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 edition of HowStuffWorks,
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 fiberglass 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 aluminum with structural aluminum skins that
were much smoother. However, the rivets and seams required by
aluminum skins produce additional drag, which tends to
decrease performance. In many modern gliders, composite
construction using materials such as fiberglass and carbon
fiber are quickly replacing aluminum. 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 maneuverability 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.
Getting off the Ground Three basic forces
act on gliders: lift, gravity and drag (airplanes have a
fourth force: thrust):
Lift is the all-important force, created by the
wings and counteracting the weight, which allows an aircraft
to stay aloft. In the case of a glider, the lift is enhanced
through the use of highly efficient wings.
Drag is the force that tends to slow a plane
down. Drag reduction is critical on a glider, even more so
than on a conventional airplane. In motorized aircraft, a
pilot can simply increase the thrust (using the engines) to
overcome drag. Since there is no engine on a glider, the
drag must be minimized wherever possible or the plane won't
remain in the air for long.
Weight can be made to work for or against a
glider. A lighter overall weight, for example, may allow the
glider to stay aloft longer or travel further. A heavier
weight, on the other hand, can be an advantage if greater
speed is the objective. Many gliders contain ballast tanks
that pilots can fill with water before takeoff. The added
weight of the water allows greater speeds while in the air.
If the pilot wished to reduce his weight, he can dump the
tanks while in the air to lighten the plane.
Without an engine, a glider's first problem is getting off
the ground and up to altitude. The most common launching
method is an aero-tow. A conventional powered plane tows the
glider up into the sky using a long rope. The glider pilot
controls a quick-release mechanism located in the glider's
nose and releases the rope at the desired altitude. Right
after release, the glider and the tow plane turn in opposite
directions and the glider begins its unpowered flight. The tow
plane is then free to return to the airport and set up for
another tow.
Since the glider's wings generate more lift,
it takes off before the tow
plane.
Another popular launching method is winch launching. An
engine powers a large winch on the ground and a long cable
connects the winch to another release mechanism located on the
underside of the glider. When the winch is activated, the
glider is pulled along the ground toward the winch and takes
off, climbing rapidly. As the glider rises, the pilot can
release the winch line as in an aero-tow and continue his
flight.
Staying in the Air The wings on a glider
have to produce enough lift to balance the weight of the
glider. The faster the glider goes the more lift the wings
make. If the glider flies fast enough the wings will produce
enough lift to keep it in the air. But, the wings and the body
of the glider also produce drag, and they produce more drag
the faster the glider flies. Since there's no engine on a
glider to produce thrust, the glider has to generate speed in
some other way. Angling the glider downward, trading altitude
for speed, allows the glider to fly fast enough to generate
the lift needed to support its weight.
Why Gliders Carry
Ballast
A plane's lift, drag and glide ratio
characteristics are governed solely by its construction,
and are predetermined at takeoff. Without thrust, the
only other characteristic that the pilot has control
over (besides normal control surfaces) is the weight of
the plane.
A heavier glider will sink faster that a light
glider. The glide ratio is not affected by weight
because while a heavier glider will sink faster, it will
do so at a higher airspeed. The plane will come down
faster, but will cover the same distance (at a higher
speed) as a lighter glider with the same glide ratio and
starting altitude. In order to help them fly faster,
some gliders have tanks that can hold up to 500 pounds
of water. Higher speeds are desirable for cross-country
flying.
The downsides of heavier sailplanes include reduced
climb rates in a lifting environment (such as a thermal)
and, possibly, shorter flight duration if suitable lift
cannot be located. To prevent this, the water ballast
can be jettisoned at any time through dump valves,
allowing the pilots to reduce the weight of the plane to
increase climb rates, or to reduce speed as they come in
for a landing.
The way you measure the performance of a glider is by its
glide ratio. This ratio tells you how much horizontal distance
a glider can travel compared to the altitude it has to drop.
Modern gliders can have glide ratios better than 60:1. This
means they can glide for 60 miles if they start at an altitude
of one mile. For comparison, a commercial jetliner might have
glide ratios somewhere around 17:1.
If the glide ratio were the only factor involved, gliders
would not be able to stay in the air nearly as long as they
do. So how do they do it?
The key to staying in the air for longer periods of time is
to get some help from Mother Nature whenever possible. While a
glider will slowly descend with respect to the air around it,
what if the air around it was moving upward faster than the
glider was descending? It's kind of like trying to paddle a
kayak upstream; even though you may be cutting through the
water at a respectable pace, you're not really making any
progress with respect to the riverbank. The same thing works
with gliders. If you are descending at one meter per second,
but the air around the plane is rising at two meters per
second, you're actually gaining altitude.
