Soaring Pilot's Manual E-Book

Ken Stewart

• Herausgeber: Crowood
• Kategorie: Fachliteratur
• Sprache: Englisch

Veröffentlichungsjahr: 2014

Beschreibung

The Soaring Pilot's Manual advances the reader from elementary flying to confident soaring by clearly and precisely explaining the basic soaring mechanisms and techniques. Explanatory diagrams illustrate the text throughout, making a complicated subject simple to understand. Having covered the first steps, the book progresses to cross-country flying and the final section contains exercises that will be found useful for any glider pilot wishing to improve his ability and qualifications. The latest technology, such as GPS navigation and instrument systems is covered. Well illustrated with over 300 black & white line drawings.

Leseprobe

The SoaringPilot’s Manual

SECOND EDITION

Ken Stewart

Illustrated by

Mark Taylor

Airlife

First published in 2000 byAirlife Publishing, an imprint ofThe Crowood Press LtdRamsbury, MarlboroughWiltshire SN8 2HR

www.crowood.com

This e-book first published in 2014

© Ken Stewart 2000 and 2008Illustrations © 2000 Mark Taylor

All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopy, recording, or any information storage and retrieval system, without permission in writing from the publishers.

British Library Cataloguing-in-Publication DataA catalogue record for this book is available from the British Library.

ISBN 978 1 84797 926 1

DisclaimerThe information contained in this book is true and complete to the best of our knowledge. All recommendations are made without any guarantee on the part of the Publisher, who also disclaims any liability incurred in connection with the use of this data or specific details.

Acknowledgements

Any book on soaring has to delve, to a large degree, into meteorology. I would like to thank Tom Bradbury for his support, the time he has given to checking the text of this book, and the many suggestions he has made. Without his specialist knowledge, I am certain that the contents would not be as comprehensive and that their accuracy would have suffered.

Also, I would like to thank Peter Disdale and Diana Bartlett who spent many hours checking and correcting the text, and the many others who have applied their expertise to specific sections.

I am indebted to John Williamson for allowing me to use the JSW Final Glide Calculator in illustrations.

Lastly, I would like to thank all those who, over the years, have passed on their hard-gained knowledge to improve others. Without them, this book would never have been written.

Ken Stewart

Units Used

Let us start with a short discussion on the units used in gliding and throughout this book. If there is any one item that is designed to confuse someone beginning gliding, it is the continual hopping between the multitudes of different units used by glider pilots in a way that would be pounced upon by any decent mathematics teacher.

So, in an attempt to clarify why metric, statute, and even nautical units are often used in the same sentence, here are some reasons.

In the UK and several other countries, the instruments in gliders are calibrated to indicate airspeed and rate of climb (or descent) in nautical miles per hour (knots). Altimeters are calibrated in feet. Fortunately, one knot is equal to almost exactly 100 feet per minute and so rate of climb and gain in altitude are easily cross-checked.

Glider instruments in continental European countries are calibrated in metric units (kilometres per hour for airspeed, metres per second for rate of climb, and metres for altitude).

However, as gliding awards and records are recognised worldwide, they have been standardised by the Fédération Aéronautique Internationale using metric units.

Therefore, it is not uncommon to hear a British glider pilot say something like, ‘I was 20 miles from home on the last leg of a 300 kilometre flight when I found a 6 knot thermal which took me to 6,000 feet and I was then able to glide home at 90 knots’.

Throughout this book, I will attempt to standardise as follows:

*

Distances for navigational purposes will be quoted in nautical miles (nm)

*

Airspeeds will be given in knots (kts)

*

Rates of climb or descent will be given in knots

*

Heights and altitudes will be given in feet

*

Only the length of tasks, average speeds achieved, and heights for international or nationally recognised awards or records will be given in metric units.

I hope the reader will appreciate that these eccentricities are not of the author’s choosing but that he promises to try to keep the various international standards separated as far as possible in the text.

Preface

Having gained the judgement and learned the skills that are necessary to fly a glider safely, most glider pilots wish to progress to become soaring pilots. This book assumes that these basic skills have been mastered, and covers the knowledge and the many different skills that must be gained if a pilot is to be successful at soaring.

