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Thermal Soaring


Introduction to Glider Flying > Soaring Techniques > Thermal Soaring

Soaring flight, maintaining or gaining altitude rather than slowly gliding downward, is the reason most glider pilots take to the sky. After learning to stay aloft for two or more hours at a time, the urge to set off cross country often overcomes the soaring pilot. The goal is the same whether on a cross-country or a local flight— to use available updrafts as efficiently as possible. This involves finding and staying within the strongest part of the updraft. This chapter covers the basic soaring techniques.

In the early 1920s, soaring pilots discovered the ability to remain aloft using updrafts caused by wind deflected by the very hillside from which they had launched. This allowed time aloft to explore the air. Soon afterward, they discovered thermals in the valleys adjacent to the hills. In the 1930s, mountain waves, which were not yet well understood by meteorologists, were discov-ered leading pilots to make the first high altitude flights. Thermals are the most commonly used type of lift for soaring flight, since they can occur over flat ter-rain and in hilly country. Therefore, we will begin with thermal soaring techniques.

As a note, glider pilots refer to rising air as lift. This is not the lift generated by the wings as was discussed in chapter 3—Aerodynamics of Flight. The use of this term may be unfortunate, but in reality it rarely causes confusion when used in the context of updrafts. This chapter refers to lift as the rising air within an updraft and sink as the descending air in downdrafts.


When locating and utilizing thermals for soaring flight, called thermalling, glider pilots must constantly be aware of any nearby lift indicators. Successful ther-malling requires several steps: locating the thermal, entering the thermal, centering the thermal, and finally leaving the thermal. Keep in mind that every thermal is unique in terms of size, shape, and strength.

In the last chapter, we learned that if the air is moist enough and thermals rise high enough, cumulus clouds, or Cu (pronounced ‘q’) form. Glider pilots seek Cu in their developing stage, while the cloud is still being built by a thermal underneath it. The base of the Cu should be sharp and well defined. Clouds that have a fuzzy appearance are likely well past their prime and will probably have little lift left or even sink as the cloud dissipates. [Figure 10-1]

Figure 10-1. Photographs of (A) mature cumulus likely producing good lift, and (B) dissipating cumulus.

Judging which clouds have the best chance for a good thermal takes practice. On any given day, the lifetime of an individual Cu can differ from previous days, so it becomes important to observe their lifecycle on a par-ticular day. Agood looking Cu may already be dissipat-ing by the time you reach it. Soaring pilots refer to such Cu as rapid or quick cycling, meaning they form, mature, and dissipate in a short time. The lifetime of Cu often varies during a given day as well; quick cycling Cu early in the day will often become well formed and longer lived as the day develops.

Sometimes Cu cover enough of the sky that seeing the cloud tops becomes difficult. Hence, glider pilots should learn to read the bases of Cu. Generally, a dark area under the cloud base indicates a deeper cloud; therefore, a higher likelihood of a thermal underneath. Also, several thermals can feed one cloud, and it is often well worth the deviation to those darker areas under the cloud. At times, an otherwise flat cloud base under an individual Cu has wisps or tendrils of cloud hanging down from it, producing a particularly active area. Cloud hanging below the general base of a Cu indicate that that air is more moist, and hence more buoyant. Note the importance of distinguishing fea-tures under Cu that indicate potential lift from virga. Virga is rain or snow from the cloud base that is not yet reaching the ground and often signals that the friendly Cu has grown to cumulus congestus or thunderstorms. [Figure 10-2] Another indicator that one area of Cu may provide better lift is a concave region under an otherwise flat cloud base. This indicates air that is especially warm, and hence more buoyant, which means stronger lift. This can cause problems for the unwary pilot, since the lift near cloud base often dra-matically increases, for instance from 400 to 1,000 (fpm). When trying to leave the strong lift in the con-cave area under the cloud, pilots can find themselves climbing rapidly with cloud all around—another good reason to abide by required cloud clearances.

Figure 10-2. Photographs of (A) cumulus congestus, (B) cumulonimbus (Cb), (C) virga.

