Soaring: Difference between revisions

[[File:Aerotow2.jpg|thumb|270px|[[Bocian]] being towed by a [[Piper J3 Cub]].]]

'''Gliding''' or '''soaring''' is a recreational activity and competitive air sport in which pilots fly unpowered [[aircraft]] known as gliders or sailplanes using naturally occurring currents of rising air in the atmosphere to remain airborne.<ref>{{cite web |url=http://en.wikipedia.org/wiki/Gliding |title=Gliding |work=Wikipedia }}</ref>

In a broader sense, this article covers how to enjoy flights with gliders, motorgliders, hanggliders and paragliders by understanding how to plan unpowered flights, how to set uo weather conditions for reasonable lift, how to launch gliders and how to make use of lifting airmasses.  

== Basics of glider flight ==

Any aircraft in flight at constant airspeed must balance thrust against drag and lift against gravity. Usually, the engine supplies the required energy. A glider instead uses its altitude (i.e. potential energy) to overcome drag and gravity. Thus, from any given altitude, a glider can only get so far. The glide ratio determines how far a glider gets for losing altitude. A high-performance plane with a glide ratio of 1:50 for instance can cover 50 m for every  meter altitude lost.  

=== Influence of wind ===

Wind has a crucial influence on the glide ratio that can be achieved. Usually a glider pilot is interested if he will reach a certain location on the ground (a good airfield to land, a mountain pass to cross a range,...). Suppose we have 1000 m altitude to spare and an optimum glide ratio of 1:40 at 110 km/h - this means we would reach an airfield 40 km away. However, with a 40 km/h headwind our groundspeed will just be 70 km/h and thus the effective glide ratio will be reduced by 70/110 to about 1:25 even maintaining the optimum airspeed.  

=== Influence of lift and sink ===

The vertical motion of air has likewise a pronounced influence on the effective glide ratio. Assume we're in an airmass rising at 0.5 m/s flying again at 110 km/h. This increases the glide ratio by 1/0.5 to 1:80 because we're actually sinking only 0.5 m/s. However, we can do better - suppose we reduce the airspeed to 70 km/h to get the sinkrate to 0.7 m/s. In still air, this would correspond to a glide ratio of 1:36, i.e. not worth doing. But since the air rises, efectively we sink only 0.2 m/s now, and so the glide ratio goes to about 1:125. The opposite is true in sinking air.

== Sources of lift ==

There are three main sources of lift for gliders: Thermals, ridge lift and wave lift. All of them are (in principle) available in FlightGear, when using the [[A local weather system|advanced weather system]] (previously called "local weather").

=== Thermals ===

Convective clouds form when the sun heats up a thin layer of air in contact with the ground. As warm air is lighter than cold air, eventually the situation becomes unstable and pockets of air start rising, creating a column of lifting air, a thermal. If this column reaches above the condensation level, a Cumulus cap cloud forms, marking its position.

=== Ridge lift ===

When a sufficiently strong wind meets rising terrain, the airstream is forced upward and thus a lift component is created at the windward slopes of a range. However, behind the ridge, the airstream turns down into the valley, and thus at the leeward side of a range a strong sink appears. For sufficient ridge lift conditions, winds stronger than 10 kt need to be perpendicular to a slope. Stronger winds create stronger lift, but make flight planning in general more difficult (see above).

[[File:Ridge_lift01.jpg|400px|Riding the ridge]]

[[File:Ridge_lift02.jpg|400px|Staying close to the ground to get ridge lift]]

=== Wave lift ===

When, in strong winds, the air descends behind a mountain range, it can 'bounce back' from the ground and form a pattern of rising and falling waves in the lee of a mountain range. These waves provide lift with almost no turbulence which can reach very high - more than 10 km altitude have been reached in waves.  

== Setting up a glider flight ==

With a glider, it is a good idea to have a firm idea what you want to do before starting FlightGear, since you have to make some choices in the commandline. It's also not a bad idea to think about what type of lift you want to utilize - ridge lift is only available in mountain regions, you need to be able to reach it from your starting location and thermal lift is strongest in the afternoon and not available over open water.

=== Starting location ===

Not every glider can be started from a runway, so you may need to add extra statements to the commandline. One possibility is to start the glider in the air by adding altitude and airspeed like

(this is close to Half Moon Bay in the default scenery around San Francisco).

=== Launch methods ===

Most available gliders do not need to be initialized in the  air but can be launched as in reality by either winch or aerotow.  

==== From the ground ====

The JSBSim version of the airwaveXtreme is launched from the ground by running down a steep slope. Refer to the aircraft documentation and help for details.

==== In air ====

Gliders that use the [[UIUC]] FDM are not (yet) capable of winch or aerotow launches. For such gliders it is necessary to start in the air.

== Setting the weather ==

The next step is to configure weather. In reality, you would take a look at the weather and decide what is possible to do with a glider. In Flightgear, it is possible to choose the weather based on what you would like to do. Dependent on the weather system used, the steps are a bit different. In general, it is recommended to use Advanced Weather for glider flights.

=== Basic Weather ===

[[File:Pinzgauer.jpg|thumb|270px|[[Schleicher ASK 21]] gliding in the [[Pinzgauer Spaziergang]] thermals scenario]]

In Basic Weather, thermals and sinks can be used but must be defined individually in a thermal scenario file. To see how this is done it would be best to examine the file called <tt>[[$FG ROOT]]/AI/thermal_demo.xml</tt>, which sets up 11 thermals and 6 sinks around San Francisco Bay. To learn more about AI scenarios in general, see the related article called [[AI Systems]]. Note that the thermals and sinks exist independently of FlightGear's Basic Weather system, so it's possible to have cloud layers that don't match your thermal heights or thermals which do not move with the wind. To prevent this you may want to manually set the cloud layers to match your thermals and match AI-defined winds with the winds set in the weather. Also, cap clouds do not match the default clouds in shape.

=== Advanced Weather ===

The [[A local weather system |Local Weather]] package (for Flightgear 2.0.x and 2.4.x) or Advanced Weather (2.6.x) has the option to automatically generate thermals along with the convective clouds. For this, the checkbox 'generate thermals' has to be selected. The slider 'thermal properties' modifies the behaviour of the thermals - for 'low convection' it will generate little turbulence, thermals with a large radius and only modest lift, for 'rough day' it will generate strong lift and turbulence in narrow thermals.

== Learn the theory ==

For those wishing to gain a more in-depth knowledge of correct glider operation, the [http://www.faa.gov/regulations_policies/handbooks_manuals/aircraft/glider_handbook/ FAA glider handbook] makes good reading.

== Related content ==

* [[Improving Glider Realism]]

[[Category:Aviation]]