Lift (soaring)

Lift, or more precisely "static lift", is rising air used by soaring birds and by humans in gliding, hang gliding and paragliding to make extended soaring flights. There are four principal types of lift:^[1] thermals, ridge lift, wave and convergence. Dynamic soaring has also been attempted, but only on a few occasions.

Thermals

Thermals are streams of rising air that are formed on the ground through the warming of the surface by sunlight.^[2] If the air contains enough moisture, the water will condense from the rising air and form cumulus clouds. This is the type of lift most commonly used in modern soaring. Once a thermal is encountered, the pilot usually flies in circles to keep the glider within the thermal, so gaining altitude before flying off to the next thermal and towards the destination. This is known as "thermalling". Climb rates depend on conditions, but rates of several meters per second are common. Thermals can also be formed in a line usually because of the wind or the terrain, creating cloud streets. These can allow the pilot to fly straight while climbing in continuous lift.

When the air has little moisture or when an inversion stops the warm air from rising high enough for the moisture to condense, thermals do not create cumulus clouds. Without clouds or dust devils to mark the thermals, the pilot must use his skill and luck to find them using a sensitive vertical speed indicator called a variometer that quickly indicates climbs or descents. Typical locations to find thermals are over towns, freshly ploughed fields and asphalt roads, but thermals are often hard to associate with any feature on the ground. Occasionally thermals are caused by the exhaust gases from power stations or by fires.

As it requires rising heated air, thermalling is only effective in mid-latitudes from spring through into late summer. During winter the solar heat can only create weak thermals, but ridge and wave lift can still be used during this period.

Although thermal lift was known to the Wright Brothers in 1901, it was not exploited by humans until 1921 by William Leusch at the Wasserkuppe in Germany.^[3] It was not until about 1930 that the use of thermals for soaring in gliders became commonplace.^[1]

Ridge lift

ridge soaring or Orographic lift is caused by rising air on the windward side of a slope allowing slope soaring. With winds of 20 to 25 knots (46 km/h), it is possible to soar at an altitude up to two times the height of the obstacle. It can also be augmented by thermals when the slopes also face the sun.^[4] In places where a steady wind blows, a ridge may allow virtually unlimited time aloft, though records for duration are no longer recognized because of the danger of exhaustion.^[5]

Wave lift

Atmospheric waves can carry gliders into the stratosphere. With a wind of 25 knots (46 km/h) blowing over an obstruction, and winds increasing with altitude, it is possible to reach heights of 10 times the obstacle height or more in the waves downwind. The powerfully rising and sinking air in mountain waves was discovered by a glider pilot, Wolf Hirth, in 1933.^[6] Gliders can sometimes climb in these waves to great altitudes, though pilots must use supplementary oxygen to avoid hypoxia. This lift is often marked by long, stationary lenticular (lens-shaped) clouds lying perpendicular to the wind.^[7] Mountain wave was used to set the current altitude record of 50,699 feet (15,453 m) on August 29, 2006 over El Calafate, Argentina. The pilots were Steve Fossett and Einar Enevoldson, who were wearing pressure suits.^[8] The current world distance record of 3,008 km (1,869 statute miles) by Klaus Ohlmann (set on 21 January 2003)^[9] was also flown using mountain waves in South America.

Convergence zones

The boundaries where two air masses meet are known as convergence zones.^[11] These can occur in sea breezes or in desert regions. In a sea-breeze front, cold air from the sea meets the warmer air from the land and creates a boundary like a shallow cold front along a shear line. This creates a narrow band of soarable lift with winds as light as 10 knots (19 km/h). Glider pilots can gain altitude by flying along the intersection as if it were a ridge of land. Convergence may occur over considerable distances and so may permit virtually straight flight while climbing.

Dynamic soaring

Glider pilots have been able to use a technique called "dynamic soaring",^[12] where a glider can gain kinetic energy by repeatedly crossing the boundary between air masses of different horizontal velocity. However, such zones of high "wind gradient" are usually too close to the ground to be used safely by gliders. Albatrosses and model gliders use this phenomenon.

References

^ ^a ^b Welch, John (1999). Van Sickle's Modern Airmanship. City: McGraw-Hill Professional, pp. 856-858. ISBN 0070696330. "There are four main kinds of lift which the soaring pilot may use...."
^ "Diagram of thermals". Retrieved on 2006-09-05.
^ Irving, Frank (1998). The Paths of Soaring Flight. City: World Scientific Publishing Company, p. 53. ISBN 1860940552. "Thermals were known to the Wright Brothers in 1901, but were first really discovered in 1921 by William Leusch at the Wasserkuppe..."
^ "Diagram of ridge lift". Retrieved on 2006-09-05.
^ "Duration record". Retrieved on 2006-08-24.
^ "Article about wave lift". Retrieved on 2006-09-28.
^ "Diagram of wave lift". Retrieved on 2006-09-05.
^ "Fossett, Enevoldson Bringing Record-Setting Glider to EAA AirVenture Oshkosh". Retrieved on 2008-01-11.
^ "Distance record". Retrieved on 2006-08-24.
^ "Morning Glory". Retrieved on 2006-09-27.
^ Bradbury, Tom (2000). Meteorology and Flight: Pilot's Guide to Weather (Flying & Gliding). A & C Black. ISBN 0-7136-4226-2.
^ Reichmann, Helmut (2005). Streckensegelflug. Motorbuch Verlag. ISBN 3-613-02479-9.

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