Gliding?

A short overview

For many people, flying has been a dream since their childhood. Who hasn’t ever imagined what it would be like to fly above roofs, forests and mountains? Nowadays, it is almost natural for us to use a plane to get somewhere, but this “busride in the sky” can not convey the real fascination of flying. When gliding, the plane is only “propelled” by the power of the sun and you are gliding nearly silently through the air. Without any engine, flights of many hours and hundreds of miles are possible.

Some records for gliding:

The highest flight was conducted by Robert Harris (USA) with a Grob G102 on 1986-02-17. He reached a height of 14,938m, this is higher than most airliners fly!
On 1972-04-25, Hans Werner Grosse flew from Lübeck (Germany) to Biarritz (France) with an ASW 12 – with a total distance of 1460km. This record has been outperformed since then, on round courses with three turning points, the record is 3009km, flown in Argentina with a Nimbus 4 by Klaus Ohlmann (Germany).
The longest flight that we were able to find was by to French men, Betrand Dauvin and Henri Couston, who stayed in the air for 57 hours and 10 minutes in 1954 with a Kranich III – reminder: without any engine!

 

How does a sailplane fly without an engine?

Many of us have already folded a paper plane. The wings “carry” the plane, until it arrives on the ground. The wings of gliders (as well as the wings of any other plane) have a special shape – the so called “profile”. If the plane moves through the air, this profil makes the air on the upper side pass faster than on the lower side. Because of this, a lower pressure on the upper side and a higher pressure on the lower side result. This induces a force, that “pulls” the wing upwards, the so-called uplift. Still, the plane has a certain weight and experiences drag when moving through the air, this is the reason why a plane constantly loses height without propulsion.

Still, the sailplane doesn’t just fall from the sky. The intrinsical descent of a glider is typically about half a meter per second. Depending on the performance of the glider, it covers much more distance in horizontal than in vertical direction while sinking. This ratio is described by the glide ratio, for example 1:40. This means that the glider can travel 40 meters while losing one meter of height. For 1000 meters height, this means a range of 40 km. “Typical” gliders have a glide ration between 1:25 and 1:60.

To get airborne, the sailplane will be pulled into the sky with a winch or with a motor plane up to a height of 300-500 meters. Still, this doesn’t explain the reachable distances of 500 and more kilometers, that sailplanes can travel. To cover these distances, the plane has to gain height in between. Consequently, the pilots tries to fly where air is soaring in order to use these updrafts. In principle, there are three types of updrafts:

1.) Slope updrafts

At the beginning of gliding, only slope updrafts were used. If a wind goes toward a mountain, on the windward side of the mountain, the air has to soar. In this updraft, planes fly quite close to the mountain to gain height. At the end of the hillside, they turn around and keep on soaring.

2.) flying “wave”

With really strong winds, also on the lee of a mountain, an updraft system can occur. Depending on shape of the mountains and the wind strength, you can reach really hight altitudes using this “wave”. Even Central German Uplands such as the “Haardt” which is only a short hop from our airfield, height of over 5000 meters can be reached. What is more, the striking effect of flying without moving relatively to the ground while soaring can happen if the wind is fast enough. When flying wave, the air soars quietly and constantly in a certain spot, while the flight is quite bumpy around. The lee wave can be observed from the ground when seeing a characteristical, lenticular cloud, the Cumulus Lenticularis

Flug

3.) Thermals

Today, mostly thermal lifts are used to soar. If the sun heats the ground, also the air above the ground is getting warmer. Some surfaces get hot faster than others (e.g. fields, concrete surfaces, sunny side of the mountain). As the density of warm air is lower, the warm air will have to soar at some point. During soaring, the air is cooling down an the contained water has to condensate as it can contain less water at lower temperatures. This is when a cloud develops.

Pilots try to find the characteristical cumulus clouds that look like flying cauliflower, as they promise good thermals. Unfortunately, this effect only works when the air is wet enough. On dry days, there are still thermals, but the cumulus cloud cannot occur as there is not enough water for the clouds to occur. Consequently, these “blue thermals” are not really liked by pilots, as they are way more difficult to find.
Thermal updrafts (called “beards” among pilots) are quite narrow in many cases and can be imagined like hose going from the ground all the way up to the cloud. In this updraft, the pilots circles to gain height. Very often, the thermals are not very uniform so that the pilot has to adjust the bank of the airplane quite often. Beyond that, the air next to the updraft usually goes down, as there wouldn’t be any air left on the ground if the air was only going up.
When passing from an upwind to a downwind, the commonly known “airhole” occurs, as many flight passengers know it from airlines. Of course there is no real hole in the air 🙂

What if I don’t find enough thermals?

Sometimes, you just don’t find any more thermal or the weather has changed. Then, you have to land somewhere, even if there is no airfield in reach. For this case, you are well prepared in flight training for gliders. For an off-field landing, the pilot chooses an appropriate field to land. Especially at gliding competitions, where large distances are flown, off-field landings are quite common. In this case, the plane will be unmounted (can be done within some minutes by 2-3 people), put on a trailer and brought back to the airfield.

What is a sailplane made of?

In the early days, gliders were mostly made of wood. For fuselages and wings, a wooden framework was covered by a special fabric. Later, the fuselage was made of a stell space frame, covered by fabric as well. One main benefit of these planes is the easy maintenance and repair. This is why this type of construction can still be seen on many airfields, even though their performance is not comparable to newer architectures.

Modern gliders are mainly made using fiber reinforced polymer composites such as carbon or glass fiber reinforced epoxy. As the outside of these planes completely consists of a painted composite material, the shape of the planes can freely be chosen in a wide range and their surface is very sleek. What is more, wingspans of over 25 meters can be achieved!