Neil,
Your second sketch above is closer to what it would take to keep the air attached.
Quote:
Originally Posted by NeilBlanchard
Hi,
The top and the bottom of my sketch are arcs, so the angle of change is constant.
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I should mention that a constant rate of change does not necessarily guarantee to keep the air attached, if the angle from horizontal is still too steep.
Take a look at the many NACA airfoil shapes from the 64, 65, and 66 series airfoils. Those tails are quite long to their full length!! Note that the length/width ratio for these airfoils is one average at least 4:1, if not longer. That means if your car width is four feet, the total length of the car body should be at least, four times this, or 16 feet long at a minimum.
When making boat-tailed car bodies, our only practical way of handling the situation is to truncate the airfoil at some practical length, and provide what else... but a Kamm back tail piece to allow the air to detach cleanly.
Once we understand the fundamentals of airfoil design, then it becomes quite easy to determine what our limited length boat-tails should look like.
Hope this helps, Jim.