The word soaring is often used interchangeably
with gliding when referring to the sport. However, more precisely
soaring is the art of using air currents to stay airborne.
you get up there?
click in image to animate -
Winches or bungees can be
used to launch gliders and sailplanes off the sides of a hill. Nowadays,
aerotows are more common. A small powered aircraft tows the sailplane
upwards using a 200' long polypropylene rope, about 3/8" thick. Once the
planes reach 2000' (the common height for release) the sailplane releases
the rope and begins what is generally referred to as "free flight".
Lift in the atmosphere
occurs in three primary forms. Thermals are by far the most prevalent of
lifting sources available to the soaring pilot. They are generated by the
heat of the ground radiating upwards to the air directly above. The heated
air rises in a vertical column, almost donut shaped, and eventually
reaches the condensation level of the airmass, and a cloud is formed. By
circling inside this rising mass of air a glider pilot can gain altitude
as quickly as 1000 feet per minute, and can reach heights of 8000 feet on
a good day.
On a summer's day, you can see birds circle
upwards without flapping their wings. They are "thermalling". A
thermal is a volume of air that has been heated by the sun more than
the surrounding air - imagine if you were standing on some sunlit
concrete, you would feel warm! As you know, hot air rises, and it is
circling within this air that allows birds - and gliders - to go
upwards. Next you may ask "so how do you know where the thermals
are?". Well, sometimes this is educated guess work, based on how you
imagine ground features below you are warming up. However, often
cumulus (cotton-wool type) clouds form at the top of the thermal,
marking where the thermals are.
Occasionally, you may be joined in
the thermal by a bird - from a swift to an eagle. You can never do
it as well as they can, but it's great fun trying! Thermals are used
in cross-country flying - you climb in a thermal to gain the height
to move forwards to the next thermal on track (or thereabouts). The
largest flight in the UK was done like this. Just over 1000
kilometres were covered in the flight
which took about 12 hours.
Another way of staying up requires a hill
(ridge), and the wind to blow against the face of it. Try to imagine
this scenario - when the wind hits the hill, it gets forced upwards.
Again, it is this upward movement of air that allows gliders to stay
airborne. With a long ridge, it's possible to do large distances
without turning, generally flying fast and low to stay in the best
"lift" close to the ridge.
Similar to ridge lift is a phenomenon called
"wave lift". This is a little harder to imagine. It arises from the
wind blowing against a hill again, but this time the air comes back
down (on the far side of the hill) and "bounces" off the ground and
goes back up again creating a very smooth upwards flow of air.
Often, this form of lift is capped by a cigar shaped "lenticular"
This wave may go back down and up again for
several cycles, meaning that you don't actually have to be close to
any hills to use it! The furthest flights in gliders have been done
using this lift - the best being 2463 kilometres (1530 miles) which
was done along the Andes, all in one flight and one day. Wave lift
is also known to go very high - the world height record in a glider
is just a little short of 50,000 feet!
you get down?
Landing an airplane is the
same, whether you have an engine or not, but in soaring you only get one
shot at it!! Most pilots use a circuit to align themselves with the runway
they want to land on. They fly parallel to the runway (downwind leg), then
turn across the end (base leg), and then turn final and begin their
approach. As the sailplane is already descending the pilot only has to
modify his decent rate using dive brakes or spoilers. The pilot has to
correct for drift due to cross-winds and once done the pilot will flare
the plane and then touch down onto the runway. Momentum ensures that the
plane rolls evenly on its mainwheel. Only when the plane stops rolling
does a wing gently touch the ground.