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Aviation History
1911
1911 - 1071.PDF
DECEMBEF 16, 191 I. [/OGHT] Eg O&B, By DrRNJiankin. MA. DSc. ^ (Copyright Reserved) CHAPTER XXXIX.—"Ergaer," the Physical Basis of Soarability. IT will be of interest to review briefly the facts that have been •described in order to see what general conclusion can now be drawn as to the nature of soarability. The first and chief question is what is the source of the energy involved ? In answer to this question, there are, in the first place, two distinct possibilities to be considered. Either the energy is furnished by the bird, or, on the other hand, it is furnished by the -air. The idea that the energy of soaring is due to the bird, that is to say, that it is derived from minute and undiscovered movements of the wings is, I think, excluded by the following facts. Firstly, soaring flight usually commences at a definite time of day, differing for each species of bird. It is impossible to imagine why a bird •should be able to make minute movements, as suggested, after a definite time and not before it. Secondly, the idea is excluded by the complicated relation of cloud shadow to soarability. Thirdly, the idea does not harmonise with the discovery of different kinds of soaring flight in which the wings have different dispositions, and in which, as has been proved, different amounts of energy are involved. Therefore, we are obliged to accept the alternative, namely, the view that the soaring bird gets its energy from the air. Here again we are confronted by two possibilities. Before the bird makes use •of this energy, this energy must be in the air. It may be present either as the kinetic energy of moving masses of air, or, on the other hand, it may be present in the form of potential energy, that is to say, stored up in some structure, which structure, by decom posing, liberates the energy required for soaring flight. Let us consider these possibilities in order. The suggestion that we are dealing with the energy of moving masses of air must be split up into two possibilities, which we may consider separately. Firstly, it is conceivable that the soaring bird takes advantage of ascending currents reflected upwards from the walls of high buildings, •&c. This suggestion becomes improbable in view of the extraor dinary regularity of soaring flight as was exemplified in Chapter XI in a description of the circling of cranes. Secondly, the statement often dogmatically made that soaring birds show skill in finding and ttaking advantage of ascending currents is absolutely opposed to the facts in the case of vultures and adjutant birds. In Agra, if there is a wind, ascending currents are reflected upwards from the walls of high buildings such as the Taj and the Fort. The lighter birds, namely, cheels and scavengers, do take advantage of these currents especially when the air is not soarable. But this is not the case with the heavier birds. Vultures 'and adjutants, if they show any skill in the matter, do so, not by finding, but by avoiding such currents. On one occasion I saw some vultures apparently circling in the ascending current over the Fort. I at once went in my motor to investigate, and found that the vultures were not over the Fort but were circling a few hundred yards beyond it. They had merely appeared to me to be over the Fort because my point of observation, the Fort, and the vultures, were in one straight line, which line made a right angle to the wind direction. Vultures may be seen •circling everywhere over the city of Agra with the single exception that they avoid the ascending current reflected upwards from the fort walls. It would be a very difficult proposition to defend that vultures, that avoid with skill ascending currents known to exist, have skill in finding other ascending currents that are not known to exist. The second possibility is that soaring birds take advantage of those ascending currents that I have described as "heat eddies." These ascending currents are of small size, and so far as the evidence goes, uniformly distributed. In these two respects heat eddies fulfil a necessary character of the physical basis of soarability, as the regularity of soaring flight is explicable if due to minute and uniformly distributed rising currents. I have shown that the morning development of soarability frequently coincides in time with the development of heat eddies. But we have been brought to the conclusion that this correspondence is due to the fact that both soarability and heat eddies are due to the same cause, namely, sun energy. That there is no direct causal relation between the two phenomena is proved by the following facts. Under certain con- editions air may be completely unsoarable in the presence of heat eddies. Secondly, air may be soarable in their apparent absence. This has been observed not only in Agra but also in Naini Tal, 1079 where the presence of the slightest eddy movement in the air, had such existed, would have been revealed by the movement of the cloud masses. Therefore it is impossible to see how heat eddies can be the cause of soarability. It is highly improbable, and in a sense inconceivable, that there should be a second set of ascending currents, also of small size, and also uniformly distributed, that could subserve soaring flight. We have seen that the relation of the centre of area of the wing to the centre of gravity in slow flex-gliding could be explained by ascending currents. But the same line of argument that leads to this conclusion also leads to the conclusion that the relation of the area to the centre of gravity cannot be explained by ascending currents in the case of fast flex-gliding. The disposition of the wings of the bird when in an ascending current of moderate strength has been described, and has been shown to be different from the disposition assumed in circling and fast flex-gliding. On one occasion in a stormy wind I saw a cheel travelling horizontally for a few seconds with wings dihedrally down and with the tail furled and elevated. This is the disposition usually employed for gliding downwards at speed. Its use in this case may be explained by supposing that the bird was, for a few seconds, in a strong ascending current. Thus, observation of the disposition of the wings may, to a certain extent, give information as to whether or not the bird is subjected to an ascending current, and, in fact, such observations show that in ordinary soaring flight ascending currents, capable of affecting the bird, are absent. Therefore there is no evidence whatever in favour of the view that the energy of soaring flight is derived from the kinetic energy of air in movement independently of the bird's wing. There is a good deal of evidence against this view. Thus we are led to the conclusion that the energy used in soaring flight is stored up in the air in potential form. There must be some substance or structure in which this energy is stored. For this unknown physical basis of soarability I propose the name " ergaer." The name no more implies that the matter is understood than does the name " protoplasm " imply that we know the nature of life. In one case, as in the other, the name stands for an unknown thing which is a subject of discussion. In an earlier chapter I permitted myself to make a sarcastic remark about an observer who said that so 1 ring flight was due to " levitation." Perhaps a critic may make a sarcastic remark about another observer who says that soaring flight is due to "ergaer." But the difference between the two cases is this. The author of the idea of levitation regarded it as a final explanation of soarability and- implied that further research is unnecessary. On (he other hand, ergaer is presented as the subject of a research that has only just begun, a research too in which I have little hope of being able to play a part. Mere scraps of time, such as I have been able to devote to cataloguing the bate facts relating to soarability, are quite insufficient for carrying out the serious experimental research that is now urgently required if the matter is to be carried further. The resources of a physical laboratory are needed to throw a light on the hundred questions that demand an answer. Is ergaer an unstable gaseous compound, or som- allotropic modification of one of the gases of the air ? Or does it consist of clusters of molecules that can fly apart, liberating energy, when disturbed by the passage of a vulture's wing? Or are we dealing with minute eddies whose circular motion is changed to tangential motion by the disturbance in question ? What are the exact conditions under which ergaer is formed or decomposed ? Perhaps the answers to such questions may lead to results of practical importance. If so, after the observer and experimenter, it will be the turn of the engineer, who, perhaps, cannot experiment, but who can design and construct. And, lastly, the general public, who can neither observe nor experiment, nor design, will reap the benefit. Thus we have reached the conclusion that the air under the wing of a soaring bird is undergoing a change of the nature of a sort of continuous explosion. This view is perhaps unexpected and surprising. But the question arises whether it is the only case in which a change of potential to kinetic energy takes place in the air. The appearance of a gust of wind is only less surprising than soaring flight because it is so familiar. On a calm morning, before heat eddies have developed, a gust of wind arises and dies away in the absence of any apparent cause or reason. Evidence is completely lacking that energy from any external source is the cause of the
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