Chapter 16 SOARING WEATHER While horse racing may be the "Sport of Kings," of the craft depends on the weather and the skill soaring may be considered the "King of Sports." of the pilot. Forward thrust comes from gliding Soaring bears the relationship to flying that sailing downward relative to the air the same as thrust bears to power boating. Soaring has made notable is developed in a power-off glide by a conven­ contributions to meteorology. For example, soar­ tional aircraft. Therefore, to gain or maintain ing pilots have probed thunderstorms and moun­ altitude, the soaring pilot must rely on upward tain waves with findings that have made flying motion of the air. safer for all pilots. However, soaring is primarily To a sailplane pilot, "lift" means the rate of recreational. climb he can achieve in an up-current, while "sink" A sailplane must have auxiliary power to be­ denotes his rate of descent in a downdraft or in come airborne such as a winch, a ground tow, or neutral air. "Zero sink" means that upward cur­ a tow by a powered aircraft. Once the sailcraft is rents are just strong enough to enable him to hold airborne and the tow cable released, performance altitude but not to climb. Sailplanes are highly 171 r efficient machines; a sink rate of a mere 2 feet per second. There is no point in trying to soar until second provides an airspeed of about 40 knots, and weather conditions favor vertical speeds greater a sink rate of 6 feet per second gives an airspeed than the minimum sink rate of the aircraft. These of about 70 knots. Some two-place training craft vertical currents develop from several sources, and have somewhat higher sink rates. these sources categorize soaring into five classes: In lift, a sailplane pilot usually flies 35 to 40 ( 1) Thermal Soaring, (2) Frontal Soaring, (3) Sea knots with a sink rate of about 2 feet per second. Breeze Soaring, (4) Ridge or Hill Soaring, and Therefore, if he is to remain airborne, he must (5) Mountain Wave Soaring. have an upward air cUll-rent of at least 2 feet per THERMAL SOARING Peter Dixon estimates that about 80 percent of jacent updrafts and downdrafts. The fast moving all soaring in the U.S. depends on thermal lift.* powered aircraft experiences "pounding" and tries What is a thermal? A thermal is simply the updraft to avoid convective turbulence. The slower moving in a small-scale convective current. Chapter 4 in soaring pilot enjoys a gradual change from ther­ the section "Convection," and chapter 9 in the mals to areas of sink. He chases after local section, "Convective Currents," explain the basic convective cells using the thermals for lift. principle of convective circulation. The explana­ A soaring aircraft is always sinking relative to tions are adequate for the pilot of a powered air­ the air. To maintain or gain altitude, therefore, the craft; but to the soaring pilot, they are only a soaring pilot must spend sufficient time in thermals beginning. to overcome the normal sink of the aircraft as well All pilots scan the weather pattern for convective as to regain altitude lost in downdrafts. He usually activity. Remember that turbulence is proportional circles at a slow airspeed in a thermal and then to the speed at which the aircraft penetrates ad- darts on a beeline to the next thermal as shown in figure 147. *Peter L. Dixon. SOARING, page 129; 1970; Ballantine Low-level heating IS prerequisite to thermals; Books, New York City. and this heating is mostly from the sun, although FIGURE 147. Thermals generally occur over a small portion of an area while downdrafts predominate. Updrafts in the thermals usually are considerably stronger than the downdrafts. Sailplane pilots gain altitude in thermals and hold al­ titude loss in downdrafts to a minimum. 172 it may be augmented by man-made heat sources toward the western horizon. For example, if a such as chimneys, factories, and cities. Cool air rocky knob protrudes above a grassy plain, the must sink to force the warm air upward in ther­ most likely area of thermals is over the eastern mals. Therefore, in small-scale convection, ther­ slope in the forenoon and the western slope in the mals and downdrafts coexist side by side. The net afternoon. Once a pilot has sighted a likely surface, upward displacement of air must equal the net he may look for other visual cues. downward displacement. Fast rising thermals gen­ erally cover a small percentage of a convective area Dust and Smoke while slower downdrafts predominate over the Surface winds must converge to feed a rising remaining greater portion as diagrammed in figure thermal; so when you sight a likely spot for a 147. thermal, look