After a thermal rises from the surface and reaches the Convective Condensation Level (CCL), a cloud begins to form. At first, only a few wisps form. Then the cloud grows to a cauliflower shape. The initial wisps of Cu in an otherwise blue (cloudless) sky indi-cate where an active thermal is beginning to build a cloud. When crossing a blue hole (a region anywhere from a few miles to several dozen miles of cloud-free sky in an otherwise Cu-filled sky), diverting to an ini-tial wisp of Cu is often worthwhile. On some days, when only a few thermals are reaching the CCL, the initial wisps may be the only cloud markers around. The trick is to get to the wisp when it first forms, in order to catch the thermal underneath.

Lack of Cu does not necessarily mean lack of thermals. If the air aloft is cool enough and the surface tempera-ture warms sufficiently, thermals will form whether or not enough moisture exists for cumulus formation. These blue or dry thermals, as they are called, can be just as strong as their Cu-topped counterparts. Glider pilots can find blue thermals, without Cu markers, by gliding along until stumbling upon a thermal. With any luck, other blue thermal indicators exist, making the search less random.

One indicator of a thermal is another circling glider. Often the glint of the sun on wings is all you will see, so finding other gliders thermalling requires keeping a good lookout, which glider pilots should be doing any-way. Circling birds are also good indicators of thermal activity. Thermals tend to transport various aerosols, such as dust, upward with them. When a thermal rises to an inversion it disturbs the stable air above it and spreads out horizontally, thus depositing some of the aerosols at that level. Depending on the sun angle and the pilot’s sunglasses, haze domes can indicate dry thermals. If the air contains enough moisture, haze domes often form just before the first wisp of Cu.

On blue, cloudless days, gliders and other airborne indicators are not around to mark thermals. In such cases, you must pay attention to clues on the ground. First, think about your previous flight experiences. It is worth noting where thermals have been found previ-ously since certain areas tend to be consistent thermal sources. Remember that weather is fickle, so there is never a guarantee that a thermal will exist in the same place. In addition, if a thermal has recently formed, it will take time for the sun to reheat the area before the next thermal is triggered. Glider pilots new to a soaring location should ask the local pilots about favored spots—doing so might save the cost of a tow. Glider pilots talk about house thermals, which are simply thermals that seem to form over and over in the same spot or in the same area.

Stay alert for other indicators, as well. In drier climates, dust devils mark thermals triggering from the ground. In hilly or mountainous terrain, look for sun-facing slopes. Unless the sun is directly overhead, the heating of a sun-facing slope is more intense than that over adjacent flat terrain because the sun’s radiation strikes the slope at more nearly right angles.[Figure 10-3] Also, cooler air usually pools in low-lying areas overnight; therefore, it takes longer to heat up during the morning. Finally, slopes often tend to be drier than surrounding lowlands, and hence tend to heat better. Given the choice, it usually pays to look to the hills for thermals first.

Figure 10-3. Sun’s rays are concentrated in a smaller area on a hillside than on adjacent flat ground.

Whether soaring over flat or hilly terrain, some experts suggest taking a mental stroll through the landscape to look for thermals. Imagine strolling along the ground where warmer areas would be found. For instance, walking from shade into an open field the air suddenly warms. A town surrounded by green fields will likely heat more than the surrounding farmland. Likewise, a yellowish harvested field will feel warmer than an adja-cent wet field with lush green vegetation. Wet areas tend to use the sun’s radiation to evaporate the mois-ture rather than heat the ground. Thus, a field with a rocky outcrop might produce better thermals. Rocky outcrops along a snowy slope will heat much more effi-ciently than surrounding snowfields. Though this tech-nique works better when at lower altitudes, it can also be of use at higher altitudes in the sense of avoiding cool-looking areas, such as a valley with many lakes.

Wind also 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, mak-ing them turbulent and difficult or impossible to work at all. Strong shear can break thermals apart and effec-tively 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 will sometimes organize thermals into longstreets, a joyous sight when they lie along a cross-oun-try course line. [Figure 10-4]

Figure 10-4. Photograph of cloud streets.

