So How Does A Glider Stay Up?

So how is a glider able to stay up without an engine - what happened to gravity?

Well of course gravity operates on gliders just as you'd suppose, but in the process a glider converts its potential energy due to height into the kinetic energy required for flight, so instead of falling out of the air its aerofoils enable it to travel forwards as well as down. In other words a glider acts rather like a cyclist freewheeling downhill. The pilot can control the direction and forward speed and with it the rate of descent but there must always be an average vertically downward component with respect to the airmass in which the glider is flying. However the air is seldom completely still and if the airmass is rising faster than the glider is descending through it then the glider will gain height. A better analogy than the cyclist is now a man descending a rising elevator - by controlling his walking speed he can remain stationary in the vertical direction or lose or gain height.

A glider is a motorless aircraft which has been designed to have a very flat glide angle, which is the ratio of how far the glider can fly to how far it descends in the process. In slightly more technical terms the glide angle is equivalent to the ratio of the lift provided by the wings to the drag of the aircraft through the air. This is normally expressed as a ratio such as 40:1 which would mean that in still air the glider can fly 40 times further horizontally than the height which it loses vertically. Today's gliders typically have glide ratios of about 35:1 to 45:1 and some advanced models have glide ratios of around 60:1, so it is possible to fly a modern machine about 5-10 miles for a height loss of only 1000 feet. In contrast, an average powered light aircraft would have a glide ratio of about 12:1. A glider should not be confused with a hang-glider which is typically an open design with a poor glide ratio of nearer 15:1. A glider is a normal aircraft in all respects except the motor and with a normal cockpit and conventional controls. The pilot can steer and control the speed as in a powered aircraft, the only difference being that in this case the motive force comes from gravity and to avoid stalling the glider the nose will normally point a few degrees below the horizontal.

Somewhat unimaginatively, a glider pilot speaks of air which is rising faster than his glider is descending through it as providing 'lift'. Gliders typically have a minimum sink rate in still air of about 90-130 feet per minute (this is in the region of 1 to 1.5 miles per hour so quite a modest amount of upwards airflow is required). There are 3 main sources of this precious commodity:

1. Thermal Lift.

Depending on their makeup, certain areas of ground get warmer from the sun than others. For instance, brown fields warm up more quickly than water, urban areas more quickly than woodland and so on. These warmer areas differentially heat up the air above them which then becomes less dense and rises and is replaced by relatively cooler air sweeping in from all sides below it. Once started this action, which is known as a thermal, usually continues for quite a long time forming a continuous column of air. After a bit it's quite possible that the ground will have cooled again and there can be a delay before the thermal starts again, or it may start somewhere else, or not at all. Other sorts of heat sources such as power station cooling towers can also generate thermals but predominantly they are formed directly by the sun. More often than not the thermal is sufficiently wide for the glider to keep within, provided the glider is turned in fairly tight circles. By doing this the pilot can effectively spiral upwards. Thermals do not go on upwards forever. As the air rises its pressure falls and so therefore does its temperature and density and when it reaches equilibrium with the surrounding airmass it slowly peters out.

Often the air in the thermal is moist, but this moisture is not visible as it is in the form of a gas called water vapour which is colourless. However, if the air cools enough, which it does as it rises to higher levels, the moisture condenses and becomes visible in the form of tiny water droplets which form a cloud. So a thermal is often signposted by the fluffy cauliflower-shaped cumulous clouds which are so common on warm summer days. These clouds continuously form and dissipate and a glider pilot becomes experienced in recognising which clouds are growing (generally more solid-looking with a well-formed, perhaps concave dark underside), indicating that they are supported by an active thermal. The action of the water vapour condensing out gives the thermal another boost due to its latent heat of evaporation and as a result of this there can be quite an upward surge in the cloud itself which experienced pilots can use. Thermals need not have clouds to signpost them - this only happens when there is sufficient humidity - when the air is dry you get what are known as 'blue thermals' which are still useable but much harder to find! By flying from thermal to thermal the glider pilot is able to make his way across country, climbing on each thermal in turn and then gliding 'downhill' towards the next.