Gliding has developed into a sport with a large number of levels of achievement and many different goals. For instance, many pilots are content just to soar locally within range of their own airfield, while others favour cross-country flying, measuring their achievement by the distance covered. Many prefer cross-country speed tasks, flown either individually or against others in competitions. Some are more satisfied by high-altitude flying. Because of all this variety, this book has been written in an attempt to cover all the known variations of soaring. It starts with basic soaring, continues through the various levels, and attempts to cover most of the avenues of soaring that are practised today.

Over the years, the basic training a glider pilot receives has become well-structured and well-organised. Unfortunately, after a pilot’s first solo flight, the learning of soaring techniques is often left to the individual. This means that each new soaring pilot has to negotiate all of the pitfalls that more experienced pilots negotiated previously. Ideally, their experience would be passed on by some form of training – but alas, it seldom is. This manual attempts to offer some of the information that the author has learned, either from his own experiences or from the many other experienced pilots who have been willing to share the secrets of their success.

Some parts of this book refer to weather patterns and phenomena. It would be an impossible task to detail the weather aspects in all of the countries where soaring takes place. This has led to a more localised emphasis, dealing mainly with British soaring weather, with references to conditions in other countries. However, most of the general aspects will be pertinent in many countries.

Contents

Section 1 – Basic Soaring

  1   The Principle of Soaring

  2   Thermals

  3   Thermal Soaring

  4   Hill Lift

  5   Hill Soaring

  6   Lee Waves

  7   Wave Soaring

  8   Sea Breeze Fronts

Section 2 – Cross-Country Soaring

  9   Task Selection

10   Preparation for Flight

11   Cross-Country in Thermals

12   Speed Flying

13   Dolphin Flying

14   Water Ballast

15   Cross-Country in Wave

16   Navigation

17   Turning Points

18   The Final Glide

19   Landing Out

Section 3 – Personal Improvement

20   Personal Improvement

21   Badge Flying

Appendix 1 – Speed-to-Fly Ring Construction

Appendix 2 – Compass Swinging

Appendix 3 – Motor Gliders and Turbo Gliders

Appendix 4 – Useful Addresses

Appendix 5 – Conversion Factors

End Note

Index

Section 1

Basic Soaring

Chapter 1

The Principle of Soaring

The wing of an aircraft, be it an aeroplane or a glider, must move through the air if it is to produce the lift force which is necessary to balance the aircraft’s weight. On a powered aircraft, the engine can produce thrust to move the aircraft through the air, thus causing airflow past the wing. This airflow past the wing creates the lift force that allows an aeroplane to maintain level flight.

Fig 1.1 Forces in balance. In an aeroplane, lift normally balances weight, and thrust normally balances drag.

A glider, on the other hand, not having the luxury of an engine, relies on the force of gravity to propel it forward through the air. A suitable analogy is a ball rolling down a gentle slope. The ball’s forward motion is due to gravity pulling it downward. In fact, if it were not for the surface of the slope, the ball would fall vertically under the influence of gravity. Similarly, gravity would make a glider descend vertically if it were not for the lift force produced by the wing, which, being tilted forward, produces the ‘slope’ down which the glider flies (figure 1.2).

If a glider tries to fly level, then the lift force will no longer be tilted forward, giving no forward component to propel the glider through the air. It will lose airspeed and the reduced airflow past the wing will cause a reduction in the lift force, eventually resulting in the wing stalling.

Fig 1.2 Lift and weight versus drag. The resultant of lift and weight ‘propels’ a glider forward.

Therefore a glider must continually fly ‘down a slope’ to maintain flight. Put another way, it must always descend relative to the air mass in which it is flying.

If the air mass has no updraughts or downdraughts, a glider is said to be flying in STILL AIR. The rate at which a glider descends in still air is dependent on its airspeed – in normal flight, the greater its airspeed, the greater its rate of descent through the air.

As a glider will always be descending relative to the air in which it is flying, in order to maintain height it must be flown in air which is rising at a rate equal to its still air rate of descent. For instance, if a glider is flying in still air at a steady airspeed and is descending at one knot (100 feet per minute), then it will have to encounter an area of air which is ascending at a rate of one knot just to arrest its descent. This would be similar to your walking down an ascending escalator at the same rate as it ascends.

