Wind has important influences not only on thermal structure, but on thermal location as well. Strong winds at the surface and aloft often break up thermals, making them turbulent and difficult or impossible to work at all. Strong shear can break thermals apart and effectively cap their height even though the local sounding indicates that thermals should extend to higher levels. On the other hand, as discussed in Chapter 9, Soaring Weather, moderately strong winds without too much wind shear sometimes organize thermals into long streets, a joyous sight when they lie along a cross-country course line. [Figure 10-4]
In lighter wind conditions, consideration of thermal drift is still important, and search patterns should become “slanted.” For instance, in Cu-filled skies, glider pilots need to search upwind of the cloud to find a thermal. How far upwind depends on the strength of the wind, typical thermal strength on that day, and distance below cloud base (the lower the glider, the further upwind the gliders needs to be). The task can be challenging when you add to this the fact that windspeed does not always increase at a constant rate with height, and/or the possibility that wind direction also can change dramatically with height.
Wind direction and speed at cloud base can be estimated by watching the cloud shadows on the ground. The numerous variables sometimes make it difficult to estimate exactly where a thermal should be. Pay attention to where thermals appear to be located in relation to clouds on a given day, and use this as the search criterion for other clouds on that day. If approaching Cu from the downwind side, expect heavy sink near the cloud. Head for the darkest, best defined part of the cloud base, then continue directly into the wind. Depending on the distance below cloud base, just about the time of passing upwind of the cloud, fly directly into the lift forming the cloud. If approaching the cloud from a crosswind direction (for instance, heading north with westerly winds), try to estimate the thermal location from others encountered that day. If only reduced sink is found, there may be lift nearby, a short leg upwind or downwind may locate the thermal.
Thermals drift with the wind on blue days as well, and similar techniques are required to locate thermals using airborne or ground-based markers. For instance, if heading toward a circling glider but at a thousand feet lower, estimate how much the thermal is tilted in the wind and head for the most likely spot upwind of the circling glider. [Figure 10-5] When in need of a thermal, pilots might consider searching on a line upwind or downwind once abeam the circling glider. This may or may not work; if the thermal is a bubble rather than a column, the pilot may be below the bubble. It is easy to waste height while searching in sink near one spot, rather than leaving and searching for a new thermal. Remember that a house thermal will probably be downwind of its typical spot on a windy day. Only practice and experience enable glider pilots to consistently find good thermals.
Cool, stable air can also drift with the wind. Avoid areas downwind of known stable air, such as large lakes or large irrigated regions. On a day with Cu, stable areas can be indicated by a big blue hole in an otherwise Cu-filled sky. If the area is broad enough, a detour upwind of the stabilizing feature might be in order. [Figure 10-6]
The Big Picture
When the sky is full of Cu, occasional gliders are marking thermals, and dust devils move across the landscape, the sky becomes glider pilot heaven. If gliding in the upper part of the height band, it is best to focus on the Cu, and make choices based on the best clouds. Sometimes lower altitudes cause glider pilots to go out of synch with the cloud. In that circumstance, use the Cu to find areas that appear generally active, but then start focusing more on ground-based indicators, like dust devils, a hillside with sunshine on it, or a circling bird. When down low, accept weaker climbs. Often the thermal cycles again, and hard work is rewarded.
When searching for lift, use the best speed to fly, that is, best L/D speed plus corrections for sink and any wind. This technique allows glider pilots to cover the most ground with the available altitude. See Chapter 3, Aerodynamics of Flight, to review shifting of the polar for winds and sink.
Entering a Thermal
Once a thermal has been located, enter it so you do not lose it right away. The first indicator of a nearby thermal is often, oddly enough, increased sink. Next, a positive G-force is felt, which may be subtle or obvious, depending on the thermal strength. The “seat-of-the-pants” indication of lift is the quickest, and is far faster than any variometer, which has a small lag. Speed should have been increased in the sink adjacent to the thermal; as the positive G-force increases, reduce speed to between L/D and minimum sink. Note the trend of the variometer needle (should be an upswing) or the audio variometer going from the drone to excited beeping. At the right time in the anticipated lift, begin the turn. If everything has gone perfectly, the glider will roll into a coordinated turn, at just the right bank angle, at just the right speed, and be centered perfectly. In reality, it rarely works that well.
Before going further, what vital step was left out of the above scenario? CLEAR BEFORE TURNING! The variometer is hypnotic upon entering lift, especially at somewhat low altitudes. This is exactly where pilots forget that basic primary step before any turn—looking around first. An audio variometer helps avoid this.
To help decide which way to turn, determine which wing tends to be lifted. For instance, when entering the thermal and the glider is gently banking to the right, CLEAR LEFT, then turn left. A glider on its own tends to fly away from thermals. [Figure 10-7] As the glider flies into the first thermal, but slightly off center, the stronger lift in the center of the thermal banks the glider right, away from the thermal. It then encounters the next thermal with the right wing toward the center and is banked away from lift to the left, and so on. Avoid letting thermals bank the glider even slightly. Sometimes the thermal-induced bank is subtle, so be light on the controls and sensitive to the air activity. At other times, there is no indication on one wing or another. In this case, take a guess, CLEAR, then turn. As a note, new soaring pilots often get in the habit of turning in a favorite direction, to the extreme of not being able to fly reasonable circles in the other direction. If this happens, make an effort to thermal in the other direction half the time—being proficient in either direction is important, especially when thermaling with traffic.
As a glider encounters lift on one side, some gliders tend to slip laterally as indicated by the yaw string. The glider pilot must bring the climbing wing down, not just to level the wings but further to begin the turn into the lifting columns of air. In these instances, the soaring pilot should turn towards the tip of the yaw string to seek the lift.