The drag coefficient is a number which
aerodynamicists use to model all of the complex dependencies of
drag on shape,
inclination,
and some flow conditions.
The drag coefficient Cd is equal to the
drag D
divided by the
quantity:
density r
times
reference area A
times one half of the
velocity V
squared.
Cd = D / (.5 * r * V^2 * A)
This slide shows
some typical values of the drag coefficient for a variety of shapes.
The values shown here were determined experimentally by placing
models in a
wind tunnel and
measuring
the amount of drag, the tunnel conditions of velocity and density, and
the reference area of the model. The
drag equation
given above was then used to calculate the drag coefficient.
The projected frontal
area of each object was used as the reference area.
A flat plate has Cd = 1.28, a wedge shaped prism with the wedge facing
downstream has Cd = 1.14, a sphere has a Cd that varies from .07 to .5,
a bullet Cd = .295, and a typical airfoil Cd = .045.
We can study the effect of shape on drag by comparing the values
of drag coefficient for any two objects as long as the same reference
area is used and the
Mach number
and
Reynolds number
are matched.
All of the drag coefficients on this slide were produced in low speed
(subsonic) wind tunnels and at similar Reynolds number, except
for the sphere. A quick comparison shows that a flat plate gives the highest
drag and a streamlined symmetric airfoil gives the lowest drag, by a
factor of almost 30! Shape has a very large effect on the amount of
drag produced.
Comparing the flat plate and the prism, and the sphere and the
bullet, we see that the downstream shape can be modified to reduce
drag.
The drag coefficient for a sphere is given with a
range of values because the drag on a sphere is
highly dependent
on Reynolds number.
You can further investigate the effect of airfoil shape and the other
factors affecting drag by using the
FoilSim III Java Applet.
You can also
download
your own copy of FoilSim to play with
for free.
Activities:
Guided Tours

Factors that Affect Drag:

Falling Objects:

Flaps and Slats:

Forces on a Model Rocket:

Rocket Modeler Talk:
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