Cross Country
by Dave Bromley, derived from an article written by Al Nunn
In previous articles you have seen how gliders can stay up, this article looks at how gliders cover distance; that is to go cross-country. In the early days cross-country flying often involved seeing who could fly the furthest from their starting point. This is called 'free-distance' and the result is the pilot, and glider, need to be recovered from their eventual location, potentially many hundreds of miles away from their start point. For this reason modern cross-country flying now generally comprises of closed tasks, often several hundreds of kilometres in length and at average speeds in excess of 100 kilometres per hour. These tasks use a number of turning points to define each leg of the route, usually a visible ground feature such as a cathedral or motorway junction are used as turning points. The weather forecast and the performance of the glider, as well as the skill of the pilot, dictate the length of the task.
How does a glider fly cross-country?
Very simply, by finding air which is going up faster than it is going down! In short, a glider gains height (and therefore, its potential energy) by flying in up-currents of air that are rising faster than it is descending. Although glider racing generally uses thermals, slope and wave lift can also be used.
Thermal cross-country
Thermals are basically rising currents of warm air. When air near the ground is heated to a higher temperature than its surroundings, it will tend to rise when disturbed. It will continue to rise if the air around it is cooler, as it is less dense. For instance a cornfield, or a town, will tend to generate and trap a lot of warm air in the sunshine. When the warmer air is gently blown towards another feature, such as line of trees, or a lake (which has cooler air over it), it will then separate and rise. The best days are often those that have fair weather cumulus; a cumulus cloud is formed by the water in the rising air condensing out as the air cools. However, even on cloudless days (called 'blue days'), there are usually thermals. There are no clouds because the rising air doesn't cool enough to form a cloud (or it contains too little water).
These thermals supply the energy a glider pilot needs to go cross-country. On a good day, thermals can be found every few miles, depending on the terrain and how high they are going. Generally speaking, the higher the better. In the UK, this can be anything from 1,000ft to 10,000ft, depending on the time of year, how much moisture is in the air and how cold the air is. Unfortunately, what goes up, must come down. Between the thermals, the air is usually coming down (and is known as 'sink'), so pilots try and minimise the time they spend in the sink and maximise the time they spend in the 'lift'. Thus, to go cross-country, a pilot will circle in the rising air to gain height, then cruise at a higher speed to the next thermal (and will go especially fast in the sink, thus reducing the time in bad air). The speed at which the pilot goes cross-country depends on many factors.
Cross-Country Competitions
It may surprise you, but gliding can be a competitive sport. Ever since man first took to the air, intrepid pilots have tried to out-do each other. Initially with gliding, it was who could fly the furthest, the highest, and the longest. However, as gliders got better, speed became important and duration less so: people ended up soaring hills for days, only stopping when they crashed after falling asleep. Not surprisingly, duration as competitive sport has tended to die out. Although there are still distance, speed and height records to be broken, when competitive gliding is discussed, it is usually referring to pilots racing against each other to see who can complete a cross-country task the fastest.
Racing cross-country
The biggest single deciding factor on how fast a glider can fly around a given task is the strength of the thermals (and thus, the available energy). The stronger the climbs, the faster height is gained, and the quicker the pilot can fly to the next thermal (the inter-thermal cruising speed can also be increased to make use of the extra energy). The geographical pattern of the thermals is also extremely important, as this can lead to a type of flying called 'dolphin flying'. Rather than stopping and circling in lift, the pilot more or less flies in a straight line, slowing down in the thermal to regain lost energy from the glide and speeding up in the sink on the other side. If the conditions are right, the glider can gain enough energy from each thermal and maintain its overall height for long distances without circling. Thus, viewed from the side, the glider dives down to gain speed (to get through the sinking air as quickly as possible), and then pulls up in the rising air to maximise the recovery of energy (thus mimicking the motions of a dolphin). Windy days are good for this, as the thermals often line up into 'streets', enabling the pilot to race along an aerial motorway without having to turn.
Another method of going faster, which is often used on good days, is to make the glider heavier by filling it with water (as much as 400lbs in some cases). Although this may sound counter-intuitive, it enables the glider to fly faster for the same glide ratio (the glider also comes down faster, but the same glide angle now occurs at a higher speed). In effect, a heavier glider can pull more energy out of the system and convert it into extra speed. The penalty of this is that it will not climb so well, but if the conditions are right for dolphining, or the thermals are big and strong, then the benefits of a higher cruising speed greatly outweigh the penalties.
Competitions
A competition normally consists of up to 50 gliders. Each day, a briefing is held to give the pilots the days task, the weather and any airspace restrictions they need to avoid. The aircraft themselves are formed into a grid at the airfield ready to be launched.
As soon as the gliders can safely stay airborne, they are stream launched as fast as possible by being towed up by numerous light aircraft (or 'tugs'). The gliders have to release at 2000 ft, usually in a similar predetermined geographical position. With good organisation, the grid can be launched in well under an hour.
Once all gliders are airborne, a start line (or zone) will be opened to allow gliders to set off on their task. Once opened, it is up to the pilot to decide when to start. Pilots are timed as starting from their last point in the start zone. The start time is critical and many pilots often play a waiting game. Starting a bit later than other competitors may often confer an advantage, both mentally (starting 5 minutes after your main rival and then catching him is very demoralising for your opponent!), as well as physically (pilots who started earlier often mark the good thermals, so saving the later pilot from wasting his or her time in weaker ones). However, starting too late may also be a mistake, as the weather may deteriorate and the pilot may not get back.
Tasks themselves vary in length and in shape. Competition task can range from 100km to over 500 km depending on how good the day is, and can take many hours to complete. Pilots use GPS to prove they have completed the task correctly, rounding all the required turning points. Gliders also carry barographs (which continuously record height) to make sure that they have not landed halfway around and taken another launch.
As the pilot get close to the end of the task they will reach the point at which the glider is able to glide back to the airfield finish line without needed to take another climb. This is called the final-glide and is an art in itself. The pilot has to determine when he or she has enough height to safely glide home. If the pilot is too cautious and takes more height than they really need, then they waste valuable time, equally, if they don't take enough, then they risk landing short of the airfield. One of the most exhilarating moments of the flight is the finish itself. If the pilot has judged it right, they will cross the finish line low and fast (often over 150 mph) to do a spectacular pull up and small circuit before landing (although, ideally, if they have judged their final glide absolutely perfectly, then they will cross the finish line with just enough energy to do a safe landing ahead). At the same time, if they have water on-board they will dump this ballast before landing, with a spectacular water trail streaming from the glider. It is the moment that most competition pilots live for! After several hours of complete concentration, tactical decisions, weather interpretation and non stop flying, the relief and exhilaration of passing the finish line is unequalled.
Winning
Very simply, the pilot who only flies in the strongest thermals and does not waste time in weak ones, who flies at the optimum speed between the thermals, who follows the straightest course while using the best energy lines (i.e., minimises the time spent in air going down), and when near home, gauges the correct height to make his final glide back to base, will often complete the task the fastest and win the day in a competition. But to win a whole competition, a pilot has to fly consistently over several days. The best pilots are the ones who win by not making mistakes. Quite often, competitions are won or lost by seconds over nine days of flying! Taking one weak thermal, or even a single turn in the wrong place, can lose the competition.