INSTRUMENTS AND TECHNIQUES FOR LOCATING AND EXPLOITING THERMALS

Abstract

A prior study surveyed methods for locating thermals remotely, and for assessing thermal characteristics when flying near or in them so as to permit full exploitation of the thermals. The present study examines further the devices which now, with present technology, seem to be most practical. For locating thermals from a distance, use is made of the fact that some thermals carry space charge aloft from the surface. This superimposes a horizontal potential gradient on the normal vertical fair-weather field, and it is possible that the horizontal field can be measured at a sailplane several kilometers away. Suitable instrumentation is discussed. There are several techniques which should be capable of showing which way to turn from the edge of a thermal toward its center. All involve devices showing the lateral variation of some variable. Practical contenders are versions of the aforementioned potential gradient device, and a system showing the gradient of wet bulb temperature between the wing tips. The strength of a thermal along the flight path is derived from a rate-ofclimb indicator. Compensation methods are discussed for rnaking this indication more accurate. The buoyancy of the thermal can help indicate whether the thermal is growing or decaying. Relative buoyancy can be given, to adequate accuracy, by an instrument combining temperature and altitude information. Visualization of the thermal is facilitated by introducing markers (long-lasting bubbles, or smoke put into the tip vortex) or by particular cockpit displays of vertical velocity which aid the human memory.