Fig 1.3 Rising air arresting a glider’s descent. A glider will constantly descend unless it is flown in air that is rising at a rate equal to its still air descent rate. This is similar to walking down an escalator at the same rate as it ascends.

To gain height, the air mass in which the glider is flying will need to be ascending at a rate greater than the glider’s rate of descent in still air – that is, in this example, greater than one knot. (Now the escalator has increased its ascent rate.)

Fig 1.4 Rising air causing a glider to gain height. Air rising at a rate greater than a glider’s still air descent rate will result in an increase in the glider’s height. This is similar to walking down an escalator which is ascending faster than your walking speed.

Successful soaring depends on your finding and using such areas of rising air. The first task, finding a suitable updraught, can be challenging enough, depending on the nature of the up-current. Often both experienced and inexperienced glider pilots will fortuitously stumble into an area of rising air. This is often the way in which many pilots achieve their first soaring flights. However, becoming a good soaring pilot depends on learning to consistently seek out rising air. Once a suitable up-current is found, you will need to be able to fly accurately in order to climb efficiently. This will require many of the handling skills that you have gained during your basic training.

NOTE: Often when discussing rising air currents, glider pilots refer to these simply as LIFT. This expression, which will be used extensively throughout the text, should not be confused with the aerodynamic force called ‘lift’, which will be used to a much lesser extent. The context in which the word is used will, hopefully, make its meaning obvious. The expression that is commonly used to describe descending air is SINK.

Various atmospheric phenomena cause air to rise in quantities large enough and at a fast enough rate to keep a glider airborne. The main causes of this lift are:

*

THERMALS, which are parcels of warm air which rise much as a hot air balloon rises

*

HILL LIFT, where the wind is deflected upwards when it meets the face of a hill or mountain

*

MOUNTAIN LEE WAVES, which are caused by the deflection of the air mass after it has flowed over a line of hills or mountains

*

CONVERGENCE LIFT, where two air masses meet, causing air to be forced upwards

All of these are discussed in the chapters that follow.

Chapter 2

Thermals

The earth’s atmosphere

The earth is surrounded by an envelope of gases, which we call the atmosphere. We live in that atmosphere. Not only do we live in it, we live under it. By living on the earth’s surface, virtually all of the atmosphere is above us. The atmosphere is retained around the earth by the pull of the earth’s gravity, which gives weight to the mass of air aloft.

As an object ascends through the atmosphere, the pressure acting on it reduces. Most people who have ascended or descended quickly in an aircraft have sensed this change of pressure on their eardrums. If the object has no firm sides (such as a child’s balloon or a bubble of air), when it is relieved of some atmospheric pressure, it will expand. When air expands it cools. When air descends, it is compressed and as a result warms up. This latter fact can be experienced when using a bicycle pump – compressing the air heats up the pump barrel.

At first, these facts may seem academic, but as we go on to discuss thermals, and indeed air movement in general, remembering them will help you understand the reason why the air behaves as it does.

The environmental lapse rate

The sun emits large amounts of energy, some of which reaches the earth. Some of this energy is scattered and filtered by the atmosphere, but much of it reaches the earth’s surface and causes it to warm up. The energy that passes through the atmosphere does not heat the air directly, as it is in the form of short-wavelength radiation, which is not absorbed by the air. However, when it reaches the earth’s surface, this energy is absorbed by the land and seas, and surface heating occurs. In a sense, the atmosphere acts like the glass of a greenhouse, in that the glass itself does not get warm, but the non-transparent contents of a greenhouse do.

When the earth’s surface warms up, it re-emits some of this energy as longer-wavelength radiation, and much of this is absorbed by the atmosphere. The result is that the air close to the surface is heated due to its contact with the warm ground, and not directly by the sun.

Fig 2.1 Solar heating and the environmental lapse rate. The sun heats the surface of the ground without significantly heating the atmosphere. The warm ground then heats the air close to the ground. Therefore, the temperature of the atmosphere generally reduces as altitude increases.

