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Gliders stay airborne by converting potential energy
(i.e., height) into kinetic energy (or forward motion).
The forward motion of the glider generates airflow
over the wings, creating lift. Moderns gliders create
lift very efficiently (by some very clever wing design);
generally speaking, the longer and thinner the wings,
the better. Furthermore, both the design of the wing
and the shape of the glider produce very little resistance
to airflow and therefore generate very little drag.
In fact, the performance of a glider is described
by its lift to drag ratio, or L/D, which also indicates
its glide angle. Thus, the glide angle is also controlled
by the speed of the glider - the faster it goes, the
more its glide angle decreases (it produces more drag).
Equally, if it flies too slowly, the wing itself starts
to create more drag and eventually stops working altogether
and the glider stalls. Every glider therefore has
an optimum speed range. Something shaped like a bus
with very short wings does not glide very well!
Most modern gliders have glide angles of better than
40 to 1, that is, for every 1 unit of distance they
descend, they move forward 40 units. Some of the very
high performance gliders have better than 60 to 1
glide angles. Moreover, they also achieve this at
quite a high speed. For instance, a glider called
a Nimbus 4 (which has a 25 + metre wing span), can,
from a height of 1 mile, glide well over 60 miles
at 60 mph before coming to earth. To put this in context,
if such a glider was 2 miles above Bristol (about
a third of the height that commercial jets fly at),
it could glide into Hyde Park in London. Smaller gliders,
such as a Discus, have a 15-metre wing span, but still
achieve a 42 to 1 glide angle. In comparison, your
average light aircraft (or 'spam can'), can only manage
something like 10 to 1 when the engine is switched
off. In fact, most planes will glide, but not very
well. The fastest, and perhaps the most famous glider,
is the shuttle. Now that is a ride!
Next: Flying Cross
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