Back in September of 1976, Cycle magazine published an aerodynamics article, written by Kevin Cooper from the National Research Counsil in Canada.
While the jest of this article was written regarding road going motorcycles, many of the basic concepts for lowering air drag can be applied to any vehicle shape. With this in mind, data from this article will be presented here.
Figure 10 shows a typical 'elliptical' form and the resultant flow path around this shape.
Figure 11 shows what happens to the efficiency of an airfoil shape when the tail is truncated to various lengths.
Figure 12 is a very useful graphic. It shows two possible ways of truncating the shape of an aero device when the length is severely limited. Note how this shape can easily make our external mirrors more efficient.
The next graphic shows how the overall drag in lowered as the length/width ratio approaches 4:1. For non-optimal shapes, the length/width ratio usually needs to be longer to achieve minimum drag.
Figure 14 shows the affect of radiusing the leading or trailing edge of our aero shape.
This last picture shows an actual application for the graphic in Figure 12, where the trailing edge of our aero shape is
closing to reduce the flow separation to the minimum for this length.
Blue arrows highlight the spot on this fairing where the shape is narrowing past the ends of the handle bars.
This fairing was made around 1990 using the principles of Kevin Coopers article, and is still valid today.
I hope you find this useful.
Jim.