Therefore, air temperature decreases as altitude is increased. With minor local variations, this general rule holds good for altitudes up to around 36,000 feet. The rate at which the temperature decreases with altitude on any one day is known as the ENVIRONMENTAL LAPSE RATE (ELR).

The ELR will not only vary from day to day but will also vary as altitude increases. In fact, over certain altitude bands the temperature may stop decreasing with height, giving what is called an ISOTHERMAL LAYER. Often, the temperature trend may reverse and increase with altitude for a time. This increase of temperature with height is called a TEMPERATURE INVERSION, or simply an INVERSION.

Fig 2.2 The environmental lapse rate. Air temperature normally decreases as altitude increases.

Fig 2.3 An inversion is where air temperature increases as altitude increases.

How thermals form

When the sun’s energy strikes the earth, different surfaces will heat up at different rates, as will the air coming into contact with them. This differential heating will mean that adjacent areas of air may vary in temperature by several degrees.

As the temperature of a parcel of air increases, it becomes lighter and will want to rise. If the temperature difference between this parcel of air and the air surrounding it is great enough, then, should it become dislodged from the surface, it will ascend. This rising parcel of air is what is known as a THERMAL. As it ascends, the atmospheric pressure upon it decreases, allowing it to expand. As it expands, it cools. This cooling occurs at a rate of 3°C per 1,000 feet. This figure is known as the DRY ADIABATIC LAPSE RATE (DALR).

Fig 2.4 Dry adiabatic lapse rate. As a parcel of air rises, it cools at the dry adiabatic lapse rate.

If the DALR is greater than the ELR, then the thermal will eventually reach a height at which its temperature is the same as its surroundings. In theory, when the thermal reaches this height it will stop ascending. (In practice, as the thermal may contain as much as 50,000 tons of air rising at possibly 1,000 feet per minute, its momentum will carry it some height above this temperature equilibrium level. This is one reason why it is not uncommon to find thermals bursting through an inversion, to temporarily leave either a haze dome or a cumulus cloud showing above the inversion.)

Up to this height, this air mass is said to be UNSTABLE. Above this height, STABILITY is said to exist.

When a thermal reaches an inversion, it is entering a layer where the air becomes warmer with height. The rising thermal soon finds itself colder than its environment, loses buoyancy and starts to sink. The inversion acts like a lid, limiting the top of convection.

Fig 2.5 DALR greater than ELR. When the DALR is greater than the ELR, a thermal will eventually reach a height at which its temperature is the same as the surrounding air and it will stop ascending.

Fig 2.6 Inversion stopping thermal ascent. An inversion acts like a lid, stopping further ascent of thermals.

An inversion close to the ground may even prevent the formation of thermals, until the surface heating is great enough to warm the air in contact with the surface to a degree where it is not only more buoyant than its surroundings, but also warm enough to break through the inversion.

Figure 2.7 shows two inversions, one just above the surface and one between 3,000 and 4,000 feet. This is a common situation early in the morning. As the sun’s heating increases the ground temperature, the inversion close to the surface will break down, allowing thermals to start.

Fig 2.7 Surface inversion. Early in the morning, there is often an inversion near the surface as well as one higher up.

Cumulus clouds

If a thermal contains a reasonable amount of water vapour, then the above characteristics change somewhat, often to the advantage of the glider pilot searching for a thermal.

The amount of water vapour that any parcel of air can contain depends on its temperature – the higher the temperature, the more water that can be contained as a vapour. Because the air’s temperature decreases as it ascends, it becomes more saturated, until eventually the parcel of air can no longer contain all of its water as vapour. At this temperature, known as the DEW POINT, the water vapour will condense into water droplets and form cloud.

Fig 2.8 Relative humidity. The amount of water vapour that a parcel of air can hold decreases as its temperature is reduced.

If a parcel of air is still rising when it reaches its dew point, a cloud, known as a CUMULUS CLOUD, will form. These clouds are often called ‘fair weather clouds’ and are recognisable by their ‘cauliflowerlike’ appearance.

The height at which these clouds start to form is known as the CONDENSATION LEVEL. Cumulus clouds mark the position where there is, or perhaps has been, a thermal.

