Centring in thermals for beginners Introduction Learning to soar can be a frustrating business for the beginner. It takes practice, which means time in the air, but because you can t soar effectively you are on the ground before you have time to practice a vicious circle. These notes are designed to give you some ideas that will get you started. Take every opportunity to fly with instructors and coaches who will help you get to grips with thermalling. A couple of airmanship points, Look for your thermals up wind of the airfield and don t allow yourself to drift too far downwind. Keep a good lookout, don t get fixated on the task of thermalling and neglect this essential task. Know the rules of the air. So you found a thermal---what next Too often a beginner loses the thermal before he has really started to explore it. Your first task is to keep in contact with it. Clearly if you were to fly an accurate circle, even if half the turn is out of the thermal you would find it again automatically. Fig 1 Fig 1 Glider flies a circle and returns to the lift D.S.Puttock page 1 of 6 4 March 2010
You need to fly accurately, and understand 2 points. The radius of your circle is affected by your bank angle. The steeper the bank angle the smaller the radius of turn. The radius of your circle (for any bank angle) increases with increased speed. If either of these 2 things change, you will not have flown a circular path. Fig 2 Fig 2 Angle of bank reduces (or speed increases) during part of the turn and the glider flies a figure 6. Potentially losing the thermal. Maintaining a steady speed in the turn is a skill you need to master. This is best achieved by maintaining a steady attitude, the ASI will misread and mislead you--- only use it as a secondary reference. Equally maintaining a steady bank angle should be done with reference to the horizon. If the bank is changing in an uncommanded way, you should correct it with co-ordinated stick and rudder. The thermal will change your angle of bank As you fly a circle near the thermal, it will naturally influence the angle of bank. As the angle of bank changes the attitude will tend to change. Increased bank, the nose pitches down and the glider accelerates. D.S.Puttock page 2 of 6 4 March 2010
Decreased bank the radius of turn increases and speed decreases. This in turn will slowly and subtly take you further from the centre of the thermal. (Fig 3) Fig 3 More lift under the port wing---bank increases, and radius of turn reduces. Less sink under starboard wing---bank reduces and radius of turn increases. If the pilot fails to maintain a steady angle of bank and speed, the glider flies a series of ellipses and progressively moves away from the centre of the thermal. The thermal will provide all the influence required to move the glider away from the lift. Finding the centre of the thermal When you are able to fly accurately enough, you now need to move the glider into the centre of the thermal. It is obviously inefficient to fly half in and half out of the thermal. Unfortunately t is a delay between the glider entering an area of lift and the variometer indicating a climb. (Figure 4) D.S.Puttock page 3 of 6 4 March 2010
Fig 4 Glider starts to accelerate upwards and vario starts to indicate lift Glider now climbing and variometer indicating a climb Vario not indicating lift LIFT T is a time delay before the variometer indicates a climb A simple technique to centre in a thermal The time taken for a complete 3 turn in a thermal is about 15 to 20 seconds. The variometer lag is about 3 seconds, which represents about 1/6 of a turn or about Fig 5 Best indication Strongest lift D.S.Puttock page 4 of 6 4 March 2010
With a lag of it is important you don t straighten up when the variometer indicates maximum lift, as by then the glider is already well into the core. Straightening out at this stage may well fly you out of the lift. (Fig 7) Fig 7 Best indication Strongest lift If glider straightens up he may fly away from the centre of lift Instead you should fly a properly banked thermal turn (40 or so) when the variometer indicates maximum lift. Reduce the bank if or when less lift is indicated and increase the bank again when the lift improves. Fig 8 shows a simple technique that works, and although it may be slower to centre than the expert, it will give you more chance to practice and develop your own methods. In summary The variometer does not provide an instantaneous reading, it is telling you history. Fly accurate turns (speed and bank angle) Fly a normal thermal turn at the best vario indication. Reduce the bank slightly on the worst indication. Have fun and Fly safely D.S.Puttock page 5 of 6 4 March 2010
Fig 8 Best indication Continue turn Strongest lift Reduce bank by say 15 ---at worst indication By a series of small changes of bank, the glider will progressively move towards the centre of the thermal. D.S.Puttock page 6 of 6 4 March 2010