Since mankind began exploring the great oceans we have been fascinated by the flight of the albatross. These huge birds are seen flying at low level over the sea with a characteristic, apparently effortless, undulating flight pattern mostly without flapping their wings. Research has shown they are capable of flying great distances at high average speeds with very little effort. Their flight manoeuvres are different to those used by land birds on soaring flights, so what are they doing?
In the ornithology literature, albatross dynamic soaring is normally explained by some variation of the Wind Gradient Theory, which has been repeated without question since Lord Rayleigh’s article in the journal Nature in 1883. Although the wind-gradient does have a role to play in dynamic soaring, it is not the whole story. This website comprises a new description of dynamic soaring, the Windward Turn Theory, which explains how and why the birds fly the way they do. It will explain the true role of the wind gradient but does not depend upon it.
The Windward Turn Theory
When you look at film of albatross in flight, you will see the bird, the ocean and the sky. You will not see the air, nor will you see the birds airspeed, ground speed, drift angle or its acceleration. Everything you can see, the waves, the swell the water and you are moving relative to everything else. The normal view is from the deck of a ship, so that the up and down motion of the bird is very obvious but less obvious is the extent of the left and right turns and their angle to the wind. This is made clearer by GPS tracking of albatrosses which you can see in the Albatross Tracking Data section.
In dynamic soaring, the albatross is continuously turning, climbing and descending. It flies approximately on a cross-wind heading plus and minus about 20 to 30 degrees, turning alternately left and right. Thus, there is a windward turn, flown with a small angle of bank, at approximately constant height above the surface and lasting about 5 to 15 seconds. The windward turn starts with a tailwind component, turns across the wind and ends with a headwind component. The bird then gains height, reverses the direction of turn and flies the leeward turn as a steeply banked wing-over, in which height is gained and lost, starting with a headwind and ending with a tailwind. taking about 2 to 4 seconds before reversing the direction of turn again and commencing the next windward turn.
Any theory of dynamic soaring has to explain these manoeuvres, the direction of their large scale flight patterns (mostly downwind), their high-aspect ratio wings and their characteristic tube-nostrils
The Windward turn
In the windward turn, the albatross maintains height above the surface and therefore loses momentum due to the unbalanced aerodynamic force, a component of which acts opposite to the direction of the ground-velocity. The loss of momentum is therefore a loss of ground-speed rather than a loss of airspeed.
Airspeed is constant, or even increases slightly, because the tendency to lose airspeed due to the reducing ground-speed is balanced by the tendency to gain airspeed from the increasing headwind component, whilst turning relative to the wind-direction. This effect depends on the rate of change of the head/tailwind component which depends on the birds rate of turn. While turning close to cross-wind headings, the rate of change of the headwind component is at a maximum even though the actual head/tail-wind component is at a minimum. The wind-velocity itself need not change.
The albatross flies the windward turn with the least angle of bank which will give it a rate of turn sufficient to maintain airspeed and height and thereby maximise the distance flown. Flying close to the surface means that ground-effect improves the efficiency of the turn by reducing drag.
The Leeward turn
At the end of the windward turn, the albatross pitches-up and reverses the direction of turn, making the leeward turn as an arched turn or wing-over. A wing-over, has a partially ballistic trajectory, with the weight of the bird partly supported by a small vertical component of lift. This enables a steep angle of bank to give a large horizontal component of lift without a large increase in actual lift and without a large increase in load-factor and drag. Despite the large bank-angle the load-factor is only about 1G and therefore requires minimum effort from the bird
With a large angle of bank, the horizontal component of lift, added vectorially to the drag, creates a horizontal resultant. A large drift-angle then enables a component of this force to act in the direction of the ground velocity, causing an increase in ground-speed and momentum. The large drift angle is due to the wind being a large proportion of the birds airspeed. Airspeed is constant or reduces slightly, because the tendency for the airspeed to increase with the increasing ground-speed is balanced by the tendency to reduce due to the decreasing headwind component.
GPS tracking data clearly shows that there are times when the birds are gaining both ground-speed and height. This gain of energy can only be achieved by aerodynamic forces and not by wind-gradient effects.
