How a glider flies
How do a glider take to the skies?
Because the typical glider have no engine, gliders are usually launched into the air by using two different metods:
Aerotow:
A motorplane tows the glider up to altitude. A rope is attached between the glider and the motorplane(we call it "towplane" or "tug"). When arriving an altitude of 500-1000m the gliderpilot pull a handle in the gliders cockpit releasing the glider off the rope. The towplane returns to the airfield to launch the next glider usually queing up at the airfield when the weather is good for flying.
Photo by Kristoffer D. Samuelsen
Winch
A powerfull whinch is placed at the end of the runway. A wirecable is attached between the winch and the glider and the glider is launched into the sky like a rocket. During a whinchlaunch, the glider does 0-100km/h in only 2-4 seconds! The glider can gain 500m in 45 seconds by winchlaunch. The winchlaunch follows pretty much the same prinsiple as launching a kite does. Thus making the need for a good distance between the winch and the glider to get to high altitudes. Because of the space needed and winchlaunch needing a bigger groundcrew to operate safely, aerotow is the most common way to launch gliders today.
Picture found on: https://www.bogan.ca/soaring/winch/Winch06.gif
The glider has no engine, how come it not just fall through the sky?
A glider in silent air can be compared to a bicycle going downhill. Gravity is what gives it speed and forward motion. A typical modern glider travels 40m in distance for every meter it falls in silent air (at a speed of around 100km/h). This relationship between progress(lift) and sink(drag) is called "glide ratio" or L/D ratio (lift over drag ratio). As the angle of the slope controls the downhill bikes speed, the same goes for the glider in silent air. The difference is, that the gliderpilot can control the angle of the slope himself by pushing forward or pulling backwards on his flightcontrol. By pushing forward on the flightcontrol, the glider falls steeper downhill bulding up more speed. This speed is paid for in altitude. The faster the glider flies trough the air, the shorter the range. For the gliderpilot, it's obvious that the glider needs external energy to stay in the air and move forward at high speeds.
The sun: Gliders "engine"
Thermals
A thermal is basicly a column or boubbles of hot, rising air. These vertical channels of rising air are formed by temperature differences in the air as the sun heats the landscape. We get these differences in temperature because of the uneaven ability of different surfaces on the ground to reflect the heat from the sun.
Warm air expands, making it less dense than the surrounding air. The warmer, lighter air rises upwards to everything from a few hundred meters to several thousand meters. Around the area of rising air, we find cold, sinking air. Depending on the weatherconditions, a cumulus cloud is usually formed at the top of thermals making good markers. A cumulus cloud are basicly those puffy, nice and white clouds you see during nice weather days. Most common in the summertime. They can also develop into afternoon showers and even thunderstorms! The cumulus clouds are formed as the rising air cools down and condenses at a certain altitude. Given there is some wind, thermals often form in long rows or "streets". Gliderpilots often refeer to this as "cloudstreets". The clouds aren't always there as markers though. To make use of thermals, the gliderpilots locate them by keeping an eye on the terrain and the weater conditions such as wind and sun. When arriving the thermal, the gliderpilots have to fly in a circle to stay in the column of rising air. Compared to formula 1 racing, the thermals are like pit-stops when the cars has to stop for more fuel. The gliderpilots stop in the thermals to "fuel" more altitude. Thermals can often generate strong lift. An average thermal can lift a glider up 2 meters every second! A really strong thermal can lift a glider up to 5 meters every second! When the thermals are aligned in rows, the gliderpilot can use them as "highways" towards his destination flying witout having to stop for altitude.
Pictures found on: https://www.aerospaceweb.org/question/nature/q0253.shtml
Picture found on: https://www.aerospaceweb.org/question/nature/q0253.shtml
Ridgelift
When wind blows towards a mountain or a hill, the wind is forced to go upwards. Gliders can surf these upward winds along the side of mountains or hills. The amounth of lift increases with wind increasing and also with a steeper and higher ridge. The closer the glider is to the mountain, the stronger the lift. This often makes ridgerunning as gliderpilots call it, a fast and intense excperience. When gliderpilots do ridgerunning in a competitive setting, the margins for errors are small and the skill and focus of the gliderpilot really comes to good use.
Source of photo: https://www.bannerdown.co.uk/lessons_thermals.htm
Lee Waves
During special meteorological conditions, when the atmosphere is stable and there is a certain amounth of wind which is also increasing with altitude, lee waves can form. These are not waves like at sea, but are air going up and down in a wave-like pattern. Lee waves are formed when the wind hits a disturbance under given conditions. This disturbance is often mountains but can also be formed by a disturbance in the atmosphere. Cumulus clouds marking the top of a thermal can be a disturbance of the wind good enough to form waves. When wind hits the face of an obstacle like for instance a ridge it goes up. On the other side the wind goes down. If the wind is very strong it goes up again at a distance behind the obstacle. During the right conditions this wave-motion can continue at far distances behind the obstacle and propagate upwards into the atmosphere as high as 30 000ft! Gliders can use lee waves to fly very high and also fast. Some of the most extreme gliderflights have been done using lee waves. As lee waves rarely occour during summertime it is not commonly used in glider competitions, but it happens sometimes. Depending on weather conditions, clouds may form following the wave motion. This makes the waves visual and easier for gliderpilots to make us of.
Picture found: https://ocw.usu.edu
The landing
A glider landing looks more or less like a motorplane landing. The gliderpilot uses airbrakes (spoilers) on the wing to control the glideslope towards the end of the runway. After the glider has kissed the ground, the pilot has to keep the wings level until the glider stops. This is because a glider only have two wheels in the longitudinal axis of the plane.