Fig 2.9 Formation of cumulus cloud. When a thermal reaches the condensation level, a cumulus cloud will form.

The likely height of the condensation level, and therefore the base of any cumulus clouds, can be determined by comparing the forecast maximum air temperature at the surface and the dew point temperature. The difference between the two is known as the DEW POINT DEPRESSION. For each degree Celsius difference between these two temperatures, the condensation level rises approximately 400 feet. Therefore, if the maximum temperature forecast is 20°C, and the dew point temperature remains at 10°C (a difference of 10°C), then the base of any cumulus can be expected to reach 4,000 feet. (The maximum temperature expected is normally given on television and radio weather forecasts, while the actual temperature and the dew point temperature are given in airfield reports on VOLMET and ATIS transmissions. Unless some general change in the weather is likely, the dew point will not change by a significant amount during the day.)

The condensing out of the water vapour into cloud releases heat (known as the LATENT HEAT OF CONDENSATION) into our parcel of air, giving it an added boost. This means that the air within the cloud will be capable of ascending faster, and possibly further, than it would have done if it had been too dry for cloud to form.

From the point at which cloud starts to form, the rising air cools at the SATURATED ADIABATIC LAPSE RATE (SALR) which is only around 1.5°C per 1,000 feet.

Fig 2.10 Saturated adiabatic lapse rate. When cloud forms, the air in the thermal cools at the SALR.

If the ELR is such that air continues to rise within the cloud, the small cumulus cloud can build into a huge, towering CUMULONIMBUS cloud which could result in heavy rain, hail, thunder and lightning.

Fig 2.11 Cumulonimbus cloud. If the air within the cumulus cloud continues to rise, then a large cloud called a cumulonimbus cloud may form.

Alternatively, if the growing cloud reaches an inversion, the cloud’s vertical growth may be halted. If the thermal is still feeding the cloud, the moist air will continue to arrive at the inversion level and, not having enough energy to burst through the inversion, will spread out horizontally. The result may be that the spreading cumulus may join with other cumulus clouds that have suffered the same fate, creating clouds known as STRATOCUMULUS. On some days, this stratocumulus may become widespread and deep enough to stop much of the sun’s energy from reaching the ground. If this happens, thermals may not be as strong (or may even stop developing completely), until some of the stratocumulus evaporates and disappears, thus allowing renewed solar heating of the ground. This cycle may continue for much of the day or as long as the offending inversion exists.

Days when such spreadout is likely are often heralded by cumulus starting much earlier than usual – raising false hopes about the soaring potential of the day. Another sign is the tendency of cumulus clouds to develop rapidly into narrow turrets of cloud instead of growing slowly. If small lenticular caps form at the top of growing cumulus clouds, this is a sign that the air at the level of the inversion is moist and that the formation of stratocumulus is a possibility.

Fig 2.12 Stratocumulus cloud. Stratocumulus cloud may form when an inversion prevents the vertical growth of cumulus clouds.

To summarise, some of the basic rules of the atmosphere, which affect the glider pilot, are listed below.

1.

Air that is warmer than its surroundings will want to rise.

2.

Air that is cooler than its surroundings will descend.

3.

As air rises it cools.

4.

As air descends it is heated.

5.

Air will stop rising when (or shortly after) it cools to the same temperature as the surrounding air.

6.

If the air is moist enough, cloud will form when the air rises to its condensation level.

7.

If the condensation level is below a strong inversion, a layer of stratocumulus cloud may form.

Blue thermals

The fact that on any given day cumulus clouds may not form does not mean that there are no thermals. It may be that the air is too dry for cloud to form. Remember that a cumulus cloud, when present, is a product of the thermal, and not vice versa!

If the air in the thermal does not contain enough water vapour, the thermal may reach the inversion before it cools to its dew point, therefore no cloud will form. On such days, the inversion can often be seen by a layer of haze. This haze layer is formed by dust, which has been carried up by thermals to the level of the inversion.

Fig 2.13 Blue thermals. If an inversion exists below the condensation level, thermals may stop rising before they cool enough to form cloud.

These DRY THERMALS are known as BLUE THERMALS and the days when they occur are called BLUE DAYS.

