4. LESSON 7 – THE EFFECT OF MOVING AIR
- Air moves, so you won’t always go in the direction you are pointing, either left/right or up/down
We can understand the effect a crosswind has on a glider by imagining a small ferry crossing a river. To cross the river in a straight line, the skipper has to point the bow into the current; otherwise the ferry will drift down the stream.
The angle between where your glider’s nose is pointing (heading) and the direction in which you are moving across the ground (track) is called the drift angle.
STAYING ON TRACK DURING CROSSWIND
To better understand windy conditions, imagine how an entire air mass moves across the ground with your glider in it. It is as if you are flying in a moving block of air.
While flying straight into the wind at your normal gliding speed you will notice that progress across the ground is slower than your indicated airspeed. Conversely, in a tailwind (wind blowing from behind), the ground below will pass faster and your ground speed will be high. Finally, during crosswind, your glider will move sideways over the ground below (just like the ferry).
We often encounter situations where we need to hold a particular track regardless of the crosswind. In this lesson, we will practice this by flying parallel to a line below, which is across the wind (for example a road). You will quickly notice that, without correcting, you will soon drift away from the landmark you picked due to the crosswind.
To keep flying parallel to a road or line below, you have to correct for drift. We do this by pointing the glider into the wind until we see that we are staying on the right track, compared to the ground below. We are now applying the drift correction.
FLYING IN A THERMAL (AIR MOVING VERTICALLY)
It might be helpful to also imagine flying in a thermal as flying in a block or box of air – only this time the air isn’t moving sideways, but upwards. You have learned that a glider is constantly sinking. So even in a thermal, you will be sinking compared to the rising air around you. However, if the surrounding air rises faster than your glider sinks, you will still climb. Of course, your variometer will confirm that you are climbing and your altitude will increase.
The opposite is true for flying in an air mass that is sinking. The sink rate of the air mass now comes on top of the normal sink rate of the glider, and the vario shows a stronger sink than normal. It is best to fly faster in a sinking air mass; that way, you can leave it as soon as possible.
You first fly at a normal sink rate (1). When approaching a thermal, your sink rate will increase, because you will have to fly through colder sinking air to reach the thermal. As soon as you reach the warmer, rising air mass, your glider will gradually start to climb. In the core of the thermal you will find the strongest climb. By making circles here, you will gain height. When you leave the thermal, you will have to cross the sinking air mass once more, which will result in an increased sink before you can continue at a normal sink rate again.
If you don’t wish to make any circles in a thermal, you will still climb a little, but only while flying through the warmer rising air mass in a straight line (2).