|HOW DOES A GLIDER STAY UP?|
A glider is at all times losing energy due to drag, either sinking in the surrounding air or losing speed while climbing. In order to stay airborne energy must be obtained from another source. During the launch phase this is from the tug or winch. Once in free flight this must be from the surrounding environment. The only source currently used by gliders is energy adsorbed from rising air. If the air is rising faster than the glider is sinking, then the glider will remain airborne.
When a glider flies into an area of rising air the experienced pilot will actually feel the glider start to climb. Additionally there are instruments, such as the variometer, that will detect this and indicate it to the pilot. If flying adjacent to a land feature, or another aircraft, it can be seen by the feature falling away.
Rising air is found in the environment in a number of situations. The following are examples of the commonly used sources of rising air, or lift used by gliders.
The most commonly used source of lift for gliders, especially for cross-country flying, is the thermal.
When the sun heats the ground the adjacent layer of air is heated as well. This air expands, becomes less dense and more buoyant. It eventually breaks away from the heated ground layer to form an isolated bubble of rising air. The air on the edge of the bubble rises more slowly than that in the centre due to entrapment with the surrounding cooler air acquiring a vortex ring motion as illustrated in the adjacent schematic.
The bubble, known as a thermal rises as a whole within the atmosphere. As the bubble rises it expands with lowering air pressure, resulting in it cooling. Eventually the bubble's ascent is stopped by the air cooling to that of the surrounding air, or by encountering a layer of warmer air; an inversion.
If the rising bubble contains sufficient moisture, and the bubble rises sufficiently high, and hence cools, the contained water vapour will condense. This appears as the characteristic flat bottomed, round topped cumulus cloud.
A single trigger point on the ground may cause a number of thermal bubbles to be formed consecutively. This will result in a line of thermals, and possibly cumulus clouds, to be formed downwind. These are known as thermal streets or streeting.
The difficulty for the pilot is in finding the thermal in the first place, and then locating the core of the thermal (they are not as regular as implied). A number of environmental clues can be used to locate thermals. Ground features such as dark paddocks, industrial complex's, or ridges often act as the triggers for thermal bubbles to generate. In the air a number of bird species such as eagles or ibis are visible circling in the thermal (and invariably in the strongest part of the core). Dust devils are a less common marker for thermals. The cumulus cloud are a very visible marker for thermals, though only once they have risen to sufficient height.
The glider pilot on detecting a thermal will try to find the the central core of most rapidly rising air. When located, the glider will circle in this core and thus be carried first to the top of the thermal, and eventually with the bubble to the extent of its ascent.
Ridge lift, being more predictable, was the first form of lift discovered. It also has the advantage of not needing the heat of the sun, and in fact can continue overnight, a feature exploited by many of the early endurance record flights (before they were banned after a number of pilots went to sleep at the controls).
As the wind strikes a hill it will rise over the windward side of the hill. If the rising air is sufficiently strong to support a glider in flight, it is known as ridge or slope lift. The glider can fly along the windward slope climbing in the rising air.
This form of lift is obviously limited by the length and height of the hill, and the wind, but can be a trigger for thermal lift. Even so with some of the the longer mountain ranges of the United States and New Zealand very long flights have been made in ridge lift.
As illustrated, the air on the lee side of the hill is sinking so this area is avoided by the pilot. Also air immediately adjacent to the slope may be flowing in a turbulent manner, an eddy and also needs to be avoided.
Fast moving water flowing over a rock in a stream will lift up over the rock then dip down over the downstream side and then rise and fall a number of times as it springs up and down, forming rows of stationary ripples downstream.
Similarly a strong wind flowing over a mountain range will set up a wave motion in the atmosphere. These lee waves can extend to very great heights, and can be used by gliders. Most of the glider altitude records have been set in these conditions. The rising air, if moist, may form clouds due to cooling of the rising air. These have a characteristic lens or lenticular shape.
The lift encountered in these conditions is characteristically very smooth. However below the lee waves the airflow may be very turbulent, amongst the most turbulent conditions encountered, the so called rotor.
Wave system can extend for the length of the mountain range generating them. One flight in wave conditions in New Zealand extended from the southern tip of the South Island, to the north of the North Island, and back to the starting point, a distance of over two thousand kilometers.
Other forms of lift in the atmosphere occur and are occasionally used by glider pilots.
A mass of air of a different temperature; a front; will displace the air in front of it. If the air in front is warmer it will be displaced upwards, forming frontal lift.
A specific form of frontal lift is the sea-breeze front. As the air over land heats up the air rises sucking the colder air over the adjacent sea in over the land. This colder air displaces the warmer air up in front of it. Unfortunately the colder air suppresses any subsequent thermal activity for a period of time. Sea breeze fronts are relatively common at the Bacchus Marsh site used by Geelong Gliding Club.
Where two airstreams meet, such as may be formed by a hill in a valley splitting the airstream, then one stream may climb up over the other; orthographic lift. Due to the specific land features needed this is uncommon, but predictable.
Anabatic lift is a special form of thermal lift. The sloping hillside facing the sun will heat faster than the adjacent valley floor. The air will then tend to flow up the hillside, even without an external wind, forming lift.
As water vapour condenses it release latent heat boosting the temperature of the air in the base of the cloud. This causes convection currents in the cloud boosting the thermal feeding it. To use this lift the glider needs to fly within the cloud, which is illegal in Australia, but often practised overseas.
If the wind above a thermal street is aligned against the street it can generate lee waves in the same manner as mountain waves. These will extend to heights well above the cumulus clouds generating them.
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