How does a glider stay in the air?
To fly, a glider uses the same principles as a powered airplane (more info). But the way how it stays in the air is different, because a glider plane has no engine…
First we are going to take-off with our glider. This can happen at 2 different methods:
Launch Methods
1. Aerotowing
The glider is attached to a powered aircraft (mostly a very light aircraft) which takes off and pulls the glider with him into the air. At an arranged altitude the glider pilot releases the rope and now he has to find a rising air column (=thermical lift, see futher) to stay in the air.
2.Winch launching
The winch is mounted on a ground-based heavy vehicle. The winch pulls in the cable that is attached to the glider. This way, the glider goes into the air. At an altitude of 400m-700m the glider releases the cable. This method is cheaper than aerotowing, but the altitude is lower so the flights are shorter, unless the pilot can find quickly thermal lift.
Our glider is now in the air, but how it stays in the air? “Because of the wind” is a common heard explanation. It's totally wrong!
Thermal lift
To hold enough speed to fly, the glider has to fly (to glide) a little bit downwards. This way, the height that is obtained by take-off reduces quickly. To get higher again, the pilot has to find a column of rising air. The rising air (= thermal lift) is caused by the following process:
1) The sun heats the earth’s surface.
2)
The heat that the earth’s surface has absorbed is given to a thin ‘air layer’ above the heated surface (good surfaces are sand, asphalt, towns; bad surfaces are grass or water)
3)
This ‘air layer’ becomes hotter than the surrounding air, so it expands and becomes lighter than the cold air above it. This causes the hot air to rise = thermal lift
When the glider comes into thermal lift, it will rise if the lift is greater than the downward movement of the glider to hold enough speed for flying. The columns of rising air are circular or oval, so the glider flies in circles. Where the Cumulus cloud appears is thermal lift present. This is a good hint to find thermal lift.