The effect of the wind gradient
Climbing upwind and descending downwind through a wind gradient will tend to increase airspeed but only if actual speed is maintained. However, in an upwind climb actual speed must reduce due to gaining height against gravity. In a downwind descent, actual speed will only be maintained by an ever steeper angle of descent as the drag load increases with the increased airspeed. In other words, the wind-gradient may improve the efficiency of the leeward turn during the climb and descent by reducing the loss of airspeed but not by actually increasing the airspeed. This will be a relatively small effect because the birds fly mainly close to cross wind headings where the head-wind component is at a minimum.
Conservation of momentum
The lift and drag forces, produced by the motion of the wing through the air, are the equal and opposite reactions to momentum given to the air. When a bird or an aircraft banks in a turn, the lift force tilts with the bird and the horizontal component of lift acts as a centripetal force which makes the bird turn. Horizontal momentum is given to the air, opposite to the centripetal direction.
In the windward turn, a component of this horizontal momentum is in the same direction as the wind and gives momentum and energy to the wind. As the albatross loses momentum, so the wind gains momentum.
In the leeward turn, the albatross gains momentum and the wind loses momentum. The birds change of direction is again caused by a centripetal force, a horizontal component of the lift force, the result of the wing imparting horizontal momentum to the air. In the leeward turn, a component of that momentum is parallel to and opposite to the wind direction, therefore the momentum of the wind is reduced whilst the momentum of the bird is increased.
The bird maintains at least average speed and height during successive windward and leeward turns. For the bird, the change of momentum is about the same in each turn. However, as the bird flies, it encounters successive different units of air. The leeward turn is shorter than the windward turn and fewer unit masses of air are accelerated; compared with the windward turn which is longer and in which a greater number of unit masses of air are given less acceleration. So, per unit mass of air, the wind loses more speed in the leeward turn than it gets back in the windward turn. The difference in energy is equivalent to the birds drag losses. In effect, wind-speed has been converted into air turbulence in the wake of the bird. Ultimately that turbulence energy is converted to heat energy at the molecular level and dissipated throughout the air.
In this way, the albatrosses bias the exchange of energy in their favour and provided they gain slightly more airspeed and height in the windward turn than they lose in the leeward turn then they can continue soaring indefinitely as long as the wind blows.
The Nature of the Wind
The wind is not the same as a box of air moving with uniform velocity. The wind moves along a curved path over the surface of the Earth and is measured relative to the surface of the Earth. Wind-velocity is caused by temperature and pressure variations within the atmosphere and wind energy is derived from the energy of the sun. Momentum in the wind is gained by acceleration of the air, not by acceleration of the ground.
For the purposes of explaining dynamic soaring, the ground can be thought of as stationary and an approximate inertial frame of reference. The wind simply has a particular velocity at a particular place and time when encountered by the albatross. When the albatross passes, the air becomes accelerated and the wind-velocity at that point increases or decreases a little bit. Overall, the wind loses some speed but the energy returns to the air in the form of turbulence in the wake of the bird.
What else is in this website?
The Windward Turn Theory described in this new revision is an explanation of dynamic soaring as practised by the albatross and other oceanic birds. To understand it you only need an open mind but high school maths will help. This page is a brief introduction to the theory. The next page, Windward Turn Theory is a more detailed, verbal explanation and the Analysis page has the scary maths. But if you are familiar with the classic wind-gradient theory you might like to check out the Wind Gradient Theory page first - to purge your mind.
It should be noted that that this kind of dynamic soaring as practised by albatrosses is not the same as the dynamic soaring done by RC model glider pilots. That is described in the Lee Soaring section. However, there is a connection here. Albatrosses appear to use a variation of the lee-soaring technique when they want to soar upwind and that is described on the Upwind dynamic soaring page.
Finally there is a page discussing the Downwind Turn Myth and this contains a more generalised account of the effect of the wind on an aircraft during turning flight.
Dynamic soaring on YouTube
Search on YouTube for ‘albatross dynamic soaring’ and you will find several films on this subject
For a 15 minute animated version of this website click Dynamic Soaring The Flight of the Albatross