A
glider
is a special kind of
aircraft
that has no engine.
In flight, a glider has
three forces
acting on it
as compared to the
four forces
that act on a powered aircraft.
Both types of aircraft are subjected to the forces of
lift,
drag, and
weight.
The powered aircraft has an engine that generates
thrust, while the glider has
no thrust.
There are many different types of glider aircraft.
Paper airplanes
are the simplest aircraft to build and fly, and students can learn the
basics of
aircraft motion
by flying paper airplanes.
Toy gliders, made of balsa wood or styrofoam, are an inexpensive
way for students to study the basics of
aerodynamics, while having
fun building and flying the aircraft.
Hang-gliders are piloted aircraft that are
launched by leaping off the side of a hill or by being towed aloft.
Piloted 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 Wright Brothers perfected the design of the first airplane and
gained piloting experience through a series of
glider flights
from 1900 to 1903.
The Space Shuttle
flies as a glider during reentry and landing. During
ascent,
thrust is provided by the three Space Shuttle Main Engines (SSME's) at the base
of the orbiter and the two Solid Rocket Boosters (SRB's) that are joined to the
orange External Fuel Tank (ET).
The solids are jetisioned about two minutes into the ascent,
and the fuel tank is jetisioned as the shuttle enters Earth orbit.
So there are no propellants available to the SSME's during descent and, though the
engines are still present, the shuttle returns as an unpowered glider.
Piloted high performance gliders are designed with a high
L/D ratio (~70) to permit a low
glide angle and a long time aloft.
The shuttle was designed with a low L/D ratio (~ 1) because during the
descent the spacecraft must be slowed from about 17,300 mph to
about 250 mph at landing.
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