Aerodynamicists use a number to model all of the complex items that affect drag, including: the shape of an object, the size of an object, and the angle at which the object meets the air. The number scientists use is called the drag coefficient (Cd). It is calculated by dividing the drag (D) by the quantity, which is density (r) times reference area (A) times one half of the velocity (V) squared.
This slide shows 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 and the tunnel conditions of velocity and density. The drag equation was then used to produce the coefficient. The projected frontal area (the maximum cross-sectional area) of each object was used as the reference area (A). 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 has a Cd = .295, and a typical airfoil has a Cd = .045.
We can study the effect of shape on drag by comparing the values of the drag coefficient for any two objects as long as the same reference area is used and other variables are matched. All of the drag coefficients on this slide were produced in low speed (subsonic) wind tunnels. 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! Therefore, we can conclude that:
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.