Imagine you're a falcon. You effortlessly ride the airwaves, circling upwards in the thermals, just flicking a wing feather every now and again to change direction. For we mere humans, the closest approximation of flying like a bird is the sport of gliding. The motorless gliding aircraft, called gliders, provide the experience of flying without distracting engine noise, and allow for wonderful views of the countryside below. Reading the weather provides a real mental challenge for those piloting gliders, and yes, they sometimes do watch the birds for clues.
Early Gliders: The Colditz Cock
Early gliders made use of simple materials such as wood and fabric. The mass availability of these components made possible one of the most ambitious escape plans of the second world war inmates of Colditz prison.
With unbelievable audacity, a glider with a 32-foot wingspan was secretly built in the attic of the castle. Materials were improvised from whatever came to hand, eg bed-boards, iron bars from the windows and even the prisoners' own sleeping bags were used to provide the skin of the aircraft. After eight months of work, the glider was finished but never used.
Recently, a replica has been constructed so the surviving members of the Colditz Cock team may yet see their design tested in flight.
Modern Day Gliders
Modern day gliders are made from fibreglass rather than the original wood and fabric designs. Gliders today can fly:
Fast - some aerobatic gliders easily reach 400 km/h.
High - the world record is over 10000m altitude.
Far - In some areas, for example South America, 2000km distances are possible.
Do You Fall Out of the Sky when the Wind Stops?
Keeping a glider airborne is all about finding areas of rising air, known as lift. There are several different types of lift:
Thermals are the result of warm air from the earth escaping into the colder skies above. When you see birds circling high above you, they are taking a free lift from a thermal. At the top there will often be a large fluffy white cloud with a flat bottom. Think of a hot summer's day with blue skies and you're probably picturing the right sort of cloud. If you're anywhere near a gliding airfield, there may well be some large-winged aircraft rising dizzyingly under the clouds.
Ridge lift is caused by a long edge of hillside getting in the way of a steady flow of air. When the air stream reaches the hillside, there's nowhere to go but up. One can then fly quite happily above the ridge, supported by all that rising air. If you are lucky enough to find an airfield that can launch directly onto a ridge so with a favourable wind, you'll be able to fly back and forth for hours if you really want to.
Wave can be King of the Skies for providing strong, steady lift. It's easiest to imagine if you consider the analogy of a stone in the middle of a river. As the water flows against the stone, waves of ripples spread out from the obstruction. A similar thing happens to moving air as it hits mountains and hilltops sticking up into the atmosphere. Standing waves occur which can be seen in the sky in the form of lens-shaped clouds, lying still across the direction of the wind instead of moving with it. The stronger the wind, the bigger the waves and the faster they lift lucky glider pilots up, up and away.
If you're a glider pilot, you'll be watching for signs of these conditions and making the most of them to get you high in the air and keep you there. If there's no wind, you might still find some thermals. If there really is no rising air to be found, then you'll gently glide downwards in a controlled descent. If you've calculated your height, glide angle and distance from the airfield correctly, you'll make it back home. If you've miscalculated and are too low, too far from home - then it's time to find a friendly field you can safely land in. Glider pilots on cross-country flights will be constantly scanning the ground for suitable fields in case the weather or their judgement fails1.
If you want to know more the British Gliding Association can give you more information and, if you are based in the UK, direct you to a gliding airfield near you where enthusiastic pilots will be happy to take you for a ride.