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Old 01-24-2013, 02:37 AM   #405 (permalink)
Master EcoModder
Join Date: Feb 2012
Location: Australia
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The Truck - '02 Nissan GU Patrol ST
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Originally Posted by freebeard View Post
You're doing important work here...I think.

I can't get a mental picture of that, that is consistent with what I take to be station profiles in the illustrations.

I can say that I raised a similar question in a thread about Why does The Template have to be a half body of revolution? The relevant illustration:

It sounds like your going for the one in the middle.

Picture this: Two spheres one inside the other centered on the pivot point of the hitch (ball, pintel or 5th wheel). The inner one is convex and attached to the tow vehicle; the outer one is concave and attached to the towed vehicle. Enough clearance to maybe toss a beer bottle through and you are done.
Spot on with your guess, the image I picture is most like the one in the middle, just the upper half mounted on a standard 6'x4' box trailer, the nose cone/blunt sphere would start at the tow hitch and radiate up to the maximum diameter which would be at the front of load area of trailer and then tapering top and sides to finish flush with tailgate on trailer, this would be cutting the image you posted at the fourth segment from the back.
So the base would be std trailer and the top would be 3/4 length of the middle image.

I modeled the full hemispherical shape and it just lacks utility and the outline of my vehicle and most others is basically square/rectangular, so doing a dome would probably not benefit aero anyway.

I went back to your previous post, as for the Cd's I have no idea, but I would say the perfect tear drop, image one would be best, the vortex issue with lateral flow is a "Red Herring" in my opinion, no matter what shape is, it ultimately has to vacate the space and all air will co mingle again.
As far as vortexes go, my reading is that the square edges is where they are created, so image 3 would be the worst for that. I read back a lot of the trailer threads and a big deal was made about having a good radius all around, front, sides and back, this was primarily related to crosswinds, seems a good Cd can be blown out of the water with a crosswind, but a good radius offers some protection from crosswinds and maintains it's Cd.

Also reading about template application there seems to be some contradictions,
Firstly there is the application/overlay that everyone uses, but then I have read over & over, the lesser dimension between height and width governs the taper, well unless you drive a very low sports car, width will be the lesser 9 times out of 10. But then the 22 degree rule comes back, if you base taper on sides, then vertical profile will easily exceed the magic 22*, I still haven't found an appropriate answer to this.

So going through my process, because I do want utility I am happy to be conservative with the Aero guidelines, the shape I have outlined is a good compromise, provides a good deal of usable space, relatively easy access and use and potentially some Aero benefits and at worst, no Aero Penalties.

As for important work, don't know bout that, it helps me understand it all a bit better and I hope it leads others to better ideas.

I stood outside by the vehicle for a long time playing with tape measures, protracters, markers, sticks etc. and it just seemed obvious that working out all the dimensions, ironing out the bugs and producing a full sectional plan on the computer seemed like the way to go. To do that though required an equation for the curve, the key so to speak, once you have that the rest is relatively easy, the NACA 0039 that I am working with is the closest thing I have found and considering all the potential errors with trying to apply the template and scale it up to life size, I don't think my projected path with this equation is any worse.

Definately want some answers on the contradictions of top taper, side taper and the 22*rule.
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