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Aviation History
1911
1911 - 0820.PDF
12.47.—Sunshine on opposite side of ravine. No cheels up on my side where there was still thin cloud. Four cheels circling down the valley in or near sunshine. 12.50.—Lighter. Cheels circling near, and two vultures gliding. 12.52.—Sunshine. One cheel circling high. Wind imperceptible. Small patches of cloud lying on opposite bank of ravine snowed scarcely perceptible movement. 12.55.—Cheels were circling, and white scavengers and vultures were gliding. A Lammergeyer circling and another flex-gliding. i.O.—Cljud in ravine disappearing. Sunshine and patches of blue sky. The birds had mostly flex-glided away to a distance, or had circled to a level with the tops of the neighbouring hills. These observations may be briefly summarised as follows. At the beginning of the period of observation, in spite of an ascending current of air (the bottom of the ravine having a rate of ascent of about I in 5), the air was not soarable unless there was either sunshine or else a strong glare of light. Towards the end of the observations, as cloud cleared off, the air became sufficiently soarable to permit not only circling but also flex-gliding, although the ascending wind had so far ceased that its movement was imperceptible. I made a few observations in Naini Tal on the formation of " heat eddies." As in Agra, these could be seen rising from the tops of houses or from the top of a stone wall in sunshine, and also in thin cloud, provided there was a strong glare of light. If the glare diminished from accumulation of cloud overhead, the eddies ceased. Apparently sunlight reflected from a cumulus cloud, or reflected from the snow ranges some 40 or 50 miles away, was not capable of producing heat eddies. Soarability seemed also to need direct action of sun energy. For instance :— June 27th, 1910.—At Ballia Ravine.—3.22.—Slightly more light. A vulture started and, after gliding about IOO metres, returned and settled. Another started and returned after going about 300 metres. There was thick cloud behind me covering the sun. The glare was mostly by reflection from a cumulus cloud down the valley. But it is difficult to see how heat eddies can be assumed to be the source of soarability. They only appear to be formed when sunshine (or glare) strikes solid objects. In the presence of strong glare, when birds are circling in thin cloud, not a trace of any eddy movement or anything resembling heat eddies can be seen anywhere near the birds. The thin cloud is usually not homogeneous, but in more or less discrete masses, so that the movement of every cubic foot of air relatively to neighbouring masses of air can be observed. Sometimes the air in the Naini Tal valley is filled with aerial seeds, similar to thistle down (but derived from a tree). These float in the air sometimes almost as thickly as snowflakes in a snowstorm. Their movements serve to indicate the direction of the wind as it flows regularly over the level surface of the lake, or as it is deflected as it meets the sides of the hills. But these aerial seeds show, so far as I have been able to observe, the same irregularity of movement after sunset, when the air is no longer soarable, as they do in the middle of the day when the air can support soaring flight. On one occasion I was so fortunate as to observe a cheel circling and gaining height when enveloped in thin cloud and in a descending current of air. The cheel was gliding at first in an ascending current of air over the top of Sher-ka Danda Mountain (height 7,520 ft.). It came down the leeward side of the mountain, past where I was standing, at a point 7,400 ft. above sea level, and descended to about 30 ft. below me. The air current was just enough to gently move leaves, and was descending probably at an angle of about 150 with the horizon. The cheel then began circling in this descending current and gained height. On the windward side of one circle it made three flaps Otherwise, without flapping, it regained a position over the top of Sher-ka Danda, and then glided out of sight. The total gain of height in the descending current of air must have been about 150 ft. During the greater part of its circling the cheel was enveloped in thin cloud, in which, as usual, not a trace of eddy movement was visible. 1 recorded in my notes that " at the time the cheel was gaining height, it was in cloud sufficiently thin to let through enough sua energy to make heat eddies, judging from the amount of glare at the time, and from the results of observation of heat eddies that I had made two hours previously." CHAPTER XVI.—Proofs that more Energy is required for Flex-gliding than for Circling. I have already stated that when, in the evening, soarability decreases, cheels and scavenger vultures are in the habit of collecting at the Agra Fort, and gliding in the ascending current of air over the windward battlements. With a certain strength of wind these birds occasionally glide along the battlements for long distances, keeping uniformly at a height of about 4 ft. or 5 ft. above the parapet. The distance along the battlements from a bastion near to the Delhi Gate to the next is 108 metres. On April 15th and 16th, SEPTEMBER 23, 1911. 