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 dis-tance below cloud base (the lower the glider, the fur-ther upwind the gliders needs to be). Add to this the fact that wind speed does not always increase at a con-stant rate with height, and/or the possibility that wind direction also can change dramatically with height, and the task can be challenging.

Wind speed and direction at cloud base can be esti-mated by watching the cloud shadows on the ground. With all the variables, it is sometimes difficult to esti-mate 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 crite-ria 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 right into the lift form-ing the cloud. If approaching the cloud from a cross-wind 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, so a short leg upwind or downwind may locate the thermal.

Of course, thermals drift with the wind on blue days as well, and similar techniques are required to locate ther-mals 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 likely be downwind of its typical spot on a windy day. Only practice and experience enable glider pilots to consistently find good thermals.

Figure 10-5. Thermal tilt in shear that (a) does not change with height, and that (b) increases with height.

As a note, 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]

Figure 10-6. Blue hole in a field of cumulus downwind of a lake.

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 glid-ing 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 will cause glider pilots to go out of synch with the cloud. In that circum-stance, 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 day 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 amount of ground with the available altitude.

Once a thermal has been located, enter it properly, so as not to lose it right away. The first indicator of a nearby thermal is often, oddly enough, increased sink. Next a positive G-force will be 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, hence 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, and at the right time in the antici-pated lift, begin the turn. If everything has gone per-fectly, the glider will roll into a coordinated turn, at just the right bank angle, at just the right speed, and be per-fectly 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 var-iometer 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 is trying 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 direc-tion. If this happens, make an effort to thermal in the other direction half the time—being proficient in either direction is important, especially when thermalling with traffic.

Figure 10-7. Effect of glider being allowed to bank on its own when encountering thermals.

Optimum climb is achieved when proper bank angle and speed are used after entering a thermal. The shal-lowest possible bank angle at minimum sink speed is ideal. Thermal size and associated turbulence usually do not allow this. Large-size, smooth, and well-behaved thermals can be the exception in some parts of the country. Consider first the bank angle. The glider’s sink rate increases as the bank angle increases. However, the sink rate begins to increase more rapidly beyond about a 45?bank angle. Thus, a 40?to a 30? bank angle may increase the sink rate less thanthe gain achieved from circling in the stronger lift near the center of the thermal. As with everything else, this takes practice, and the exact bank angle used will depend on the typical thermal, or even a specific ther-mal, on a given day. Normally bank angles in excess of 50?are not needed, but exceptions always exist. It maybe necessary, for instance, to use banks of 60?or so to stay in the best lift. Thermals tend to be smaller at lower levels and expand in size as they rise higher. Therefore, a steeper bank angle is required at lower altitudes, and shallower bank angles can often be used while climbing higher. Remain flexible with techniques throughout the flight.

If turbulence is light and the thermal is well-formed, use the minimum sink speed for the given bank angle. This should optimize the climb because the glider’s sink rate is at its lowest, and the turn radius is smaller. As an example, for a 30?bank angle, letting the speed increase from 45 to 50 knots increases the diameter of the circle by about 100 feet. In some instances, this can make the difference between climbing or not. Some gliders can be safely flown several knots below mini-mum sink speed. Even though the turn radius is smaller, the increased sink rate may offset any gain achieved by being closer to strong lift near the thermal center.

There are two other reasons to avoid thermalling speeds that are too slow: the risk of a stall and lack of controllability. Distractions while thermalling can increase the risk of an inadvertent stall and include, but are not limited to: studying the cloud above or the ground below (for wind drift, etc.), quickly changing bank angles without remaining coordinated while cen-tering, thermal turbulence, or other gliders in the ther-mal. Stall recovery should be second nature, so that if the signs of an imminent stall appear while thermalling, recovery is instinctive. Depending on the stall charac-teristics of the particular glider or in turbulent thermals, a spin entry is always possible. Glider pilots should carefully monitor speed and nose attitude at lower alti-tudes. Regardless of altitude, when in a thermal with other gliders below, maintain increased awareness of speed control and avoid any stall/spin scenario. Controllability is a second, though related, reason for using a thermalling speed greater than minimum sink. The bank angle may justify a slow speed, but turbu-lence in the thermal may make it difficult or impossible to maintain the desired quick responsiveness, espe-cially in aileron control, in order to properly remain in the best lift. Using sufficient speed will ensure that the pilot, and not the thermal turbulence, is controlling the glider.

