How Gliders Fly
Gliders fly in the same way as powered aircraft; airflow over their wings creates lift which balances out their weight. As most gliders don’t have engines they generate airflow by flying “downhill” – typically losing height at 150 to 200 feet per minute, which generates an airspeed of 45 to 55 knots.
Sources of Lift
The really clever bit is to find a parcel of air that is rising and fly in that. It the air is rising at more than 150 to 200 feet per minute then the glider will gain height while flying downhill. There are three main types of rising air:
- Thermals – Sometimes the sun heats one patch of the earth more than another making the air above it is relatively hot and less dense. It therefore rises. Mature thermals have cumulus clouds (the fluffy white ones) above them. Thermals generally have strong lift and go all the way up to the base of the clouds (and in fact up though them – although few gliders fly in cloud), possibly 5,000 feet or more. Note that if there are thermals about there are also places where the air will be coming down (sink). Glider pilots seek to avoid these if possible.
- Ridge – if the wind at Challock is from the southwest at over about 25 knots the ridge deflects the air upwards, usually at about 200 to 400 feet per minute. Gliders use this lift to fly along the ridge. The lift usually lasts until about 1,300 feet above the top of the ridge.
- Wave – sometimes distant hilly area set up an oscillation in the atmosphere which creates large areas of rising air. Their presence is indicated by lenticular clouds. although there is sometimes wave over Challock it is too high and in controlled airspace.
There are several methods of getting gliders into the air. At Challock we use the two most common methods:
- Aerotow. The glider is literally towed into the air behind a powered aircraft which has been modified to tow gliders and is flown by a specially qualified tug pilot. These usually go to 2,000 feet above the airfield, but of course can go higher.
- Winch. The glider is connected to a powerful (approx 500 horsepower) winch. When the pilot is ready the winch operator pulls in the cable and the glider accelerates – far faster than a sports car. The pilot puts the glider into a steep (45 degree) climb and up they go. At the top of the launch, typically 1,000 feet, the cable releases automatically and the flight begins.
Gliders land much like any other aircraft. They are fitted with air-brakes to enable them to come down steeper than a normal aircraft but the final touch down should be very gentle.