1910, I noticed that cheels glided this distance in 13, 14 and 14 sees.; this corresponds to a speed of 7 7 metres per second. Scavengers did the same distance in 113, uj, 12 and 12 sees.; this is equal to a speed of 9 metres per second. On May nth, 1910, when seated on the Delhi Gate at a point slightly above the level of the battlements, I made the following simple observation, which led to results of some importance :— 5.30.—Cheels noticed that were gliding beam on to the wind, parallel to the battlements, and at a height of 3 ft. or 4 ft. above them. The secondary quills of the leeward wing appeared re laxed—the hinder ends of these feathers, that is to say, were higher than the ends of the feathers of the windward wing. The difference in level was probably 1 centimetre, perhaps as much as 2 centimetres. The birds were gliding on a level keel. This observation led me to notice the position of the secondaries under different conditions. On the following day I was watching cheels " wind-facing" over the battlements in a light wind Suddenly the wind increased in strength. Immediately the cheels relaxed their secondaries and increased the flexing of their wings— that is to say, instead of ease-gliding they were flex-gliding. Their speed had increased pari passu with the increase of speed of the wind, so that they retained their position over the battlements. Hence the peculiar appearance presented by the wings of cheels in flex-gliding is due to the fact that, concomitantly with the decrease in span, there is a relaxation of the secondaries, which, as I shall show later, is equivalent to a decrease in camber in the case of slow flex-gliding. In the case of fast flex-gliding, the camber of the inner part of the wing is not only decreased, but actually abolished. When a cheel is gliding with wings extended, the posterior margin of the wing (formed by the free ends of the secondaries) forms a straight line. When flex-gliding, the posterior margin is no longer a straight line, but forms a curved line with the convexity upwards. In cheels, when flex-gliding, the relaxation affects mostly the more centrally-placed of the secondaries. In vultures, when flex-gliding, all the secondaries appear relaxed to the same extent. The evidence in my possession goes to show that a particular amount of flexing of the wing and relaxing of the secondaries corresponds to a particular speed. For instance :— August 28th, 1910.—At 11.40.—A vulture slow flex-gliding with wings slightly flexed, was seen to make a double dip. During the up stroke of this double dip, the wings were seen to acquire extra flexing. This extra flexing was retained, and was followed by an immediate increase of speed. If, as frequently happens, flexing is increased without a double- dip movement, then the consequent increase of speed is gradual instead of almost instantaneous, as in the above case. That the increase of flexing in such cases is accompanied by increase of relaxation of the secondaries will be proved on a later occasion. The above facts give a further insight into the nature of flex- gliding. It is now necessary to consider facts that prove that more energy is required for flex-gliding than for circling. I have observed several instances in which the development ot cloud shadow (in Agra), in cases in which the sun is only obscured by a thin layer of cloud, may cause flex-gliding to cease, while permitting birds to continue circling. Though I have only recorded a few such cases, it is probably not an infrequent occurrence. An unaccustomed observer, on seeing circling with gain of height, going on in the absence of sunshine, might infer that cloud shadow has no effect on soaring. I was for some time in this position, and it was only after more lengthy experience that I realised the different effects of thin cloud shadow on circling and flex-gliding. Examples of decrease of soarability of this nature are as follows :— March 9th, 1910.—At 12.10.—Wind north. Leaves still. A thin layer of cloud. No birds up except cheels. These were either circling or flex-gliding. No ease-gliding seen except apparently on windward side of fort. 12.30.—-Still cloudy. Scavenger vulture seen circling, with occasional flapping. 12.34.—Sunshine. 12.35.—Cheels seen flex-gliding, but with loss of height. No flex-gliding had been seen previously. March 12th, 1910.—At 3.0.—Thin cloud, but sun making faint shadows. Heat eddies strong. Vultures were flex-gliding and circling. 4.0 to 5.0.—Stronger cloud shadow and heat eddies ceased. Vultures, if at low level, were flap-gliding. If at higher level, they were circling. July 22nd.—At 8.15.—Cheels near me had been flex-gliding. Shade came over. Then the cheels that were flex-gliding tightened their secondaries, but for a little time continued gliding up wind. Then they ceased such gliding, and confined their movements to circling, or if at low level to flap-circling. A little later flex-gliding at high level was seen.
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