2. Hill Lift.

If a steady wind blows against an obstruction such as a hill the air is forced around and over it. The air which is forced up and over the top creates a band of rising air immediately in front of the hill. By flying along the front edge of the hill in this band of rising air the pilot can stay up. Depending on the strength of the wind, this lift can rise to 2 or 3 times the height of the feature which caused it. Of course the air will come down again behind the front edge of the hill and so this is an area best avoided.

The best kind of hill formation for this to happen is when the hill is long in the direction across the prevailing wind because in this case most of the air is forced over the top instead of being able to find its way around the sides. These kinds of hill formations are known as ridges and provide relatively effortless flying conditions.  For more experienced pilots it gives the opportunity to cruise around without loss of height whilst looking for the best conditions to make a start to a cross-country flight

3. Mountain Wave.

Under certain meteorological conditions, once the air has gone over the top of the hill or mountain, it cascades down the other side (the leeward side) with considerable energy and then 'bounces' up again. This process can be repeated and go on for a considerable distance behind the obstruction, the oscillations in the airstream being rather like those which you could form by rapidly jerking one end of a long rope up and down.

In this airstream the wave oscillations often go to great heights - far greater than that of the hills or mountains originally causing them - as the air above the first layer follows its actions up and down. By flying in the up-going parts of the wave system the glider pilot may be able to climb to considerable heights, or he may travel many miles along the wave system parallel to it. Wave lift is generally characterised by powerful but smooth upcurrents rising to many miles in the air (or if you are in the wrong place equally powerful downdraughts!) and gliders have been known to climb to the height of passenger jet aircraft in these systems - the world glider altitude record is almost 50,000 feet! There is a project underway (the Perlan project) in which Steve Fossett, perhaps better known for his balloon exploits, plans to fly a specially-designed glider to 100,000 feet.

Although wave systems may sometimes be invisible, more often than not they are marked along their length by long clouds (lenticulars) which lie approximately across the wind direction and which may be characteristically very smooth and even in shape. These clouds tend to remain static even though there may be a strong wind and they continuously form and evaporate on their windward and lee faces respectively.

Wave systems can become extremely complex. The wind can vary in both strength and direction at different altitudes as different airmasses are encountered and wave systems can build in different orientations above the primary lower one. Unlike thermal or hill lift, wave conditions can be dramatically affected by slight changes in the atmosphere resulting in abrupt phase change or even total collapse. A phase change can result in a glider which had been in a strongly rising air current suddenly being in a strongly descending one and prompt action can be needed to avoid an unexpected enforced landing.

Other Sources of Lift

Aside from the three main lift sources above, a number of other atmospheric effects are worth knowing and can sometimes provide enough lift to make that little essential bit of difference. For instance:

Different air masses have a curious unwillingness to mix (this is one of the reasons why thermals do what they do instead of simply rapidly equalising with their surroundings). As a result of this when air masses converge there is a tendency for the less dense to ride over the other producing an effect not unlike hill lift. This can happen for instance when a convection-driven sea breeze meets a prevailing wind and can form a ridge of rising air along the incoming front. Unlike hill lift this is not fixed in position but generally moves slowly inland during the afternoon and evening, and also unlike hill lift its likely position can be hard to determine visually depending on how different the visibilities in the 2 air masses may be.

In the same way as a thermal sucks in colder air from around itself, precipitation can force warmer air out from beneath and it is possible to find quite strong lift in front of a squall. Of course this is not always the best place to be and a sharp eye needs to be kept open for a safe escape and to avoid being sucked into the oncoming cloud.

During the evening it is possible for the upper slopes of a hill to cool more rapidly than the valley below due to radiation effects with the opposite being the case during the daytime. As a result of this katabatic winds (down the slope) and anabatic winds (up the slope) can occur. In the UK these are normally slight effects and can only be detected for short periods quite close to the surface, but in some mountainous parts of the world these effects can be powerful and sustained (the French Mistral is a katabatic wind).

Page last updated by on 28/11/2009