How Do Gliders Fly?

Like all other aircraft, the basic principle is to keep the pressure of the air above the wings lower than that below them and literally hold yourself up by suction. To do this needs forward motion, and without the luxury of an engine the glider has to follow a downward sloping flight path. The steeper this is, the faster the glider goes. Modern gliders are exceedingly high-tech pieces of equipment, capable of speeds of up to 150 knots (165mph approx) and have incredible performance, some capable of gliding up to 10 miles for every 1000ft of height they have. They truly have to be experienced to be believed.

Launching

As the majority of gliders are engineless, they require other means in which to get airbourne. There are 2 common methods:

Winching

Probably the most common method of launch in the UK today. Cheap and efficient winch launching is particularly suited to pre-solo training. Winches comprise of anywhere between 1 and 6 drums holding several thousand yards of multi strand cable at the opposite end of the Airfield to the launch point. The cables are towed using a tractor or off road vehicle to the waiting gliders at the launch point. A cable is then attached, via a weak link, onto a belly hook underneath the glider and after the appropriate commands the winch driver will slowly take up any slack in the cable before initiating the launch. The glider rapidly accelerates from 0 to 60 mph in about 3 seconds lifting off after a short ground run. The pilot will maintain a shallow climb until a safe height is reached before rotating into the full climb of about 30 – 45 degrees. The height gained prior to release at the top of the launch depends on the glider, wind speed and the length of the cable run but is typically about 1000 – 1500 feet (heights in excess of 2000 feet can be reached under the right circumstances). The whole launch is over in less than a minute and is generally considered to be one of the most entertaining launch methods!

Winching

Winching

Aerotowing

Another common method of launching gliders in the UK. More sedate then the winch launch it is however much more expensive as the cost of maintaining and running the tug aircraft has to be covered. The tug, such as a Pawnee, Robin or Chipmunk, is attached to the glider by a stretch of rope a hundred feet or so in length. The glider is then towed along the ground until it has enough speed to lift off. At this stage the tug will still be on its ground run so the glider pilot must fly behind the tug maintaining just a few feet of height until the tug is airborne and begins the climb out. The glider and tug maintain formation until the glider releases in lift or reaches release altitude, normally two or three thousand feet. Aerotows have the significant advantage of a release point which can be remote from the airfield allowing the experienced tug pilot to take the glider to where the lift is. The extra height is also useful on non-soarable days for early instructional flights due to the increased flight length and for those aerobatic ‘hooligan’ flights.

Aerotowing

Aerotowing

Soaring

To stay in the air for long periods of time, a glider requires sources of rising air, commonly referred to as lift. If the air is going up faster than the glider is going down, the glider will also rise. There are three main types of lift that can be utilised

Using a Thermal

Using a Thermal

Thermal Lift

The most common type of lift is the ‘thermal’:- a rising current of warm air that has been heated by the sun. If you have ever seen gliders all circling together on a summers day they are probably all climbing in a thermal. As the sun heats the ground, the air near the surface gets hot. Once the air is warm enough, it will start to form a bubble and rise. As the air rises, it start to cool, until eventually it is at the same temperature as the surrounding air. Depending on the weather, the bubble may rise as much as 8,000ft. The bubble will have areas of lift on the inside (at the core) and associated areas of sink on the outside. By circling in the core, you can use the rising air without flying thorugh the sinking air. Although thermals are weather dependant, they can be experienced for the majority of the year, with the ‘season’ being March-October. Thermals are generally very easy to find, and you can be taught from an early stage how to use them, making you flights longer and more enjoyable.

How a thermal is created

How a thermal is created

Ridge Lift

Gliders can also use the into-wind side of mountains and hillsides to find rising air. As the wind approachs the side of the mountain, or ridge, the terrain forces the air upwards. By flying close to the side of the mountain in this rising air, you will gain height. As ridge lift is not temperature dependant, it will occur all year round. icGC carry out a winter expidition to a UK ridge site very year, but ridge lift can also be experienced more locally on the South Downs, which are within gliding range of Lasham. By utilising ridge lift you can travel thousands of kilometres around mountain ranges. The current world distance record of 3500km was achieved using ridge lift in the Andes, South America.

Ridge Lift

Ridge Lift

Wave

Wave is the most unusual type of lift because it is very dependant on the landscape. However, using wave it is possible for height gains in excess of 20,000ft. As the wind passes over a mountain range, the currents start to oscillate. The wavelength and magnitude of the oscillations are dependant on the terrain over which the air has just passed. By flying in the up flowing wave bars, very strong lift can be experienced. Whilst there are only certain places that experience wave lift, there are numerous gliding clubs that have been located in good wave areas. Whilst icGC do not also go to these sites, Lasham organise 3 trips to wave sites every year, 2 to Jaca in Spain and one to Aboyne in Scotland. As an icGC member you are able to sign up for these trips.

Wave Lift

Wave Lift