An excellent way for students to gain a feel for
aerodynamic forces
is to fly a
kite.
Kites
fly
because of forces acting on the
parts
of the kite.
Though kites come in many
shapes and sizes, the
forces which act on the kite are
the same for all kites.
You can compare these forces to the
forces that act on an airliner in
flight and you will find that, with the exception of thrust,
they are exactly the same.
Since the forces on a kite are the same as the forces on an airplane,
we can use the mathematical equations developed to predict airplane
performance to predict the aerodynamic performance of a kite.
The
aerodynamic force
on a kite is a
vector quantity
having both a magnitude and a direction.
The aerodynamic force is
resolved
into the
lift
which acts perpendicular to the wind direction and the
drag
which acts along the wind direction.
There are several factors that affect the magnitude and the direction
of the aerodynamic force.
On this page we show the effects of inclination angle on the
lift and drag.
The graphic shows a side view of the
flying kite with the aerodynamic lift
shown by the blue vector, and the drag by the red vector.
The wind is blowing parallel to the ground.
The kite is inclined to the wind at an angle of attack, a, which
affects
the lift and drag generated by the kite.
Other factors affecting the lift and drag include the
wind velocity,
the
air density,
and the
downwash
created by the edges of the kite.
For any object, the lift and drag depend on the
lift coefficient, Cl,
and the
drag coefficient, Cd
of the object.
These coefficients are usually determined experimentally for aircraft,
but the aerodynamic surfaces for most kites are simple, thin, flat
plates. So we can use some experimental values of the lift and drag
coefficients for flat plates to get a first order idea of our kite performance.
For a thin flat plate at a low angle of attack
the lift coefficient Clo is equal to 2.0 times pi (3.14159)
times the angle a expressed in radians (180 degrees equals pi radians):
Clo = 2 * pi * a
The drag coefficient Cdo is equal to 1.28 times
times the trigonometric sine,
sin,
of the angle a:
Cdo = 1.28 * sin(a)
We use Clo for the lift coefficient and Cdo for the drag coefficient
because there is another aerodynamic effect present
on most kites. If we think of a
kite as an aircraft wing, and use the
terminology associated with aircraft wings,
most kites have a low wing span or distance from side to side.
Near the tips of a wing the flow spills from the under side to the top side
because of the difference in pressure. This creates a
downwash
which changes the effective angle of attack of the flow over
a portion of the wing and affects the magnitude of the lift and drag.
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