Soaring pilots’ opinions differ regarding how long to wait after encountering lift and before rolling into the thermal. Some pilots advocate flying straight until the lift has peaked. Then, they start turning, hopefully back into stronger lift. It is imperative not to wait too long after the first indication that the thermal is decreasing for this maneuver. Other pilots favor rolling into the thermal before lift peaks, thus avoiding the possibility of losing the thermal by waiting too long. Turning into the lift too quickly will cause the glider to fly back out into sink. There is no one right way; the choice depends on personal preference and the conditions on a given day. Timing is everything and practice is key.

Usually upon entering a thermal, the glider is in lift for part of the circle and sink for the other part. It is rare to roll into a thermal and immediately be perfectly cen-tered. The goal of centering the thermal is to determine where the best lift is and move the glider into it for the most consistent climb. One centering technique is known as the “270?correction.” [Figure10-8] In this case, the pilot rolls into a thermal and almost immedi-ately encounters sink, an indication of turning the wrong way. Complete a 270turn, straighten out for a few seconds, and if lift is encountered again, turn back into it in the same direction. Avoid reversing the direc-tion of turn. The distance flown while reversing turns is more than seems possible and can lead away from the lift completely. [Figure 10-9]

Figure 10-8. The 270?centering correction.

Figure 10-9. Possible loss of thermal while trying to reverse directions of circle.

Often stronger lift exists on one side of the thermal than on the other, or perhaps the thermal is small enough that lift exists on one side and sink on the other, thereby preventing a climb. There are several techniques and variations to centering. One method involves paying close attention to where the thermal is strongest, for instance, toward the northeast or toward some featureon the ground. To help judge this, note what is under the high wing when in the best lift. On the next turn, adjust the circle by either straightening or shallowing the turn toward the stronger lift. Anticipate things a bit and begin rolling out about 30?before actually heading towards the strongest part. This allows rolling back toward the strongest part of the thermal rather than fly-ing through the strongest lift and again turning away from the thermal center. Gusts within the thermal can cause airspeed indicator variations; therefore, avoid “chasing the ASI.” Paying attention to the nose attitude helps pilots keep their focus outside the cockpit. How long a glider remains shallow or straight depends on the size of the thermal. [Figure 10-10]

Figure 10-10. Centering by shifting the circle turn toward stronger lift.

Other variations include the following. [Figure 10-11]

Figure 10-11. Other centering corrections.

1. Shallow the turn slightly (by maybe 5?????? ?????) when encountering the weaker lift, then as stronger lift is encountered again (feel the posi-tive g, variometer swings up, audio variometer starts to beep) resume the original bank angle. If shallowing the turn too much, it is possible to fly completely away from the lift.

2. Straighten or shallow the turn for a few seconds 60?after encountering the weakest lifts or worst sink indicated by the variometer. This allows for the lag in the variometer since the actual worst sink occurred a couple of seconds earlier than indicated. Resume the original bank angle.

3. Straighten or shallow the turn for a few seconds when the stronger seat-of-the-pants surge is felt. Then resume the original bank. Verify with the variometer trend (needle or audio). For the new glider pilots, it is best to become profi-cient using one of the above methods first, and thenexperiment with other methods. As an additional note, thermals often deviate markedly from the conceptual model of concentric gradients of lift increasing evenly toward the center. For instance, it sometimes feels as if two (or more) nearby thermal centers exist, making centering difficult. Glider pilots must be willing to constantly adjust, and re-center the thermal to maintain the best climb.

