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 Lord Rayleigh

 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) 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..Pelicans by David Stephens

 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

     ‘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. 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

     ‘the difference of horizontal velocities which we know to exist at different levels.

 Which was probably the experience of the balloonists of the day.

 What Peel observed was clearly  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

      ‘In a uniform wind the available energy at the disposal of the bird depends upon his velocity relative to the air about him.

 This is 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 this only applies within the air mass. However much airspeed you have, if the air mass within which you are flying is moving downwards fast enough, you may not gain any height at all.

 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, 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

    ‘...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’ will increase drag and reduce actual speed and therefore reduce kinetic energy. It can only be achieved by firstly converting airspeed to height. 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 energy which may offset drag losses but without actually increasing total energy.

 Later in the article 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. It does appear to relate to albatross flight but albatross 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.

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, 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.

 When measuring height gains relative to the ground (potential energy) you should consider kinetic energy relative to the ground and this must relate to ground-speed which comprises both airspeed and a component of the wind. In the Windward Turn Theory, it can be shown by analyzing kinetic and potential energy levels, that, in a uniform horizontal wind, a bird can, by turning from a tailwind to a headwind, convert the energy in the wind to turbulence in overcoming drag, without loss of airspeed.

 Since 1883 many different kinds of soaring have been described including thermal soaring , hill soaring and atmospheric wave soaring. The Rayleigh cycle has been used to explain dynamic soaring as practised by albatross and the like but it is not clear that he was trying to explain the flight of these sea birds. 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

 

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