Wherever you look for references to dynamic soaring, whether on the net or in Wikipedia or in books on ornithology or on the mechanics of bird flight, you will find reference to Lord Rayleigh’s article THE SOARING OF BIRDS in Nature of April 1883. John Strutt, Lord Rayleigh was a well regarded scientist and educator who won the Nobel prize for his discovery of argon gas. His article in Nature is not long or detailed and does not attempt to explain the flight of the albatross or the giant petrel. Rather, it is an early attempt to explain avian soaring in general. His theory describes a kind of dynamic soaring in which the bird exploits a wind shear or wind gradient.
Some authors say that ever since that time dynamic soaring has been understood and variations of his theory are repeated without question. Other authors attempt to explain dynamic soaring in terms of the wind gradient and end up introducing basic errors in an attempt to make their particular theory work. Recently model glider pilots have discovered a way of dynamic soaring in the lee of a hill as opposed to slope soaring on the upwind side of the hill and they too are invoking the Rayleigh cycle of dynamic soaring to explain what is happening but they too are falling into similar traps.
So, what did Lord Rayleigh actually write in his 1883 article in Nature?
- ‘I premise that if we know anything about mechanics it is certain that a bird without working his wings cannot, either in still air or in a uniform horizontal wind, maintain his level indefinitely…. Whenever …a bird pursues his course for some time without working his wings , we must conclude either (1) that the course is not horizontal, (2) that the wind is not horizontal, or (3) that the wind is not uniform.
This statement recognises that (1) gliding involves a loss of height and that (2) a wind with a vertical component could cause a bird to rise. Clause (3) is strictly correct; if the wind is uniform then there is no change of wind-speed and no exchange of momentum with the bird and no energy is available to the bird to overcome drag. However, aerodynamic forces are caused by wings imparting momentum to the air and that will cause the air to be accelerated and therefore non-uniform. The wind could be uniform but the passing of the bird could cause it to become non-uniform as an exchange of momentum takes place.
Rayleigh then proposes a non-uniform wind by describing a two-layer system with the two layers moving at different speeds. However, the two wind layers and the bird are all moving with constant velocity and he does not describe an exchange of momentum between the wind and the bird. The loss of energy in the wind-system is a matter of the interaction of the wind and the ground and has already happened before the bird enters the scene. His theory was made without a complete knowledge of either the nature of the air currents in the atmosphere or how to navigate those currents or, in other words, how to fly. In 1883 the only person to have flown a heavier-than-air craft was George Cayleys understandably reluctant coachman. Of the earliest pioneers of manned flight, Otto Lilienthal began gliding only in 1894 and the Wright brothers achieved their first glides in 1902.
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In the article Rayleigh refers to another author Mr S E Peel who observed, in Assam, pelicans and other large birds soaring in circles to heights of 8000 ft and drifting downwind. Rayleigh says this may happen when there is a wind and,
- ‘That birds do not soar when there is no wind is what we might suppose, but it is not evident how the existence of a wind helps the matter. If the wind were horizontal and uniform it certainly could not do so.
He goes on to say that,
- ‘it does not seem probable that at a moderate distance from the ground there could be a sufficient vertical motion of the air to sustain the birds.
Neither of these two statements is correct. We now know that what Peel observed were birds thermal soaring, circling in rising columns or bubbles of air. If the air is rising faster than the bird is descending then the bird will gain height. The vertical currents of air which the birds are exploiting are caused by atmospheric instability and are trigged by surface effects like localised solar heating or orographic features. Thermals do not require a wind, although they will normally generate a local wind near the surface due to the inflow of surrounding air to replace the air which is rising. Thermals not only enable birds to soar but also lead to the formation of cumulus clouds; close observation of which will reveal the atmospheric motion, which is not only sufficient to create the clouds but also strong enough to support birds in soaring flight.
He then correctly writes of,
- ‘the difference of horizontal velocities which we know to exist at different levels.
This was probably the experience of the balloonists of the day. They found that as they gained height and drifted downwind, their track over the ground turned to the right (in the northern hemisphere) and as they descended the track turned to the left. This is due to the natural variation of wind-direction and speed with height.
Rayleigh then describes what we nowadays call a kind of dynamic soaring. He says that,
- ‘In a uniform wind the available energy at the disposal of the bird depends upon his velocity relative to the air about him.
He is assuming that kinetic energy is proportional to airspeed so that, if you have excess airspeed, you can zoom into a climb and gain height just like on a roller-coaster. This is true up to a point. However, if the air mass within which you are flying is moving downwards fast enough, you may not gain any height at all. Also increased velocity relative to the air means increased drag, requiring a steeper angle of descent to overcome the drag.
