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How do Gliders Stay Up?

So how can a glider stay airborne without an engine? What about gravity? The basic principles are easy to grasp, you do not need to learn a lot of technical theory to fly a glider.

A glider is nothing more nor less than a refined paper dart. If it is launched to a given height in still air conditions it will move forward horizontally relatively rapidly and descend vertically relatively slowly in rather the same way as a cyclist can freewheel down a hill. To fly faster it has to descend more rapidly and to stay up as long as possible it flies relatively slowly. It can be turned during its flight and its descent can be controlled, but its glide path remains essentially downwards through the surrounding air. Gliders are designed so that they can fly quite a long way with a relatively small loss of height - in still air a modern training glider can fly about 5 or 6 miles at about 55mph for a height loss of only 1000 feet.

Fortunately, however, this air is never perfectly still, and the glider pilot can maintain or even actually gain height if he finds some air that is going up faster than the glider is coming down through it. The principle is the same as a man trying to walk down an ascending escalator. If the escalator is moving fast enough, then in spite of his efforts, the man will be carried to the top. Most large birds spend quite an appreciable part of their flying time gliding. If you see a bird circling gently or cruising along without flapping its wings or perhaps just the odd flap to change direction or accelerate, then it's gliding using this same principle to stay airborne. Birds do it much better than we can but with a little experience it's possible for a glider pilot to stay up for long periods of time using air currents.

The action of the air in keeping the glider airborne or even gaining height is called (rather unimaginatively) 'lift' and there are three main sources of this precious commodity which the glider pilot can use.

Ridge Lift

animation of glider in hill lift

If a wind blows against an obstruction such as a hill, some of it is forced up and over the top creating a band of rising air immediately in front of the hill. Provided the glider pilot keeps flying within this band of lift, he can stay up all day and all night (in theory) if he wishes.

The best kind of hill formation for this to happen is when the hill is long in the direction across the prevailing wind because in this case most of the air is forced over the top instead of being able to find a way around the sides, Portmoak is ideally situated for ridge flying with lengthy soarable ridges (Benarty, the Bishop and West Lomond) for wind directions from just east of north, through west, to just east of south. This gives an ideal training environment in that the average time in the air is much greater that that achieved by "non-hill" sites and even in the hands of inexperienced learners ('ab-initios' in gliding terminology) long flights are easily possible. For more experienced pilots it gives the opportunity to cruise around quite effortlessly looking for the best conditions to make a start to a cross-country flight.

However, for cross-country soaring or soaring where there are no hills, the glider pilot must look for other lift sources.

Thermal Lift

 animation of glider climbing in thermal lift

Certain areas of ground that get warmer from the sun than others heat up the air above them which then rises. Once started the action usually continues for quite a long time forming a continuous column of air. More often than not such a column (known as a thermal) is sufficiently large for the glider to keep within, provided the glider is rotated in fairly tight circles. By doing this the pilot effectively spirals upwards.

Often the air in the thermal is moist, but this moisture is not visible as it is in the form of a gas called water vapour. However, if the air cools enough, which it generally does at higher levels, the moisture becomes visible in the form of myriads of tiny water droplets which we then see as a cloud. So a thermal is often signposted by a fluffy cauliflower-shaped cumulous cloud, so common on warm summer days. These clouds continuously form and dissipate and a glider pilot becomes experienced in recognising which clouds are growing, indicating that they are supported by a thermal. Thermals need not have clouds to signpost them - when the air is dry you get what are known as 'blue thermals' which are still useable but much harder to find!

By flying from thermal to thermal the glider pilot is able to make his way across country, climbing on each thermal in turn and then gliding 'downhill' towards the next. But sometimes, especially in winter, when the heat from the sun is too weak to form useable thermals, the glider pilot needs to seek an alternative form of lift in order to climb high or fly cross-country.

Wave Lift

animation of glider climbing in wave system

Under certain meteorological conditions, once the air has gone over the top of the hill or mountain, it cascades down the other side (the leeward side) with considerable energy and then 'bounces' up again. This process can be repeated and go on for a considerable distance behind the obstruction, the oscillations in the airstream being rather like those which you could form by rapidly jerking one end of a long rope up and down.

In the airstream the wave oscillations often go to great heights with successively higher layers of air following the contours of those below - far greater than that of the hills or mountains originally causing them. By flying in the up-going parts of the wave system the glider pilot may be able to climb to considerable heights, or he may travel many miles along the wave system. Wave lift is generally characterised by powerful but smooth upcurrents rising to many miles in the air (or if you are in the wrong place equally powerful downdraughts!) and gliders have been known to climb to the height of passenger jet aircraft in these systems. Heights approaching 40,000 feet have been achieved in the UK and glider pilots planning to climb in this way generally carry oxygen systems for use once they are above 10,000 to 12,000 feet.

Although wave systems may sometimes be invisible, more often than not they are marked along their length by long clouds (lenticulars) which lie approximately across the wind direction and which may be characteristically very smooth and even in shape. These clouds are continuously forming at the upwind side and decaying on the downwind side and give the impression of remaining stationary although the wind may be strong.