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Natural Thermalling - What is it ?

by Bomber

Have you ever wondered when you hear the likes of Thomas Suchaneck talk about natural climbers like Manfred Rumour and Larry Tudor what he’s talking about ? Ever wondered why an eagle can centre quickly in lift and rise above you within a few turns ?

On a flight back from the eastern states I started to think about what a natural climber would feel when he or she is thermalling that allows them always to be in the best lift or searching for the best lift. One possible answer maybe they are more sensitive to the sensation of acceleration. I have though about this and developed my ideas below. It may be right or wrong, but at the very least its a view on a topic of interest to most of us. I’ll start with a preamble before I discuss my thoughts on the sensation of acceleration.

Preamble

When I’m thermalling I’m always trying to move my circle into the best part of lift. I know for certain that I’m able to move my circle, but I’m guessing that I’m moving my circle to the best part of the lift. This is were I decided to figure what sensations trigger me to move my circle.

Generally, if I’m thermalling and the lift is strong on one side of the circle I’ll think about moving my circle towards the area of greater lift. What I really saying here is when my instruments are chirping loader or I feel that sudden upward burst I’ll move my circle. However, when you start to analysis this I have concluded the following. The vario has a delay of approx. 1 second. This means that I’m hearing the lift I passed through 1 sec ago. Thermalling at 30 Km/h this means the lift I travelled through 8.3 m. An additional delay also experienced by pilot and glider response say 2 seconds. This means a further 16.3 m. This means a total delay of 3 seconds in what could be a 10 second circle. 3 seconds in a 10 second circle translates to an adjustment in my circle of 108 degree from the point in which the best lift actually occurred. As a result I could be moving my circle away from the best lift.

Even I don’t rely on my vario and I instantly feel the increase in lift as I accelerate to the new vertical velocity my pilot and glider delay of 2 seconds could see me past 72 degree past the point of best lift. So what I’m trying to explore is that when I move my circle am I doing it at the right time and for the right reasons.

Acceleration

Now what I’m about to explore may change the way you think about when it is that you are in the best part of lift in a thermal. I’ll use the figures below to illustrate my thoughts - here goes.

Figure 1 below shows the profile of the text book thermal looking at it from above. We generally model the thermal as having a core (the area of greatest upward velocity), in figure 1 this is represented by “E”. The lift in the thermal generally reduces as we move way from the core in any direction. For this example lets assume that this holds true and the thermal is perfectly circular. We’ll assume that at point “A” and “I” the lift drops off to zero.

Imagine a glider flying from point “A” to “I” through the centre of the thermal. Figure 2 shows the expected upward velocity that the pilot would experience. We can expect that the vario response would be similar but subject to the vario’s delay. If we know look at figure 3 this shows the acceleration sensation that the pilot would feel on his flight path from point “A” to “I”.

In figure 3 we can see that the pilot experiences no feeling of upward acceleration at point “A”,”E” or “I”. This is because that these point there is not change in upward velocity.

• As the pilot moves from point “A” to “C” the upward velocity is rapidly increasing and the corresponding acceleration is high. The pilot would feel pressure in their chest as you body tries to accelerate down through their harness.
• As the pilot passes through point “C” towards “E” he is still accelerating but the acceleration is reducing. The pilot still feels pressure on their chest, but its reducing from point “C”.
•   At point “E” the pilot has maximum upward velocity but zero acceleration. The pilot feels no pressure on his chest.
• As the pilot moves from point “E” to point “G” still in good upward lift they feel the sinking feeling that accompanies the downward acceleration. At point “G” the downward acceleration is a maximum - however, the pilot is still in good lift.
• As the pilot moves from point “G” to “I” the downward acceleration reduces.

So what ?

So what does this all mean to a hang glider pilot. Well I for one know that I usually assume I’m in the best lift when I feel the sensation of maximum acceleration. Using the model above this means I’m trying to centre my turn at point “C”. In which I would be constantly falling in and out of the best lift or so it would feel. In the model above the natural climber would be able to centre their circle around point “E”.

So what would we feel like to centre a circle around “E”. Lets image the thermal is large enough to allow you to centre around “E” with a circular path with outer limits of  “C” and “G” as shown in figure 4 below.

As we fly towards the thermal from “A” to “E”, where we want to start our turn , we should experience the following:

• Rapid increase in chest pressure from “A” to “C”, a reducing chest pressure from “C” to “E”, at the point were feel neither lift nor sink (eg pressure on the chest or that weightless feeling is were we want to turn.

• In figure 5 we see our lift profile in terms of upward velocity. Our climb rate increases until we reach a maximum at point “E” and then since we follow the lift profile around the circle described by “C” and “G” our vertical velocity stays constant.

• As we enter better lift we should feel ourselves accelerate to the higher velocity and hence feel the increased