Field Observations of Thermals and Thermal Streets, and the Theory of Cross-Country Soaring Flight

摘要: The classical theory of cross-country soaring flight postulates climbs in discrete thermals, separated by glides through stationary air, and prescribes an "optimum" speed for the inter-thermal glides, based on performance glider or bird, rate climb thermals. However, it is known that pilots usually fly slower than theoretical optimum between often achieve faster speeds maximum predicted theory. reasons this are analysed relation to a survey real thermals over Serengeti National Park, Tanzania. About 1500 km was flown straight lines, Schleicher ASK-14 motor glider, measuring airmass lift (vertical velocity air) at 15-s intervals. It shown how bird pilot can increase mean sample actually used, so speed, selecting below supposed optimum. If air not stationary, time lost circling be further reduced variations during glides. Eventually, if organised into lines "streets", eliminated altogether, linear becomes possible. Linear also sometimes possible along ridges escarpments. Soaring may seen as one end spectrum techniques, with other end, giving much higher speeds. In flight, flapping members another spectrum, connected various intermediate techniques. A migrating line reduce its power requirements flying forms rising air.