Often, a day which started off with cumulus cloud, will see a reduction of the amount and depth of such cloud and, as the day progresses, cumulus cloud may disappear completely. This is often the result of the surface temperature increasing during the day, to the extent that when a thermal breaks away from the ground it is warm enough to reach the inversion before it cools to its dew point. When this situation occurs no cloud will form.

A similar situation occurs when an area of HIGH PRESSURE (known as an ANTICYCLONE, or RIDGE) is approaching the region. Anticyclones tend to lower the inversion. If the inversion becomes lower than the condensation level, then thermals may not go high enough to cool to their dew point.

The opposite situation may occur on some days. Late in a day that has had only blue thermals, small cumulus clouds may start to form. This occasionally occurs on hot days and is a result of a gradual lifting of the inversion to a height above the condensation level. This lifting of the inversion is a result of thermals reaching and pushing into the inversion, causing mixing of the warmer air above the inversion with the air below the inversion. This increases the temperature of the air below the inversion, and eventually the height of the inversion itself.

Fig 2.14 Inversion level rising. As the day progresses, the mixing of air as thermals push into the inversion may cause its level to rise.

Not all thermals require a large volume of air to be heated by a ‘hot spot’ on the ground. Any air that is warmed enough to become buoyant will rise. Cumulus clouds and cloud streets observed far out over a sea or ocean are an example of streams of rising air that develop above a surface of uniform temperature. Such rising air is unlikely to keep a glider airborne, except perhaps at cloud base. For this reason, the descriptions of ‘thermals’ in the remainder of this book refer to thermals of a size and strength likely to be usable by a glider pilot – what could be called ‘soarable thermals’.

The source of a thermal

Normally, for a thermal to form, the air must be heated as a result of its contact with the ground. It therefore follows that the warmer the ground, the warmer and more buoyant the air will become. It is also necessary for the air to heat up differentially; that is, for neighbouring areas to heat up at different rates. Fortunately, the surface of the earth varies immensely and so every surface has its own characteristics as a thermal source, ranging from poor to excellent.

Poor thermal sources would include marshy areas, wetland, lakes or generally areas which have a large water content. This is because, as the sun’s energy starts to heat such areas, much of the energy goes into evaporating moisture, or is conducted deeper into the ground or water, rather than warming up the surface. As a result, the surface may never become warm enough to heat the air above it to produce a reasonable thermal. Another problem of wet areas is that there will be a certain amount of reflection as opposed to absorption of the sun’s energy.

Surfaces which tend to be good thermal sources therefore tend to be those which are dry, and are often of a colour which will absorb the sun’s energy more readily. For instance, a bare, brown field will display totally different thermal producing qualities when it is dry from when it is waterlogged.

Another criterion that will affect the quality of a thermal is how well its source area retains the parcel of air next to the warm ground so that heating of the air can occur. The longer the air is stationary over the thermal source, the warmer it will become. For instance, in strong winds, a parcel of air may only be over an area of warm ground for a short time before it is blown past it. Some areas will trap air for considerable periods, allowing it to increase in temperature. Crop fields and areas protected from the wind by hills or buildings may allow the air to remain in contact with the surface just long enough to create a better than average thermal.

Fig 2.15 Wind shadow. Areas sheltered from the wind by a hill will allow air to remain in contact with the warm ground for longer, increasing the chances of a good thermal forming.

No matter how good the thermal source, it still needs the all-important solar heating to raise its temperature. Areas under cloud shadow or in the shadow of a hill or mountain will not warm up as much as land that is receiving direct sunlight.

On the other hand, if the surface is inclined in such a way as to receive the sun’s rays at a more direct angle, then the surface will be a much more efficient thermal source. Hillsides and mountain slopes facing the sun will provide such sources.

Fig 2.16 Sun angle. The angle of the sun’s rays. The more directly the sun’s rays strike the ground, the greater the surface heating.

Triggers

Once our parcel of air is heated by the thermal source, it will not necessarily break away from the ground immediately. It needs something to dislodge it. The ascent may be caused by the parcel of air becoming so warm that its buoyancy is too great to allow it to remain close to the ground. Alternatively, some external influence (known as a TRIGGER) may cause the beginning of a thermal.

