How Does It Stay Up?

 Of course, without an engine a glider will always descend. It has to be flown fast enough to ensure that the air flowing over it is strong enough to enable the control surfaces to work, and in still air, from a height of 2000 feet would reach the ground in around ten minutes. However, if rising air is encountered a pilot can cause his craft to gain height - a good example would be of a person trying to walk down an up escalator.

There are three ways in which a glider can stay up. The first is thermals, the second is ridge lift and the third is wave.

Thermals are the name given to rising air that is heated from the action of the sun on the ground. Certain areas of ground that get warmer from the sun than others heat up and a column of air begins to rise. If a glider flies into this rising air, the pilot begins to circle to remain inside it, in doing so, the rising air will carry him upwards - often to over 5000 feet. Often a cumulous cloud will mark the top of such a thermal. This type of lift is best during long summer days, beginning from around 10 am

and finishing before sunset. (Although there can be some very good days in early spring) Pilots regularly travel many miles in these conditions. Booker is in an ideal part of the country for thermals - The Chiltern Hills are a good source of lift - beginning early and finishing late.

Ridge lift, or hill lift, is encountered when the prevailing wind encounters a steep hillside at right angles to it.

The airflow is diverted upwards and a glider can soar in this air for as long as the wind continues to blow in the same direction. There is a ridge near Booker on the scarp of the Chilterns near Stokenchurch that we can soar when the wind is northwesterly.

Wave is usually only found in mountainous areas. Under certain meteorological conditions, air deflected upwards by a mountain, cascades over the other side and then 'bounces' upwards again, often to many thousands of feet higher than the original mountain. A similar phenomenon can be seen by placing a pebble in a shallow fast flowing stream. A large wave is created over the pebble . Air behaves much like water and a skilled pilot can soar to tens of thousands of feet in this way. Often, smooth, cigar shaped clouds (lenticular) appear at the top of each wave 'bar'at right angles to the airflow - they appear to be

stationary but are created by the moisture in the air condensing into water droplets at the leading edge and dissipating at the trailing edge. Pilots can travel many miles using these wave systems.

At Booker we organise expeditions to Aboyne in Scotland which enjoys some of the best wave conditions in the UK at the end of each season.