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?
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) suggests that some kind of dynamic process is involved but is 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. Of the earliest pioneers of manned flight, Otto Lilienthal began gliding only in 1894 and the Wright brothers achieved their first glides in 1902..
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. Rayleigh says this may happen when there is a wind and
He goes on to say that
Neither of these two statements is correct. The Windward Turn model of dynamic soaring works in a uniform horizontal wind and there certainly is sufficient vertical velocity in thermals to cause birds and gliders to rise. Thermals can occur in the absence of wind although the thermals will tend to generate their own winds. He then correctly writes of
This was probably the experience of the balloonists of the day. They found that as they gained height as they 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.
We now know that what Peel observed was birds thermal soaring in rising columns or bubbles of air which are typically caused by variations of heating of the surface by the sun. If the air is rising faster than the bird is descending then the bird will gain height. Thermals are typically capped by cumulus cloud, careful observation of which will reveal the rising air currents.
Rayleigh then describes what we nowadays call a kind of dynamic soaring. He says that
This appears to be the roller-coaster effect where, if you have excess airspeed, you can zoom into a climb and gain height. This is true up to a point and has been repeated often since then. 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.
The Rayleigh cycle involves the bird gliding downwind and descending through a horizontal shear layer where the wind speed reduces,
No, loss of height does not necessarily involve a gain of airspeed. Airspeed is maintained because aerodynamic drag is balanced by a component of weight and drag losses are equivalent to height lost
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’ 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.
Rayleigh’s proposition is full of holes but it does work in one sense, which is that flying through a wind gradient may add some airspeed which may offset drag losses but not necessarily increasing total energy.
Later in the article he recognises that
which is true. He then says
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. His soaring model does appear to relate to albatross flight because of the up and down motion and he made this connection in a later letter. However, albatross do not make 180 degee turns in dynamic soaring and do not get anywhere near to an upwind or downwind heading. Nor do they achieve dynamic soaring by circling. Observers of the time did not see or understand this. Later he writes
He is saying that not only is the wind gradient insufficient to enable the flight of the pelican (or the albatross) but 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 component. However, as explained by thre Windward Turn Theory, in a uniform horizontal wind, the variation of the wind that the bird is exploiting is not an intrinsic part of the wind itself, a wind gradient, but rather 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 albatross and the like 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’