A glider is a special kind of
aircraft
that has no engine.
Paper airplanes are the most obvious example, but gliders come in a wide
range of sizes. Toy gliders, made of balsa wood or styrofoam, are an excellent
way for students to study the basics of
aerodynamics.
Hang-gliders are piloted aircraft that are
launched by leaping off the side of a hill.
The Wright brothers perfected the design of the first airplane and
gained piloting experience through a series of glider flights from 1900 to 1903.More sophisticated gliders are
launched by ground based catapults, or are towed aloft by a
powered aircraft then cut free to glide for hours over many miles.
The Space Shuttle flies as a
glider during reentry and landing; the rocket engines are used only
during liftoff.
Compared to a powered aircraft,
a glider has only three main forces acting on it:
lift,
drag, and
weight.
Forces are
vector quantities
having both a magnitude and a direction.
The weight acts through the
center of gravity
and is always directed towards the center of the earth.
The magnitude of the weight is given by the
weight equation
and depends on the mass of the vehicle plus its payload.
The lift and drag are
aerodynamic forces
and act through the
center of pressure.
The drag is directed opposite to the flight direction,
and the lift is directed perpendicular to the flight direction.
There are many factors
that influence the magnitude of the
lift
and
drag
forces.
In order for a glider to fly, it must generate lift
to oppose its weight. To
generate
lift, a glider must move through the air. But the motion of a
glider through the air also generates drag. In a powered aircraft, the
thrust
from the engine opposes drag. But a glider has no
engine to generate thrust. With the
drag unopposed,
a glider quickly slows down until it can no longer
generate enough lift to oppose the weight.
So how does a glider generate the velocity needed for
flight?
The simple answer is that a glider trades altitude for velocity. It
trades the potential energy difference from a higher altitude to a
lower altitude to produce kinetic energy, which means velocity.
Gliders are always descending
relative to the air in
which they are flying.
How do gliders stay aloft for hours if they constantly
descend?
The answer is that they are designed to be very
efficient, to descend very slowly. If the pilot can locate a
pocket of air that is rising faster than the glider is descending,
the glider can actually gain altitude, increasing its potential
energy. Pockets of rising air are called
updrafts. Updrafts are found when a wind blowing at a hill or
mountain has to rise to climb over it.
Updrafts can also be found over dark
land masses that
absorb heat
from the sun.
The heat from the ground warms the surrounding air, which
causes the air to rise. Rising pockets of hot air are called
thermals. Large gliding birds, such as owls and hawks, are
often seen circling inside a thermal to gain altitude without
flapping their wings. Gliders do exactly the same thing.
Activities:
Guided Tours
-
Forces on a Glider:
-
Gliding Flight:
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