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.
Hanggliders 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.
If a glider is in a steady
(constant velocity and no acceleration) descent, it loses altitude as
it travels. The glider's flight path is a simple straight line,
shown as the inclined red line in the figure. The flight path
intersects the ground at an angle a called the
glide angle.
If we know the distance flown and the altitude change, we can
calculate the glide angle using
trigonometry.
The tangent tan of the glide angle a is equal to the
change in height h
divided by the distance flown d:
tan(a) = h / d
Considering the balance of forces
for a glider, we get two
vector component equations.
The lift L times the cosine cos of the glide angle plus
the drag D times the sine sin of the glide angle is equal to
the weight W:
L * cos(a) + D * sin(a) = W
The lift times the sine of the glide angle
is equal to the drag times the cosine of the glide angle.
L * sin(a) = D * cos(a)
With these three equations we can experimentally determine the
lift
and
drag
of a glider.
If we test fly the glider and measure the
distance along the ground, the height at which the glider is
released, we can use the first equation to give us an average value of the
glide angle. With the glide angle known and a measurement of the
weight,
we have two remaining equations in two unknowns which can be solved for
the lift and the drag.
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