" by James Parle--a very technical exploration of dynamic soaring on the lee side of a hill. For more see this link
Dynamic soaring "type 2": using heading changes and "zoom climbs" for "cross-country" travel within the wind gradient over flat ground or over the ocean, as practiced by the albatross :
"Dynamic Soaring"--a simple article, with an animated illustration, about the form of dynamic soaring practiced by the albatross
"How Flies the Albatross: the flight mechanics of dynamic soaring" by J. Phillip Barnes--a fascinating, in-depth article about the form of dynamic soaring practiced by the albatross
An exploratory analysis of dynamic soaring: trajectories in shear layers" by Misty Davies -- a very technical exploration of the form of dynamic soaring practiced by the albatross
Dynamic soaring "type 3": various ways to use transient gusts, and to use the transition between still air and lift or sink, far from the earth's surface, such as are now being explored by full-scale sailplane pilots:
"Dynamic Soaring and Sailplane Energetics" by Taras Kiceniuk--an article introducing the basic principles of soaring without lift
"Calculations on Soaring Sink" by Taras Kiceniuk--an in-depth article on extracting energy from the transitions between still air and sinking air
"Dynamic Soaring of Sink Pockets" by Taras Kiceniuk--an illustration for the above article
"Vector diagram of dynamic soaring: showing how a glider can get energy from a downward gust" by Taras Kiceniuk--an illustration for the above article
"Side gust soaring" by Taras Kiceniuk--an illustration showing how a glider can extract energy from a sideways gust
Dynamic soaring "type 4": combining various ways to use transient gusts and transitions between still air and lift or sink, with a knowledge of fine-scale meteorology:
"Understanding Microlift" from September 6 2002 OZ report--notes from G. Osaba
More notes on putting various types of "dynamic soaring" into practice in full-scale sailplanes:
Extracts from "For Pilots" discussion group: #1
Or in spacecraft...
More dynamic soaring links:
Autonomous dynamic soaring platform for distributed mobile sensor arrays" by Mark B.E. Boslough -- a very technical exploration of various types of dynamic soaring, including the technique used by the albatross and the technique used by RC glider pilots on the lee side of hills