What! Flying without an engine? You must be mad!
Well, quite possibly, but leaving the engine behind has many benefits: how many Cessnas can fly for eight hours without refuelling, or reach 38,000 feet in the UK?
As a glider must trade height for flying speed, glider pilots seek out rising air. However, first consider the case where the air is stationary. The glider will sink down through the air at a rate depending on how fast it is flying; a typical sink is about 1.5 knots at normal flying speed. For the glider to remain at a constant height it is necessary for the air around it to be rising at the sinking speed of the glider (1.5 knots in this case). This is akin to walking up a down escalator, but not getting any closer to the floor above. If the air mass is rising faster than the sinking speed of the glider, the glider will climb.
So where do I find this rising air?
Well that's a very good question, and is the challenge all glider pilots face. The most common form of lift is generated by the sun warming areas of the ground. This warms the air above it, which rises upwards as a plume. If the air is buoyant enough, flying within this rising plume of air (by turning tightly) will propel the glider skywards. Mother nature is usually kind and puts a nice white fluffy cumulus cloud signpost at the top of the rising airmass or thermal, although sometimes she creates the dreaded 'blue' day with thermals but no marker clouds.
Lift is also generated by air meeting a hill or ridge line and being deflected upwards. This lift can be quite strong, but is localised and depends on the wind strength and direction. A further source of lift is a phenomenon known as wave. A large geographic feature such as a mountain range can, under certain conditions, produce an atmospheric oscillation in its lee. Gliders can ride the rising crests of these wave to great heights. The UK glider height record of 11,570 metres (37,959 feet) AGL was set in wave at Abyone in Scotland.
The challenge in gliding is to find this rising air to stay airborne, proving that Newton did not know everything! However, flying over the same bit of ground around your local airfield for hours is considered by some to be slightly tedious, so this lift is used by the more intrepid pilots to go somewhere else. These cross-country jollies typically last for hours and cover hundreds of kilometres. In competitions, pilots race each other around lengthy courses, aiming to complete the course in the shortest time.
What happens when the wind doesn't blow?
Glider pilots smile! The lack of wind allows more progress to be made over the ground, with strong and well formed thermals. Pilots seeking lift from ridge and wave sites will disagree because they need wind for this type of lift to work.