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Convective clouds form when the sun heats up a thin layer of ai 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.
Convective clouds form when the sun heats up a thin layer of ai 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.


Glider pilots can enter the thermals circle in them to gain altitude. A good thermal might have a radius of about 1000 m and provide 1 - 3 m/s of lift. However, where there is lift, there is also sink: thermals are usually surrounded by a region of sinking airmass, and the convective upward motion of air also creates turbulence.
Glider pilots can enter the thermals circle in them to gain altitude. A good thermal might have a radius of about 1000 m and provide 1 - 3 m/s of lift. In general, that means that one has to fly quite tight turns - a 45 degrees bank angle isn't very unusual in a thermal. However, where there is lift, there is also sink: thermals are usually surrounded by a region of sinking airmass, and the convective upward motion of air also creates turbulence.





Revision as of 11:58, 17 July 2012

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.[1]

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.

Note on units: In many areas of the world, glider pilots use metric units and instruments show altitude in meters and velocity in m/s - we will in this article follow this convention.

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.

The glide ratio isn't a constant however but depends on the airspeed. Each glider has an optimum airspeed at which the glide ratio is maximal (usually between 90 and 120 km/h). The glider can fly faster, but this increases the sinkrate. The glider can also fly slower, and this decreases the sinkrate, but since the airspeed also drops, the glide ratio can still deviate from the optimum. In still air, a glider must thus be flown at its best glide ratio for best performance.

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.

This can have dramatic consequences for hanggliders with much lower optimum speeds - a hangglider flying at 35 km/h against a 35 km/h headwind remains stationary in the air and has an effective glide ratio of zero - in order to get anywhere, it must fly faster. For slow gliders, it is crucial to factor the wind direction and strength into any decision.

In general, in order to have the best effective glide ratio, one must fly somewhat above optimum speed against the wind and below optimum speed with the wind.

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.

The rules derived from these examples are: Fly slower than optimum speed in rising air, faster than optimum speed in sinking air. If your minimum sinkrate is equal to the lift, you do not lose any energy at all. Consequently, in any airmass providing net lift, the glider is flown at the velocity of minimum sinkrate (which is always lower than the optimum speed) to maximize the altitude gain. This makes it also easier to fly tight turns in small thermals.

In practice, wind and lift tend to be variable and one doesn't calculate the glide ratio but just estimates based on experience.

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.

Thermals

Convective clouds form when the sun heats up a thin layer of ai 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.

Glider pilots can enter the thermals circle in them to gain altitude. A good thermal might have a radius of about 1000 m and provide 1 - 3 m/s of lift. In general, that means that one has to fly quite tight turns - a 45 degrees bank angle isn't very unusual in a thermal. However, where there is lift, there is also sink: thermals are usually surrounded by a region of sinking airmass, and the convective upward motion of air also creates turbulence.


Strong Cumulus development indicating good thermals Entering a cap cloud reduces visibility dramatically

While the lift column reaches deep inside the cap cloud, a glider can usually only climb to the cloudbase. Entering the cap cloud is quite dangerous, as the visibility deteriorates rapidly and orientation can be completely lost. The Cumulonimbus clouds characteristic for thunderstorms have very powerful updrafts of air, but for obvious reasons (strong turbulence, possibility of hail and icing,...) they should not be used by glider pilots.

Gliders in FlightGear

FlightGear has several glider models and winch, AI aerotow or MP aerotow launching methods (in addition to the normal "in-air start" method).

Winch launches

Winch launches are currently available with the Bocian, the ASK21, the ASK-13 and the DG-101G. With the Bocian, it is possible to click in the scenery on a point where you would like to place a winch; with both, you can use Ctrl-w to place a winch directly in front of the glider. Press w to start the launch (in the ASK you need to hold it down) and, once at the top of the tow, release the cable with W.

Aerotows

For aerotows, two types are possible - AI or human pilot (via multiplayer). To get an AI aerotow, select either the ASK, Bocian or DG-101G, choose KRHV as your airport and select the KRHV_towing_demo in the "Scenario" list box in FGRun. You should see a J3 Cub wobble its way towards you from a nearby taxiway, and pause close to your aircraft. Press control-o to hook on to it, and hold tight... the O key releases the cable.

For a multiplayer aerotow, you obviously need to arrange a tow with a human pilot - full instructions are available at doing aerotow over the net.

The DG-101G implements a third type for aerotowing: a drag robot. To setup the drag robot press D. Then use the key sequenze as for AI aerotowing to attach to the drag robot. Then press d to start the robot.

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.

Thermals and sinks

Schleicher ASK 21 gliding in the Pinzgauer Spaziergang thermals scenario

Thermals and sinks are modeled, but they must be defined individually in a thermal scenario file. To see how this is done it would be best to examine the file called $FG ROOT/AI/thermal_demo.xml, 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 weather system, so it's possible to have cloud layers that don't match your thermal heights. To prevent this you may want to manually set the cloud layers to match your thermals. Thermal cap clouds are available since about one month after 0.9.10 was released. If you are using 0.9.10 or earlier you can make cap clouds work by a) fetching the cap cloud model from CVS, and b) adding a <z-m> offset to the cap cloud wrapper file to put the cloud at the right altitude.

Extra soaring locations

If you'd like to discover the Austrian alpine region, you might want to read Pinzgauer Spaziergang.

The 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. Thermals and cap clouds optionally also evolve in time.

Learn the theory

For those wishing to gain a more in-depth knowledge of correct glider operation, the FAA glider handbook makes good reading.

Related content

References