There are three main types of rising air used by glider
pilots to increase flight times:
Thermals
Ridge lift
Wave lift
Thermals Thermals are
columns of rising air created by the heating of the Earth's
surface. As the air near the ground is heated by the sun, it
expands and rises. Pilots keep an eye out for terrain that
absorbs the morning sun more rapidly than surrounding areas.
These areas, such as asphalt parking lots, dark plowed fields
and rocky terrain, are a great way to find thermal columns.
Pilots also keep a look out for newly forming cumulus clouds,
or even large birds soaring without flapping their wings,
which can also be signs of thermal activity.
Once a thermal is located, pilots will turn back and circle
within the column until they reach their desired altitude at
which time they will exit and resume their flight. To prevent
confusion, gliders all circle in the same direction within
thermals. The first glider in the thermal gets to decide the
direction -- all the other gliders that join the thermal must
circle in that direction.
Ridge Lift Ridge lift is
created by winds blowing against mountains, hills or other
ridges. As the air reaches the mountain, it is redirected
upward and forms a band of lift along the windward side of
slope. Ridge lift typically reaches no higher than a few
hundred feet higher than the terrain that creates it. What
ridge lift lacks in height however, it makes up for in length;
gliders have been known to fly for a thousand miles along
mountain chains using mostly ridge lift and wave lift.
Wave Lift Wave lift is
similar to ridge lift in that it is created when wind meets a
mountain. Wave lift, however, is created on the leeward side
of the peak by winds passing over the mountain instead of up
one side. Wave lift can be identified by the unique cloud
formations produced. Wave lift can reach thousands of feet
high and gliders can reach altitudes of more than 35,000 feet.
Detecting Lift Columns
and bands of rising air obviously benefit any glider pilot,
but how can you tell if you are flying in one? The answer is
the variometer, a device that measures the rate of
climb or descent. The variometer uses static pressure to
detect changes in altitude. If the glider is rising, then the
static pressure drops (because air pressure decreases the
higher you go). If the glider is sinking, then the static
pressure rises. The needle on the variometer indicates the
rate of change in altitude based on the rate of change of
static pressure. When flying through a rising mass of air
(like a thermal), the needle on the variometer will jump (and
usually beep to notify the pilot) before any change on the
altimeter is even noticeable.
Detecting Yaw The glider
is yawing when it is not pointing exactly in the direction it
is flying (relative to the air around it). Instead the glider
is angled sideways and is "slipping" or "skidding" through the
air. The string on the windshield indicates whether the glider
is flying straight (string straight) or whether it is yawing
(string left or right). The glider produces the least drag
when it flies straight through the air. When it is yawing, the
drag increases -- so in general, glider pilots try to keep the
string straight.
The string on the windshield tells the pilot
if the plane is yawing
Landing
Glider World
Records (as of March
2001)
Absolute Altitude: 49,009 ft
Speed over 100 km triangular course: 135.09 mph
Free Distance: 907.7 mi
Distance around a triangular course: 869.52 mi
Free distance with up to three turning points:
1,272.70 mi
Landing a glider is much like landing a conventional plane,
except there is usually a single small wheel located directly
under the pilot. The wings on gliders are very strong, and the
tips are reinforced to prevent damage in case they scrape
along the ground during a landing. Even so, pilots can usually
manage to keep both wing tips off the ground until the plane
has slowed sufficiently (kind of like riding a fast bike down
the runway). Glider tails typically have a tiny wheel that
prevents the tail from scraping while on the ground.
When landing the glider, the pilot needs to be able to
control the glide path (the rate of descent relative to
distance traveled) in order to bring the glider down in the
right location. The pilot has to be able to reduce the amount
of lift produced by the wings without changing the speed or
attitude of the glider. He does this by deploying spoilers on
each wing. The spoilers disrupt the airflow over the wing,
drastically reducing the lift it produces and also increasing
the drag.
Note the raised spoiler on the wing during
landing
On July 23, 1983, a brand new Air Canada Boeing 767
was forced to glide to a landing after running out of
fuel in midair. The plane essentially became an enormous
glider. Even descending at a paltry glide ratio of about
11:1, the pilots managed to land safely at an abandoned
airport in Gimli, Canada. The story of why the plane ran
out of fuel is a long one, but it was partly due to an
error in confusing English units with metric units.
If you are interested in learning more about this
incident, you can read more by searching the Web for
"Gimli glider" or by clicking here.
