How a Glider Flies

How a Glider Flies . . .

Using rising currents of air to keep the sailplane airborne, the experienced glider pilot is able to soar like a bird, silently exploring the skies and often covering many miles before returning home several hours later. Modern gliders, or sailplanes as they are frequently called, are built of fibre glass, they have wingspans of between 15 and 25 metres and their advanced design enables them to glide long distances - up to 30 miles from 5,000'.

Gliders . . .
Gliders are essentially light aircraft without engines. With the same basic controls as any other aeroplane, they are highly manoeverable and extremely controllable. Unlike most powered aircraft, however, modern gliders have long slim wings which are capable of generating large amounts of lift and their overall design is highly streamlined to reduce drag.

For many years most gliders were made from wood and fabric, however the development of modern materials such as glass fibre, carbon fibre and kevlar has revolutionised glider design resulting in today's high performance sailplanes.

Getting Airborne . . .
There are several methods of launching gliders, but the most common are by a static winch or a tow behind a light aeroplane (aerotow).

Winch:
The winch is positioned at the upwind end of the airfield, the glider launch point at the other end. When the pilot is ready to launch a steel cable is attached to the glider and then wound onto the drum at the winch, this pulls the glider forward and the wing generates lift enabling the pilot to climb, very much like launching a kite.

The height achieved on a winch launch varies and is dependent on a number of factors - the length of the field and therefore how much cable can be pulled out, the power of the winch and the strength of wind. At Cross Hayes we average around 1,300' to 1,600' although on some stronger wind days 2,000' is possible.

Aerotow:
The glider is towed into the air behind a light aircraft (tug). During the tow, the pilot of the glider flies in formation behind the tug and is able to choose the point at which he releases the tow rope.

Bungee:
Used in the early days of gliding, the glider is catapulted of a hillside using an elastic rope. Bungee launching is now rarely used.

Staying Airborne . . .
Following its launch, the glider embarks on a gradual descent path through the surrounding air. Fortunately, however, the air is never perfectly still - in places it will be rising, in others it will be descending. By flying in air which is rising faster than the glider is descending the pilot can gain height. The principle is the same as that of a person trying to walk down an escalator that is travelling in the opposite direction - if the escalator is moving upwards fast enough, the person will be carried to the top despite their efforts to go down. There are three main sources of rising air, or lift as it is more commonly referred to:

Thermal:
This is the most common form of lift used by pilots flying from Cross Hayes. Differential heating of the ground causes some surfaces to heat the air above them more quickly than surrounding areas. Eventually this warm air will start to rise, producing a column of rising air, known as a thermal. On a summers day these ‘thermals’ are often marked by small cumulus clouds.

Having found a thermal, the glider pilot will circle in the rising air, often turning quite tightly, as the glider is carried upwards. By staying within the area of rising air the glider pilot will be able to gain height, often reaching the bottom of the clouds above, and by moving from one area of lift to the next, can cover many miles in a single flight.

The glider on the right is climbing in a thermal near Cross Hayes Field and is achieving a vertical climb rate of 415 feet per minute!


		Hill Lift: When the wind blows against an obstruction such as a hill or ridge of higher ground, the air is forced upwards as it follows the contours. This creates a band of lift parallel to the ridge and the glider pilot can fly back and forth in this lift, maintaining or gaining height, for as long as the wind carries on blowing.			Wave: Most commonly found in mountainous regions, wave occurs when the air blowing over a mountain or hill sets up an oscillatory motion, similar to that achieved by rapidly jerking a long rope up and down. These oscillations often go to great heights - far in excess of the height of the original obstruction - and cover large areas. By flying in the up going part of the system the glider pilot can often make extremely high altitude climbs as well as being able to travel great distances along the system.

Gliding Records . . .
There are national and international records for gliding covering overall distance, time taken to complete closed circuits of set distance and for absolute altitude and gains of height. The UK distance record for Gliding currently stands at 1,108 km (688 miles). The fastest recorded cross country to be flown in the UK was a 100 kilometre flight which was completed at an average speed of 199 kph (124 mph). The UK height record currently stands at 11,570 metres (37,700 feet), which was achieved in wave over Scotland during 1995.