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Aviation History
1949
1949 - 0111.PDF
January 20th, 1949 FLIGHT 75 soaring pilots decide whether joy is obtainable. In the use of this simple and accurate device some caution is needed because of differences, not only of location but also of time ; the ascents given on the broadcast may not be representa- tive of the area over which it is intended to soar. Cross-country thermal soaring flights are made by circling in thermals to gain as much height as possible (or as neces- sary), and then moving on to another thermal, losing again on the way as little height as possible or as necessary—the two do not always coincide. Several ingenious rules have been evolved and tables computed to find the most efficient forward and sinking speeds applicable to the various por- tions of the air and their different conditions. Clues lead- ing one to expect the presence of thermals are found in the physiognomy of the sky, with its clouds and the aspect of the ground. Soaring Possibilities—with reservations So much for soarable air and soaring. Let us now assume that fair sampling of the air in which soaring is intended has been achieved and soarability assessed. It is then natural amongst glider pilots to make their enthusiasm for the potentialities of the day directly proportional to the depth of the soarable layer. If that depth is 4,000ft they say that it is not very good; if it is 8,000ft it is good enough; 15,000ft is wonderful and 10,oooft terrific. The two diagrams on these pages demonstrate most effectively the differ- ences, in both formation and frequency, between conditions encountered in layers of soarable air extending to 6,000ft (lower left), and 4,000ft. As explained in the text, thermals are more frequently and more easily contacted in the 4.000ft layer, but the "streets" of cumulus in the deeper layer offer better cross-country prospects to the average pilot. Conditions approximately similar to those depicted in the sketches are portrayed in the accompanying photographs of soaring activity. This simple view cannot be accepted without the most stringent reservations. There is no doubt that, if one wishes to gain height by thermal soaring alone, the top of the unstable layer is the limit. But the reservations I have in mind spring from the limits set by the severity of the weather. To these reservations I shall give concrete expression right away by stating what I consider is the most important proposition in this essay; for the bulk of soaring pilots there is an optimum depth of soarable air for cross- country thermal flights which I would put at 6,000 and certainly not more than 7,000ft. In so saying, I must, once more, emphasize that I am discussing the conditions pre- vailing in this country. ~" Several fairly straightforward meteorological reasons will be given for thus grudging the soaring enthusiast room in the vertical in which to manoeuvre. Because complete experimental evidence is as yet far from available, I shall assume certain things and make generalizations. The first assumption is that the horizontal space between the ascend- ing currents is proportional to the depth of the unstable layer. This assumption is both bold and fairly obvious. It is bold when it implies the simple symmetry of ceDular circulation; a candid generalization which the complexity of nature defies. At the same time it is obvious to expect that, when more room is allowed, movements should be on a larger scale.
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