In reality, the energy available to a glider is height. The glider loses height at constant airspeed and the drag losses are directly equivalent to the height lost. Airspeed (squared) gives the kinetic energy of the relative airflow using the density of the air, which leads to aerodynamic forces like lift and drag. The kinetic energy of the mass of the aircraft depends on actual speed (squared) which is the same as airspeed in still-air.
The Rayleigh cycle involves the bird gliding downwind and descending through a horizontal shear layer where the wind speed reduces,
- ‘In falling down to the level of the plane there is a gain of relative velocity, but this is of no significance for the present purpose, as it is purchased by the loss of elevation;
No, for a glider, loss of height does not necessarily involve a gain of airspeed. Airspeed is maintained because aerodynamic drag is balanced by a component of weight. Drag losses at constant speed are equivalent to height lost. To increase airspeed in a dive involves diving steeper to overcome the greater drag. He continues,
- ‘...but in passing through the plane there is a really effective gain. In entering the lower stratum the actual velocity is indeed unaltered, but the velocity relatively to the surrounding air is increased. The bird must now wheel round in the lower stratum until the direction of motion is to windward, and then return to the upper stratum, in entering which there is a second increment of relative velocity. ....if the successive increments of relative velocity squared are large enough to outweigh the inevitable waste which is in progress all the while, the bird may maintain his level, and even increase his available energy, without doing a stroke of work.
This cycle is then repeated by circling to explain Peel’s observation. The problem with this is that if actual speed (ground speed) is preserved and the speed of the wind in each layer is constant, then, when descending through the shear layer, there is no acceleration and kinetic energy is unchanged. There is no gain of energy, only an increase in airspeed. The only way to sustain the increased airspeed and consequently greater drag is to dive more steeply and use up potential energy more quickly Wheeling around will result in further loss of actual speed or height.
The ‘second increment of relative velocity’ during the upwind climb, will increase drag and reduce actual speed and therefore reduce kinetic energy. It can only be achieved by firstly converting airspeed to height so that the gain of airspeed must be less than the change of wind-speed. Note that Rayleigh avoids referring to constant actual speed when returning to the upper stratum. Now the bird must wheel around again to go downwind during which its actual speed must increase but Rayleigh does not explain this.
Rayleigh’s proposition is intuitive. It feels like it might be so - that airspeed will increase with an increasing headwind. It does work in one sense, which is that flying through an instantaneous wind gradient as in turbulence will cause the airspeed to react. Passing through a wind gradient may reduce airspeed losses due to drag but not necessarily by increasing total energy.
Later in the article, referring to the wind gradient, he recognises that,
- ‘there is of course no such abrupt transition in nature …there is usually a continuous increase of velocity with height…
which is true. He then says,
- ‘…it is only necessary for him [the bird] to descend while moving to leeward and to ascend while moving to windward…
This does not explain Peels observations of circling pelicans. We know that birds or gliders in a thermal, gain height continuously and not by climbing and descending.
Rayleighs soaring model does appear to relate to albatross flight because of the up and down motion close to the surface and he made this connection in a later letter of 1898. However, albatrosses do not achieve dynamic soaring by circling or by making alternating 180 degree turns and do not get anywhere near to an upwind or downwind heading. Observers of the time did not see or understand this. Later he writes,
- ‘A priori I should not have supposed that the variation of velocity with height to be adequate for the purpose; but if the facts are correct [Peel’s observation], some explanation is badly wanted.
He is saying that the wind gradient in the atmosphere is not sufficient to enable the flight of the pelican and therefore the wind gradient theory itself is impracticable.
I think Rayleigh knew there was a weakness in his argument but he was not offering a definitive explanation of avian soaring. Rather he was just contributing to a debate and inviting the world to provide the answer. In a sense he was correct, that a bird cannot soar without a changing wind. However, as explained by the Windward Turn Theory, elsewhere in this website, a uniform horizontal wind is simply a wind with a particular velocity at the time and place it is encountered by the bird. The variation of the wind is then caused by the exchange of momentum between the wind and the bird. That non-uniformity of the wind is not an intrinsic part of the wind itself, a wind gradient, but rather an acceleration of the wind as a consequence of the way the bird turns relative to the wind.
Since 1883 many different kinds of soaring have been described including thermal soaring, hill soaring and atmospheric wave soaring. The Rayleigh cycle has been left to explain dynamic soaring as practised by albatrosses but it is not really up to the task. The pity is that that the world has taken his contribution to be the answer and has not really completed the dialogue, until now.
‘Some proofs command assent. Others woo and charm the intellect. They evoke delight and an overpowering desire to say ‘Amen Amen’
Lord Rayleigh
