Quote:
Originally Posted by freebeard
The Dymaxion isn't like that.
You'll see that when I said "what's an ellipsoid" I was being disingenuous. I didn't respond promptly because I've been under the weather (I'm not complaining because I'm down 10 pounds and a week ahead on my grocery budget  )
I did show this before, a symmetrical, prolate 4v octahedron.
I hadn't run the software that generates the primitive geometry for a year or two and I'm sure it ran in the Terminal on Mac OSX but it doesn't want to now; but finally I found my old laptop running System 9.2.1 and managed to wrestle a 6v octahedron out of it. I tried something more exotic, a octahedral Bucky ball but had no further success.
I pipelined it through a 2nd program to convert file types and took it into Wings 3D.
Half the sphere was stretched 230% to get 0° camber at 30/70 forebody/afterbody ratio.
The bottom half was selected and reduce to 25% as a rough approximation of an underbody. It looks like I missed a step where I scaled on the X axis to get a 2.5:1 fineness ratio.
The entire object was reduced on the Z axis to 62% (Golden Ratio).
4 is 2.3x2.5, so I scaled in X by 230% to get a 4:1 fineness ratio.
Finally I added some materials for better visualization.
Additional modification aft the 22° camber point could even more closely match the template. The point would be that this method can provide any level of accuracy required, or attainable. You can see the improvement in contour between 4v and 6v. At 16-20v you'd have a monster data cloud, but it could be passed directly to a 3D printer. |
Just lost the entire post somehow.
will try to repeat in brief:
I missed your ref. to the Dymaxion , but it looks like the first couple of images posted.
My main insight was that the fineness ratio was much more than a boattail, that you could take template or reverse it, or anywhere in between and get virtually the same drag.
So what does this mean, it suggests that the front profile and the overall length is just as important as the rear, yes we do need to have compound curvature, but the options are a bit broader than just boattailing.
It suggests if I build a boattail on the front of my vehicle I will get to within 15% of the drag of a boattail at the rear.
The other thing that it questions to me is the influence of the thickness of the turbulent boundary layer, this takes time to develop, and if we have longer surface contact at the front then the layer is thicker at the rear and will negotiate a steeper curve without separation, vs a short abrupt front, then a more gentle boattail curve is required.
So it’s not just about the rear departure angles, the front plays nearly as much in the overall drag, if we can reduce the intensity and depth of the pressure / bow wave at the front, then we can closer the rear much more quickly with more aggressive curvature.
The Dymaxion shape seems to be just slightly biased towards a rear boattail and maybe that is the ideal shape to be functional in the real world.
I have read many threads about the front curvature, but they are usually dismissed under the banner that the big gains are in the rear, but this information seems to suggest this is not quite the whole truth.