Quote:
Originally Posted by NeilBlanchard
Hi,
I was about to post a thought that I was having about ways to do boattails, related to their length, and it seems this thread is a good fit?
Since the top and the sides of a boattail can be up to 15 degrees, and the bottom is best if it is limited to 4 degrees, this means the car needs to have a height that is ~5/8 the width IF the boattail comes to a point.
But since we need to have the two tail lights spread apart by some reasonable distance, this increases the need to have a shorter height even than that.
So, what is a shade tree aerodynamics person supposed to do? Here's a possible solution: divide the trailing shape of the car into two parts!
Take the lower portion and taper the "top" and sides and bottom as much as possible to end the back in a "line" i.e. like the back of the Aptera.
And take the remaining top portion and taper the sides and the top at the maximum angles so it closes to a point -- but keep it joined to the lower portion.
We seen this in many cars: the 1939 Maybach is one (though it is quite different than my little xA). The original UrSaab is another example. The Volvo 3CC concept car is also similar to this idea; except it truncates it far too early to form anything like a boattail:
Am I explaining it properly? I hope to photograph my xA from above to be able to better illustrate (and test) this idea. |
I would like to comment that,we might think in terms of how far along the ideal aerodynamic pathway a particular body is,and the included angle possible at that point.
I we toss out rules of thumb ( which I'm guilty of ) and angles,we run the risk that they'll be taken out of context.
The shaded template that Darin posted on his Permanent Kammback for the Metro,or the Aerodynamic Streamlining Template are tools designed to allow members to evaluate their vehicles potential for attached/separated flow,when compared to the "teardrop curve."
From Mair's research,it appears that 22-degrees is the maximum included angle that will support attached flow.You will notice that the "template" "finishes" at 22-degrees.
That angle cannot be realized if everything ahead of it is not a gentle sweeping curve of ever-decreasing radius of curvature.
Kamm was curving in the bodies of his K-cars as much as he was curving down the roof.Again,he was using very subtle and generous curves.
And if you study the photograph of Kamm's K-3 car pictured in the archive,you'll notice that the flow does separate over the last 1-foot of roofline,as the curvature is too steep for the air to follow.
Same for EV-1,same for Ultralite,same for Opel Calibra,etc..