How does a sailplane stay up?
They use gravity and the flow of air over the wings to advantage, to overcome their own weight. The wings are designed to generate lift at very low speeds. When the lift equals the weight, the glider becomes airborne. It does this by moving through the air at various angles to the airflow, which allow it to climb or cruise. Modern sailplanes have a top speed of up to 300km/h. When thermalling they will fly as slowly as possible, around 70 to 90km/h. When cruising between thermals they will fly between 100 and 200km/h. On a day with very strong up-currents, gliders on cross-country tasks will carry perhaps 100 litres of water in tanks within the wings. This ballast improves performance in a glide, and will be jettisoned if conditions weaken, or prior to landing.