Many of these triggers are man-made. Anything that disturbs the air can kick off a thermal. Cars, trains, launching cables and tow-planes, even combine harvesters and tractors, can all stir up the air enough to start a thermal on its way.

It might be hard to believe that something as small as a car or tractor may cause a large enough disturbance to trigger a thermal. However, remember that the trigger is not forming the thermal, only stirring it into action. The trigger only needs to start a small amount of air moving, the result of which is movement of surrounding air, which may be enough to disturb our incipient thermal.

Moisture thermals

Water vapour (containing a large proportion of hydrogen) is less dense than dry air. It is therefore more buoyant. It follows that a parcel of moist air surrounded by drier air will want to rise. Whether or not this would create a usable thermal is doubtful, but the effect on a thermal of having a large moisture content will be to enhance the temperature differential between the thermal and the surrounding air. In theory, this should make for a better thermal.

The formation and structure of a thermal

To talk about a typical thermal is difficult, as any meteorological phenomenon is blessed with the ability to be infinitely variable in its structure, shape and size. The best that can be achieved here is to describe an idealised thermal, which will give a general picture of what a thermal would look like, if only the air of which it is composed were visible.

The beginning of our ideal thermal can be regarded as a volume of air next to a part of the ground which is hotter than the neighbouring surfaces.

Fig 2.17 Thermal sources. A thermal starts as a volume of air being heated by an adjacent ‘hot spot’ on the ground.

As the surface heats this local air mass, the air will become increasingly buoyant until, due to either some independent disturbance (a trigger) or its own degree of buoyancy, it overcomes its tendency to adhere to the ground and breaks away.

As this thermal ‘bubble’ rises, it will be ascending to levels where the atmospheric pressure upon it is less. Therefore, it will expand as it rises. It will also encounter some drag from the air through which it is passing. This drag will affect the outer areas of the thermal. There will also be some degree of mixing with, and entrainment of, the air through which it is passing. The result of these latter events is that the outer air of the thermal bubble will be cooling and descending relative to the centre of the thermal bubble (the CORE). The accepted shape of a thermal is therefore one of a three-dimensional vortex ring, with air rising up the centre and spilling downwards on the periphery.

Fig 2.18 Birth of a thermal. Eventually the air will break away from the ground and rise.

Fig 2.19 Thermal bubble. As it ascends, the thermal will acquire a vortex motion, with its core rising faster than its outer edges.

This entrainment and mixing with the outside air not only leads to the thermal expanding but also adds to its cooling. This cooling will reduce the thermal’s buoyancy and may eventually stop its ascent. As a result, smaller thermals, which will suffer more from this cooling effect, may not last as long as their bigger brothers.

This vortex structure will assume some horizontal movement of the air at the top and bottom of the thermal bubble, with an inflow at the bottom and an outflow at the top.

Two important points should be realised from this model of a thermal.

1.

Despite the fact that the air on the outside is descending relative to the core, the whole bubble is rising relative to the ground.

2.

From the above, it can be visualised that in the core of a thermal, the air will be ascending faster than the thermal bubble as a whole.

Whether all thermals take the form of a bubble being released in this way, or whether some are continuous streams of rising air forming a column, has been a point of debate for many years. Most thermals probably begin as a column of lift that is still being fed from its ground source, but as the reservoir of warm air at the source is exhausted, the thermal breaks away and forms a rising bubble.

If conditions are favourable, a good thermal source will fire off regular thermals, and if these are frequent enough, then the result may well give the impression of one continuous column of rising air, as each bubble released follows behind its predecessor. The spiralling nature of a dust devil may suggest that some thermals at least form the shape of a column rather than a bubble of rising air. Straw stubble and heath fires supply a continuous source of heat while they are alight, and such a thermal source is likely to form a continuous column of rising air through which will rise pulses of faster rising air.

However, all too often, it is possible to find oneself searching unsuccessfully for lift under other climbing gliders. This would suggest that the thermal bubble has ascended past your level. In other words, ‘you have missed the bubble’. Once the warm air close to the ground has ascended, it will take time for the warm ground to heat the cooler air that has replaced it. How long this takes, will depend on the prevailing conditions. If you are lucky, the next thermal bubble may be on its way up. If you are unlucky, you may watch from the field below, while your colleagues climb in this later thermal bubble. Who knows, your landing may even have disturbed the air enough to trigger this new thermal!

