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Aviation History
1949
1949 - 0112.PDF
FLIGHT January 20th, 1949 SPAYABLE AIR... ' The Official Truth, which will gradually and prudently emerge from the data on convexion phenomena now being accumulated, especially in the United States, is probably that the simple cellular pattern is there, as a framework, more or less hidden by a number of auxiliary currents due to local idiosyncrasies of the ground or to secondary circu- lations. Similarly, on our western shores the very long rollers caused by violent cyclones in the middle of the Atlantic Ocean arrive mixed with many other trains of smaller waves which, although they may obscure it, do not cancel their vast movement. This to a certain extent is illustrated by the barographic records illustrated of two slope soaring flights over the Dunstable Downs in unstable layers of widely differing depths. From this first assumption, and from what has been said about soarable air and soaring, it follows that the taller the soarable air, and the wider its gaps, the higher it is necessary to climb in order to bridge these gaps safely. I would express this in a general thermal soaring rule and say that in order successfully to go across country by thermal soaring it is necessary to climb to at least three- quarters of the depth of the soarable layer. This rule has the weakness of any such generalization. It is open to severe or even indignant criticism based on bona fide indi- vidual cases. It needs the massiveness of statistical evidence, which is not, at this juncture, forthcoming. I now propose finally to explain my meaning, by describ- ing typical conditions within unstable layers of increasing depths. However, before this is done two important Secured during the International Contests at Samedan last year, this fine study illustrates both the promise and the peril of Alpine gliding. factors which greatly in- fluence thermal soaring performance must be dis- cussed so that they might be eliminated from amongst the variables, and come under the heading: all other things being equal. The first is the humidity of the air within the soarable layer and immediately above it. This humidity controls the height of the condensation . level and the cloud cover- age of the sky. Let us assume that in the several layers to be discussed the water con- tent of the atmosphere is such that the condensation level is 3,500ft and that, over a reasonably large area, not more than half of the sky is covered by clouds. The second im- portant factor is the wind. There is no doubt that in most flights the wind carries one most of the way. But there is another important respect in which the wind affects soar- ing flights: within a certain speed range it promotes the formation of cloud "streets," these providential trade winds of the sky. Let us, therefore, add to the cloud con- ditions which I have stated a gradient wind of 25 knots; and start with a consideration of a layer 4,000ft thick. Disadvantage of the Shallow Layer There pleasant conditions are met; thermals are frequent and many are earth bound, easily contacted at low height. The cloud is thin and there are short streets with few bands of continuous lift. All this is marred by the precarious- ness of the going. The margin of reserve height and time in hand is not sufficient. Assume that the pilot has climbed to the top of the layer and that he starts gliding down on course; within five minutes, at 3,000ft just below cloud base, and whilst having a good look round, his posi- tion is already precarious. If he sees something promising two miles ahead and goes for it, he will be nearer to 2,000ft than 3,000ft by the time he gets there. And if he has been led up the garden path his situation becomes serious. Another such mistake and he has to start thinking about landing places. He may then see promising clouds or patches on the ground likely to generate thermals, but, alas, they are out of gliding range. Meanwhile, more height is lost, and only a chance meeting of a young thermal can waft him up again. Although good flights have been carried out in such a thin layer, great skill and luck is required and there is no time for relaxation. Let us now increase the depth to 6,500ft, and the first obvious advantage if again one assumes climbing to the Tracing of the barographic record of a soaring flight over the Dunstable Downs from the time of launching by winch to 90 metres to the moment when height was gained by circling in a thermal prior to soaring away. The depth of the soar- ing layer was 3,600ft, and the flight level fairly constant around 800ft.
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