for dust or smoke movement near the Since thermals depend on solar heating, thermal surface. If you can see dust or smoke "streamers" soaring is restricted virtually to daylight hours with from two or more sources converging on the spot considerable sunshine. Air tends to become stable as shown in figure 148(A), you have chosen wisely. at night due to low-level cooling by terrestrial radia­ If, however, the streamers diverge as shown in tion, often resulting in an inversion at or near the figure 148 (B), a downdraft most likely hovers over surface (see chs. 3 and 6). Stable air suppresses the spot and it's time to move on. convection, and thermals do not form until the Rising columns of smoke from chimneys and inversion "burns off" or lifts sufficiently to allow factories mark thermals augmented by man-made soaring beneath the inversion. The earliest that sources. These rising columns are positive indica­ soaring may begin varies from early forenoon to tion of thermals. They are good sources of lift if early afternoon, the time depending on the strength upward speed is great enough to support the air­ of the inversion and the amount of solar heating. craft and if they are broad enough to permit cir­ Paramount to a pilot's soaring achievement is his cling. Towns or cities may provide thermals; but to skill in diagnosing and locating thermals. use a thermal over a populated area, the pilot must have sufficient altitude to glide clear of the area in LOCATING THERMALS event the thermal subsides. Since convective thermals develop from uneven heating at the surface, the most likely place for a Dust Devils thermal is above a surface that heats readily. Dust devils occur under sunny skies over sandy or dusty, dry surfaces and are sure signs of strong Types of Terrain Surfaces thermals with lots of lift. To tackle this excellent When the sky is cloudless, the soaring pilot must source of lift, you must use caution. The thermals look for those surfaces that heat most rapidly and are strong and turbulent and are surrounded by seek thermals above those areas. Barren sandy or areas of little lift or possibly of sink. rocky surfaces, plowed fields, stubble fields sur­ If approaching the dust devil at too Iowan alti: rounded by green vegetation, cities, factories, chim­ tude, an aircraft may sink to an altitude too low neys, etc., are good thermal sources. A pilot learns for recovery. A recommended procedure is to al­ through experience the most favorable spots in his ways approach the whirling vortex at an altitude local area. But terrain features are only part of the 500 feet or more above the ground. At this altitude, story; time of day influences not only when thermals you have enough airspace for maneuvering in the form but also where. event you get into a downdraft or turbulence too great for comfort. Sun Angle A dust devil may rotate either clockwise or Angle of the sun profoundly affects location of counterclockwise. Before approaching the dusty thermals over hilly landscapes. During early fore­ column, determine its direction of rotation by ob­ noon, the sun strikes eastern slopes more directly serving dust and debris near the surface. Philip than other slopes; therefore, the most favorable Wills* quotes R. H. Swinn, Chief Instructor of the areas for thermals are eastern slopes. The favor­ Egyptian Gliding School, on approaching and en- able areas move to southern slopes during midday. In the afternoon, they move to western slopes be­ *Philip Wills. ON BEING A BIRD, page 79; 1953; fore they begin to weaken as the evening sun sinks Max Parrish and Co., Ltd. 173 r A B PROBABLE THERMAL THERMAL UNLIKELY CONVERGING DIVERGING DUST AND SMOKE DUST AND SMOKE SMOKE DUST SMOKE FIGURE 148. Using surface dust and smoke movement as indications of a thermal. When you have sighted an area which you think will heat rapidly (the red area), look for dust or smoke movement at the surface as an indicator of surface wind. Converging dust or smoke streamers (left) enhance the probability of a thermal. Diverging streamers reduce the likeli­ hood of a thermal. tering a dU'st devil: " . at around 500 feet; the pilot turns towards the dust devil and cuts his speed as he approaches it to the minimum con­ sistent with the control of the glider. As he nears the whirling column of sand he makes a circle on the outside of the dust devil against the direction of rotation, care being taken to give it a wider AGAINST ROTATION berth on the downwind side. In light of the vari­ SLOW SPEED ometer reading on the .initial ·circle, closer contact SHORT TURN is made with the column or a hasty retreat is beat to a safer orbit." FroURE 149.