Centring in thermals for beginners - Dartmoor Gliding Society

Dartmoor Gliding Society Student Notes Ab –initio –in flight Basic Soaring

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

Thermal

D.S.Puttock page 1 of 6 4 March 2010

Dartmoor Gliding Society Student Notes Ab –initio –in flight Basic Soaring

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

Thermal

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

Dartmoor Gliding Society Student Notes Ab –initio –in flight Basic Soaring
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

Thermal

More lift under the port Less sink under starboard
wing---bank increases, wing---bank reduces and
and radius of turn reduces. 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 there 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

Dartmoor Gliding Society Student Notes Ab –initio –in flight Basic Soaring

Fig 4 Glider now climbing
and variometer
Glider starts to indicating a climb
accelerate upwards and
vario starts to indicate
lift

Vario not indicating lift

LIFT

There is a time delay before the variometer indicates a climb

A simple technique to centre in a thermal

The time taken for a complete 360° 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
60°

Fig 5 Best indication
here

Thermal

60°

Strongest lift
here

D.S.Puttock page 4 of 6 4 March 2010

Dartmoor Gliding Society Student Notes Ab –initio –in flight Basic Soaring

With a lag of 60°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
here

Thermal

60° If glider
straightens up
Strongest lift here he may fly
here 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

Dartmoor Gliding Society Student Notes Ab –initio –in flight Basic Soaring

Fig 8 Best indication Continue turn
here

Thermal 60°

Strongest lift
here

Reduce bank here 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