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Aviation History
1911
1911 - 0753.PDF
SEPTEMBER 2, ign. When near my level the wings were nearly fiat (that is to say, extended horizontally, with no dihedral angle). The legs and also the body were hanging downwards below the level of the wings. All the birds as they descended glided to leeward of the dead leopard, so that when landing they were gliding up wind. The wind was west and very light, slightly moving the leaves of the trees. (How these birds knew the direction of the wind is a question more easily asked than answered.) The birds that arrived flex-gliding from a distance began to drop their legs and allow their bodies to hang down when about 50 metres up, and when from 50 to 100 metres to leeward of the leopard. In every vulture observed the "alula" was seen to be extended. In one or two instances the alula of one wing was seen to be suddenly rotated upwards. In spite of my best endeavour, it was impossible for me to observe whether this rotation affected both alulre at the same time. The observed movement of the alula must have been over 2 centimetres of its front edge. One vulture, when gliding down, suddenly slightly increased the flexing of both wings simultaneously. This produced an immediate drop of several feet, nearly vertically, obviously with the intention of getting to the ground without overshooting the mark. The drop was checked by renewed extension of the wings. The important point in this description is the fact that the diving vulture placed its wings in the dihedrally up position to cause rotation round the transverse axis. On other occasions I have seen black vultures and cheels when playing together in the air make ® ® l/USE short dives which were checked by placing the wings in the dihedrally up position. Fig. 17 shows diagrammatically the changes in the disposition of the wings that result in checking the speed of diving. The explana tion of the rotation is obvious. When the wings are placed dihedrally up, as at B, the inertia of the bird acts through the centre of gravity pulling the bird downwards or nearly downwards. The resistance of the wings must be acting in the opposite direction. The two forces do not act in the same straight line. Hence there must be a couple that rotates the bird to the position shown at C. I will now consider the effect of changes in the dihedral angle. The change from the dihedrally-up to the dihedrally-down positions can often be seen in cheels, though in these birds it is usually not very great in extent. One of the first things I noticed on beginning my observations on cheels was that the dihedrally-up position is seen in circling especially on the up-wind side of the circle. I also saw that it was assumed at the end of a horizontal glide and that it immediately resulted in a gain of height. I learnt to associate the dihedrally-down position with loss of height and increase of speed. I once saw a cheel gaining height in several successive circles, with its wings, so far as I could see, in the dihedrally-up position all round the circle. Then, without gain of height, it described a circle with the wings either flat or slightly dihedrally down. Then it made a long glide in a straight line, descending gradually, with the wings dihedrally down and with clearly seen increase of speed. To be continued.) ® ® THE AVERAGE WEATHER OF SEPTEMBER. By T. F. IN September the weather conditions begin, as a rule, to become somewhat adverse to the flying man. This amazing summer, of course, has shown no regard for averages, and how it will influence the general character of the first autumn month we cannot guess. Normally there is less rain in September than in either July or August; but this year we may reasonably calculate on a reversal of the normal relationship, and perhaps for a greater than the average rainfall. Usually the first three weeks have considerably fewer rainy days than the corre sponding period in August, but with the fourth week a decided increase sets in. The fact of most importance to the airman is the very great increase which occurs in the number of fogs and mists. Both light fogs and dense fogs are three times as numerous in September as in August, and nearly seven times as numerous as in July. They differ from winter fogs, however, in the fact that they occur chiefly in the mornings and evenings. They increase steadily from the beginning to the end of the month, and while the odds are about 2 to 1 against a fog in the first week, the chances are 11 to 10 in favour of a fog during the last week. The following figures show how badly September compares with August in this respect:— Fogs in a Hundred Years. MANNING. 1st week 2nd week .. 3rd week .. August. 23 " 13 29 Sept. 55 85 97 August. 4th week ... 41 Whole month 122 Sept no 374 Storms are almost as infrequent in the early part of September as during the summer months, but in the fourth week they rapidly increase. The fourth week over a series of years is twice as stormy as the first week; but, even so, the chances are 3 to 1 against a gale during this week in any year. In the whole month there are 11 gales in 10 years ; this is an increase of 25 per cent, over August. Hail is very rare in September—one fall in six years, and snow is scarcely ever seen in the south of England daring this month, although one or two falls have been recorded. Thunderstorms may still be expected, but they are only half as frequent as in August, and average just one for the whole month. But this phenomenon is so very uncertain that we might have several thunderstorms or none in any one September. With this month the most favourable flying season comes to an end. The falling temperature brings fogs and mists, gales increase in number and violence, sunshine and daylight decline, and cloudy or entirely overcast skies become much more common. The following figures show the relative average weather of August, September and October. They give the number of events occurring in each month during a period of ten years :— August. Ten years'gales 9 „ fogs ... 12 „ dense fogs ... ... 2 „ thunderstorms 21 ,, hail-storms ... ... \\ ,, rain-days ... ... 129 Sept. 11 37 H 10 iS 123 Oct. 16 50 10 3A if 150 Average rainfall Greenwich) • •* Mean temperature (Greenwich) ... Hours of sunshine (Greenwich) ... Degree of humidity (Greenwic Table of The figures show years. Day. 1 ... 2 ... 3 ••• 4 ••• 5 ••• 6 ... 7 ••• 1st week 8 ... 9... 10 ... 11 ... 12 ... 13- 14 ... 2nd week 15... 16 ... 17 ... 18 ... 19 ... 20 ... 21 ... 3rd week 22 ... 23 ••• 24 ... 25 ... 26 ... 27 ... 28 ... 4th week . 29 ... 30 ... in 3 O ••• 3 2 1 1 2 ••• 3 4 ... 16 2 - 5 ... 4 ... 4 ... 6 ... 4 2 ... 27 •• 3 • 5 1 •• 3 4 2 2 .. 20 2 .. 8 .. 4 •• 5 •• 5 .. 6 • 4 •• 34 .. 6 •• 9 h)... 2-34 in. 2 61-6 189 76-3 Weather Phenomena in the 1 a 8 t> 8 9 8 11 5 55 12 10 12 15 9 17 10 ~85 16 10 19 13 15 9 15 97 14 IS 12 17 14 16 22 no 13 14 lumbers of S> tuo c 0 v — Q — 6 2 2 2 - 2 3 - 11 2 3 4 2 1 2 3 '7 3 2 2 4 3 6 4 24 3 5 i 0 c if: - - - - - - - 0 - - - - - - - 0 - - - - - - - 0 - - I I - - - — 2 _ - each 33 1 - - - 1 2 1 5 - 2 - 1 - - - 3 2 1 - - 1 2 1 7 - 1 - - - - 1 — 2 1 - 15 in. : 57'2 141 Jo-2 September. event g S3 H 5 3 5 3 3 8 7 34 5 4 3 3 9 3 3 24 4 1 4 3 1 1 5 19 3 4 4 - 2 1 3 17 2 4 in one a a. 2 6 597 597 59-6 59 4 59'3 59-1 58-9 — 587 58-5 58-3 58-1 58-0 57 9 57-8 — 577 57-5 57'3 56-9 56-5 56-1 557 — 55-4 553 55"i 55-0 54*9 54'9 54-8 — 54'6 54 "4 8-58 in. 50*0 93 850 hundri w .2 01 B5W 37 4') 44 37 40 5' 40 298 40 43 4.? 32 32 27 34 251 32 35 4' 4i 37 38 54 278 40 • 47 5i 41 38 46 48 3" 49 48 112 374 66 18 100 57-2 1,235 755-
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