Do you find yourself losing a thermal after a few turns and having to reverse direction or go into search mode to find it again? Are there days when you get dumped out of thermals and have to reenter them? Or do you find that other pilots regularly get higher or out climb you? If the answer to any of these questions is yes then maybe you need to work on your thermalling technique. Not only can good thermalling technique reduce the afore mentioned problems, it is very often the only difference between getting up and sinking to the ground. OK, so you have answered yes to one or more of the questions above. Before you sell your equipment and leap from the highest available building lets have a look at some of the common causes and fixes for the problem.
Rule 1: Turn hard to establish yourself well inside the lift.
Upon entering lift it is usually prudent to make a hard aggressive turn. Even in light lift this can help to establish yourself inside the thermal. Once inside it may be necessary to slow down and reduce your bank angle but it is important to stay in the lift. A few seconds of hard maneuvering in lift is far better than the smoothest flying outside the lift.
Rule 2: Strive to fly at the bank angle that gives you the best climb rate. Once established in a thermal experiment by turning tighter. The worst that can happen is your climb rate will decrease and all you will have to do is reduce the bank a little. You may be pleasantly surprised though to find that you will actually be climbing faster.
For years we have been told that many pilots do not turn tightly enough. This is true in the sail plane world as well as in hang gliding. There are still some pilots who insist that turning flat and efficiently is the best way to thermal. While this may be the case at times, most often it is flatly untrue. I won't go into great detail here but thermals are usually stronger in the very center (commonly referred to as the core) and get weaker further out from the center. In theory this means that for each thermal and glider/pilot combination there is usually a precise bank angle that will result in the best climb rate. This bank angle is most often steeper than many if not most pilots fly in thermals. To get a better understanding of the difference in strength within a thermal and the proper bank angle to fly read Dennis Pagen's Performance Flying and/or Helmutt Reichmann's Cross Country Soaring. Both books give examples comparing thermals and glider performance.
Rule 3: Center as quickly and efficiently as possible.
There are a number of ways to center in a thermal but I will describe the one that I prefer. While in lift pay close attention to where the lift increases and decreases. As the lift increases fly a shallower bank angle to extend your flight path farther into the stronger portion of the thermal. When the lift begins to decrease turn tighter to return to the stronger portion. In my experience this is the fastest method for centering. For additional methods read the books mentioned above.
In the diagram on the left we have our pilot hitting lift at about the 7:00 o'clock position of the thermal. As the left wing comes up our pilot starts a turn to the left to enter the thermal. From the entry at point A the lift will continue to increase until its peak at point B. After passing through point B the lift will begin to decrease as our pilot flies away from the center. At this point it is prudent to sharply increase the bank angle to reduce the turn radius in order to get quickly back to the stronger lift. At C the lift will begin to increase again so it is time to reduce the bank to extend the circle into the stronger lift. At D the lift will begin to decrease slightly making it necessary to increase the bank angle again but not as much as our pilot did back at B. A constant rate turn (the inner dashed circle) will keep the glider centered in the best lift at least until the thermal changes, and it will change. Then it will be time to start the centering process again. With some thermals recentering is almost constant.
Thermals are not always perfectly round but by following this centering technique you will stay in the best portion of the thermal even though your turns may be irregular circles. Most of the time just staying in the strongest lift possible will make a large difference.
Rule 4: Recenter as often as needed to stay in the strongest lift.
Listening to pilots talk of thermalling and watching them has convinced me that recentering is the most neglected skill in flying thermals. It is common to see a pilot do a few turns at the same bank angle whether steep or shallow and then flounder around trying to get back to the center of the thermal. Sometimes they even fly completely out of the thermal and lose it. How many times have we heard pilots talking about how well a certain glider does or does not hold its bank angle in a turn. These are symptoms of what I call constant rate turn syndrome. Thermals are constantly changing and in order to utilize them you must change with them. It is rare to be in a thermal very long without having to recenter. The methods are the same as finding the center originally and recentering may be needed after a few turns or in every turn so be ever alert to changes in lift for cues.
Rule 5: Concentrate on the execution of the 4 rules outlined above.
Pete Lehmann suggested that I add this rule. He correctly pointed out to me that even short lapses of concentration while thermalling can result in a poor climb or losing the thermal entirely. The most susceptible time for loss of focus is when recentering is needed. This is a time after you have found the thermal and are in the center climbing. It is easy to think that you are home free and lose that precious concentration. The next thing you know you are out of the lift in search mode. Problem Cause and Remedy You find yourself continually reversing your turns or searching to find and reenter a thermal 1. This can be the result or decreasing the bank as the lift decreases which cause you to fly out of the lift. Use the centering method described above.
2. Another cause is flying constant rate turns. The remedy is the same as #1. You stay in the thermal but you aren't climbing as well as other pilots. This is usually caused by flying steeper constant rate turns. Your bank keeps you in the thermal but not necessarily in the best part of it. You must recenter in the stronger lift. You have a hard time getting into thermals or the first turn ends up being half in and half out of the thermal This may be caused by not turning into lift aggressively enough. Turn harder into lift.
Thermals come in all manner of shapes, sizes, and strengths. Certainly there are some exceptions to technique but I feel that the rules outlined above are a good foundation for exploiting thermals. Good thermalling requires practice, concentration, and more practice so get out there and give it a try.