Another fact which supports this bubble theory is that gliders contacting a new (lower) bubble often find that they can catch up with gliders which are hanging around in the weak lift near the top of the thermal. This is often observed in the club environment where inexperienced pilots often jealously guard their hard-won height.

Fig 2.20 Thermal column. Often a thermal source will set off regular thermal bubbles, giving the impression of a continuous column of lift.

The dimensions and rate of ascent of any one thermal are as hard to pin down as an average shape. Figures of around 1,000 feet (300 metres) in width have been suggested for ‘typical’ British thermals, with glider pilots achieving average rates of climb of 4 knots regarding themselves as having found a good thermal. On average, the height which a British thermal might reach before forming cloud would be around 4,000 feet, with an exceptional day being 6,000 feet or higher. (The greatest reported height reached by a thermalling glider in the UK was 11,000 feet during the legendary heat wave of 1976!)

These figures would be regarded as poor by glider pilots from hotter countries such as Australia, South Africa and some parts of the USA. In such countries, climb rates of over 10 knots and much wider thermals rising to well in excess of 10,000 feet may be considered the norm.

These descriptions of thermal shapes are an over-simplification to help visualise these normally invisible entities. Thermals may have two or more cores. Several thermals may feed the same cloud. These points are often manifested when two gliders, which are initially thermalling in neighbouring thermals, find themselves in overlapping circles or even in the same core. (One wonders whether thermals are like raindrops running down the glass of a window, in that they occasionally merge and increase their speed.)

Thermal streets

In certain conditions, the wind and the circulation caused by rising thermals will result in a phenomenon known as THERMAL STREETING. This is most obvious in moderate to strong winds when cumulus clouds form long lines in the sky, called CLOUD STREETS. Under such streets can be found long lines of lift while under the clear areas between them, you can expect to find large amounts of descending air. Figure 2.21 shows the circulation which causes such conditions.

Fig 2.21 Cloud streets. In windy conditions cloud streets will often form, indicating the presence of long lines of lift.

These lines of cumulus will be aligned more or less parallel to the wind direction at cloud base. The distance between the streets has been found to be around three times the height of the cloud tops. Such streeting is most likely to occur when the wind speed is increasing with height between the surface and cloud base.

Occasionally, a single ‘cloud street’ will form which differs from ‘true’ cloud streets in that it is not a result of the general circulation just mentioned. Such a line of cumulus may be caused by a thermal source which persistently fires off thermals more often than the surrounding land, or perhaps by two differing air masses converging and causing thermals to be triggered along a line.

It should be noted that thermal streeting is not restricted to days when cumulus clouds are present to show the streets. Blue thermal streets are also common, and the lack of cloud in these situations can add to the challenge of thermal finding.

Chapter 3

Thermal Soaring

It is out there somewhere but it is invisible. It is not solid; you cannot touch it but you can feel it. It changes shape, changes speed and sometimes it changes direction. It has character and, more often than not, it is antisocial. Above all, it has energy which you need to tap.

No, this is not a description from a Star Trek movie. It is the description of a thermal. ‘Character? Antisocial?’ All thermals have characteristics and many, once you enter them, will do their utmost to get rid of you by tipping your wing or apparently disappearing temporarily or changing position.

Finding a thermal

Finding a thermal is the first task that you will face when you embark on a thermal soaring flight. How difficult or easy this may be will depend very much on the local weather conditions on the day. It will also depend on how you read the signs which give clues to a thermal’s presence, and on the way in which you search for a thermal.

Ideally, you will launch into a thermal by aerotow or even from a wire launch. Therefore, the time to start looking for a thermal is while awaiting launch. Unfortunately, the average club environment does not always allow you to select the exact moment of launch. Often your glider will be in a queue and you will be forced to launch as soon as there is a tow-plane or a launch cable available. Alternatively, whichever of these you have chosen to use may not be forthcoming at the time when you consider it would be ideal to launch.