In
its simplest form, a glider is an unpowered aircraft, an airplane without
a motor. While many of the same design, aerodynamic and piloting factors
that apply to powered airplanes also apply to gliders, that lack of a
motor changes a lot about how gliders work. Gliders are amazing and
graceful machines, and are about as close as humans can get to soaring
like birds.
From
paper airplanes to the space shuttle during re-entry, there are many types
of gliders. In this article, we will focus on the most common type of
glider, often referred to as a sailplane.
Parts of a Glider A glider
has many of the same parts as an airplane:
fuselage
wings
control surfaces
landing gear
But, there are significant differences in these
parts on a glider, so let's take a look at each.
Fuselage Gliders are as small and light as possible.
Since there is no large engine taking up space,
gliders are basically sized around the cargo they carry, usually one or
two people. The cockpit of a single-seat glider is small, but it is large
enough for most people to squeeze into. Instead of sitting upright, pilots
recline with their legs stretched out in front of them. The frontal
exposure of the pilot is reduced and the cross-sectional area of the
cockpit can be substantially smaller.
The glider's fibreglass
construction enables a sleek, smooth design
Gliders, along with most
other aircraft, are designed to have skins that are as smooth as possible
to allow the plane to slip more easily through the air. Early gliders were
constructed from wood covered with canvas. Later versions were constructed
from aluminium with structural aluminium skins that were much smoother.
However, the rivets and seams required by aluminium skins produce
additional drag, which tends to decrease performance. In many modern
gliders, composite construction using materials such as fibreglass and
carbon fibre are quickly replacing aluminium. Composite materials allow
aircraft designers to create seamless and rivet-less structures with
shapes that produce less drag.
Wings If you look at a glider next to a
conventional powered plane, you'll notice a significant difference in the
wings. While the wings of both are similar in general shape and function,
those on gliders are longer and narrower than those on conventional
aircraft. The slenderness of a wing is expressed as the aspect ratio,
which is calculated by dividing the square of the span of the wing by the
area of the wing.
Glider wings have very high
aspect ratios -- their span is very long compared to their width. This is
because drag created during the production of lift (known as induced drag)
can account for a significant portion of the total drag on a glider. One
way to increase the efficiency of a wing is to increase its aspect ratio.
Glider wings are very long and thin, which makes them efficient. They
produce less drag for the amount of lift they generate.
The aspect ratio of a
wing is the wingspan squared divided by the area of the wing. The glider
has a much larger aspect ratio than a conventional
plane
Why don't all planes have
wings with high aspect ratios? There are two reasons for this. The first
is that not all aircraft are designed for efficient flight. Military
fighters, for example, are designed with speed and manoeuvrability well
ahead of efficiency on the designer's list of priorities. Another reason
is that there are limits to how long and skinny a wing can get before it
is no longer able to carry the required loads.
Control
Surfaces Gliders use the same
control surfaces (movable sections of the wing and tail) that are found on
conventional planes to control the direction of flight. The ailerons and
elevator are controlled using a single control stick between the pilot's
legs. The rudder, as in conventional aircraft, is controlled using foot
pedals.
Mouse-over the
control names to see where they're located on the glider
Ailerons Ailerons are the movable sections cut into
the trailing edges of the wing. These are used as the primary
directional control and they accomplish this by controlling the
roll of the plane (tilting the wing tips up and down). Ailerons
operate in opposite directions on each side of the plane. If the pilot
wants to roll the plane to the right, he moves the control stick to the
right. This causes the left aileron to deflect down (creating more lift
on this side) and the right aileron to deflect up (creating less lift on
this side). The difference in lift between the two sides causes the
plane to rotate about its long axis.
Elevator (horizontal stabilizer) The elevator is the movable horizontal
wing-like structure on the tail. It is used to control the pitch of the
plane, allowing the pilot to point the nose of the plane up or down as
required.
Rudder (vertical stabilizer) The rudder is the vertical wing-like
structure on the tail. It is used to control the yaw of the aircraft by
allowing the pilot to point the nose of the plane left or
right.
Landing Gear Another way to reduce the size of an airplane
is to reduce the size of the landing gear. The landing gear on a glider
typically consists of a single wheel mounted just below the cockpit.
3-view drawings of a modern sailplane.
Although somewhat simplified, the drawings show the basic structure of a
typical sailplane for the 15 metre class. There are many different racing
classes of gliders, Standard Class (15m span, no flaps), 15 Metre Class
(15m span with flaps), Open Class (Large metre spans, ranging from 15m to
25m and over), and the new World Class which uses the PW-5 sailplane
exclusively.
Glider Cockpit
Inside a typical glider cockpit, you'll find the
following:
Altimeter (to indicate your altitude)
Air-speed indicator (to tell how fast you are going)
Variometer (to tell what the air around you is doing)
Radio (to contact other planes or someone on the
ground)
Control stick (located between pilots legs)
Tow rope release knob (to disengage the tow rope)
Instrumentation on each
sailplane varies according to pilot preference, but each carries a minimum
of altimeter, airspeed indicator, a magnetic compass, a variometer (a
sensitive vertical speed indicator), and the "yaw string".
Most pilots immediately
upgrade their factory "vario" for a total energy system. Simply put, the
total energy system allows the pilot to get a more accurate reading on the
lift or sink surrounding the sailplane, but does not take into account
"stick lift" or a vertical acceleration by the pilot like an uncompensated
vario would.
The most useful instrument
by far is the yaw string. Attached to the outside of the canopy at one
end, this 3 inch piece of red yarn shows the pilot the relative
airflow of the glider. It works opposite to the more common "ball" found
in powered aircraft. Glider pilots try to keep the plane co-ordinated at
all times, but with large wings and ailerons on the planes, adverse yaw is
a factor to be reckoned with. Glider pilots are forced to use the rudder
pedals all the time to get the most from their plane.
Of course, the most
important instrument for all pilots remains on our body, the
eyeball.
