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Aviation History
1949
1949 - 0110.PDF
••<&*ASAHLE AIR Thermal Prospects for the Glider Pilot over Great Britain , _ By JACQUES COCHEME, A.F.C, B.Sc THE purpose of this article is to discuss the kinds ofBritish atmospheric conditions which lend them-selves to thermal soaring flights with a sailplane.These variable conditions hold, for pilots of different apti- tudes, the promise of different achievements. At one end of the scale there are the holders of a "C" gliding certi- ficate who, having learnt to take off, land and control a glider in the air, are getting ready to seize the opportunity to achieve the five hours of duration, one kilometre in height and fifty in distance, which will give them the silver badge to which they aspire. Next comes the majority of confirmed British soaring pilots, silver "C" holders, now 200 in number, who, given adequate conditions, will have no difficulty in exceeding again the silver "C" standards, but whose experience and aptitude for spiral climbing and navigation in cloud are limited. From this honourable collection few are the wizards, or potential wizards, who might be expected, owing to their ability, to_-fly in deep cloud amidst unpleasant conditions of icing and turbulence, retaining meanwhile an idea of their ground position. Between this select group and the mass of fair-weather soarers, the proficiency necessary to climb three kilometres and soar for three hundred, which wins a golden "C," occupies a somewhat intermediate and ambiguous position, which will presently be examined in more detail. To give a basis to this discussion, something must first be said of the nature of thermal soaring, and of its medium, soarable air. Thermal soaring is the art of gliding at the very low sinking speed of a sailplane, in air that is going up faster than the machine is descending through it because it is warmer, and consequently lighter, than the surround- ing atmosphere. This buoyancy is produced by uneven heating of the ground by the sun, because of differences in colour, texture, vegetation, orientation and many other details at its surface. The rising currents of air, or thermals, are cooled by the expansion caused by the reduction in pressure at the rate of 5.4 deg. F per 1,000ft until they are fully saturated with water vapour and cloud begins to form. From there up- wards the rate of cooling is reduced to approximately 3 deg. F per 1,oooft. Clearly the thennals will continue to rise so long as they remain warmer than the air at their level. The vertical distance over which an initial temperature difference will be maintained depends on the distribution of temperature with height of the surrounding air. This distribution is measured regularly by means of radio sondes and aircraft. It is one of the most valuable tools of the forecaster, and can easily be plotted on a Meteorological Office form called a Tephigram when, if forecast maximum temperatures for the day are also available, the depth of thermal ascent can be assessed graphically. Air within this depth is spoken of as unstable—a glider pilot might say soarable. Every morning representative ascents and forecast tem- peratures are broadcast on the Airmet frequency to help Feet 7O00 6.OO0 • S.OOO -
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