In addition to helping pilots locate lift, other gliders can help pilots center a thermal as well. If a nearby glider seems to be climbing better, adjust the turn to fly within the same circle. Similarly, if a bird is soaring close by, it is usually worth turning toward the soaring bird. Along with the thrill of soaring with a hawk or eagle, it usually leads to a better climb. Collision avoidance is of primary importance when thermalling with other gliders. The first rule calls for all gliders in a particular thermal to circle in the same direction. The first glider in a thermal establishes the direction of turn and all other gliders joining the ther-mal should turn in the same direction. Ideally, two glid-ers in a thermal at the same height or nearly so should position themselves across from each other so they can best maintain visual contact. [Figure 10-12] When entering a thermal, strive to do so in a way that will not interfere with gliders already in the thermal, and above all, in a manner that will not cause a hazard to other gliders. An example, of a dangerous entry, is pulling up to bleed off excess speed in the middle of a crowded thermal. A far safer technique is to bleed off speed before reaching the thermal and joining the thermal at a “normal” thermalling speed. Collision avoidance, not optimum aerodynamic efficiency, is the priority when thermalling with other gliders. Announcing to the otherglider(s) on the radio when entering the thermal enhances collision avoidance. [Figure 10-12]

Figure 10-12. Proper positioning with two gliders at the same altitude. Numbers represent each glider’s position at that time.

Different types of gliders in the same thermal may have different minimum sink speeds, and it may be difficult to remain directly across from another glider in a ther-mal. Avoid putting yourself in a situation where you cannot see the other glider, or the other glider cannot see you. Radio communication is helpful. Too much talking clogs the frequency, and may make it impossi-ble for a pilot to broadcast an important message. Do not fly directly above or below another glider in a ther-mal since differences in performance, or even minor changes in speed can lead to larger than expected alti-tude changes. If you lose sight of another glider in a thermal and cannot establish position via a radio call, leave the thermal. After 10 or 20 seconds, come back around to rejoin the thermal, hopefully with better traf- fic positioning. It cannot be stressed enough that colli-sion avoidance when thermalling is a priority! Mid-air collisions can sometimes be survived but only with a great deal of luck. Unsafe thermalling practices not only endangers your own safety but that of your fellow
glider pilots. [Figure 10-13]

Figure 10-13. When thermalling, avoid flying in another glider’s blind spot, or directly above or below another glider.

Leaving a thermal properly can also save you some altitude. While circling, scan the full 360?of sky with each thermalling turn. This first allows the pilot to con-tinually check for other traffic in the vicinity. Second, it helps the pilot analyze the sky in all directions in order to decide where to go for the next climb. It is better to decide where to go next while still in lift rather than losing altitude in sink after leaving a thermal. Exactly when to leave depends on the goals for the climb— whether the desire is to maximize altitude for a long glide or leave when lift weakens in order to maximize time on a cross-country flight. In either case, be ready to increase speed to penetrate the sink often found on the edge of the thermal, and leave the thermal in a man-ner that will not hinder or endanger other gliders.

The preceding pages describe techniques for locating thermals, as well as entering, centering, and leaving thermals. Exceptions to normal or typical thermals are numerous. For instance, instead of stronger sink at the edge of a thermal, weak lift sometimes continues for a distance after leaving a thermal. Glider pilots should be quick to adapt to whatever the air has to offer at the time. Just as the mechanics of simply flying the glider become second nature with practice, so do thermalling techniques. Expect to land early because anticipated lift was not there on occasion—it is part of the learning curve. If thermal waves are suspected, climb in the thermal near cloud base, then head toward the upwind side of the Cu. Often, only very weak lift, barely enough to climb at all, is found in smooth air upwind of the cloud. Once above cloud base and upwind of the Cu, climb rates of a few hundred fpm can be found. Climbs can be made by flying back and forth upwind of an individ-ual Cu, or by flying along cloud streets if they exist. If no clouds are present, but waves are suspected, climb to the top of the thermal and penetrate upwind in search of smooth, weak lift. Without visual clues, thermal waves are more difficult to work. Thermal waves are most often stumbled upon as a pleasant surprise when their presence is furthest from the pilot’s mind.

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