How do Gliders Fly?
Gliders fly using the same
aerodynamic principles as any other aircraft. The gliders wings create
lift as they move through the air. The only thing that differentiates
a glider from a powered aircraft is that, whereas the powered craft
uses its motor to move it through the air, the glider uses that old
standby, gravity. In still air, a glider is always sinking and this
is why glider pilots concentrate on finding rising air commonly
known as lift - to counteract the effects of gravity and
stay airborne for as long as possible.
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Operating a glider
is not as difficult as you may suppose. The controls quite simple
to understand. As you sit in the glider, between your legs is
a control column or stick.
This is directly connected
to the flaps on the tail and the wings. At your feet you have
two pedals which are connected to the rudder. Let me explain the
use of the stick first. If you push the stick forward the glider
will point down to the ground (as if going down a rollercoaster).
As with anything as you go down hill you pick up speed. Now if
you pull it back the glider will slow down, untill it flies too
slow and the glider falls out of the sky (don't panic its easy
to restart them flying again). If you move the stick from side
to side the glider will bank to the left or right. This will turn
the glider, however the glider will tend to try and fly sideways
through the sky which is not very efficient!
The solution to this
is to bank the glider and use the rudder (controlled by your feet)
together. This is called co-ordinated turn and is one of the first
things you learn when flying a glider.
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Most people are amazed
that gliders don't just sink to the ground. One reason is that
they are amongst the most efficient flying machines next to the
birds. The efficiency of a glider is expressed by the ratio of
height and distance. For example, a glider with a glide angle
of 1:30 when the glider is 1 mile high, means the glider can fly
for 30 miles. However in reality this can be much less becouse
of sink (falling air).
On
a good flying day with plenty of lift (see below for and explanation)
you can easily fly between thermals and travel across the country.
I've set up a game to demonstrate
this if you fancy a play. Please be aware the game is addictive
and you may end up playing for hours!!!
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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 by 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.
If you would like to
try this yourself play a game
of flight club!
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 just over 1000
kilometres and took about 12 hours.
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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.
See
this dune running video
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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" cloud.
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 3008 kilometres (1880 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!
See
the wave flying video (sorry this is quite large)
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Now we
come to landing. As you have learnt gliders can stay in the air
for a long time and travel great distances. However some people
are a little paranoid about getting back to the terra firma.
Your instructor
will be a highly experienced glider pilot with hours and hours
of flying time. In his flying career he will have made many, many
take offs and landing. During your flight he will not fly further
away from the airfild than he could glide back without finding
any lift. However suppose he was to fly accross country and find
him or herself out of flying distance of an airfield. No problem....
glider piolts are some of the most highly skilled pilots and as
part of their training is the ability to land in fields. The only
problem then is to persuade other members of the club to retrieve
you and the glider!
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Another
good page on how gliders fly
Flight Club
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