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Old 05-11-2017, 09:24 PM   #21 (permalink)
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Hard edges are very tricky because your cap would have to be very well designed to prevent vortex formation. The 12 slope is NOT what you want, it is too steep. Go with 7-
8 and you'll get more storage space, way better lift coefficient (Your truck will fell more planted and stable, and still get 90% of the optimal aerodynamic benefit. Plus it will be way more forgiving of any design errors.

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Old 05-13-2017, 01:35 PM   #22 (permalink)
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campers

Quote:
Originally Posted by freebeard View Post
As much as you can consider the front top and sides to be flat, you can make it out of three triangles (pointing back) and two simple curved conic sections (pointing forward).

Can anyone explain why the NASCAR Camping World Truck Series don't have campers on them?
*It's just a rule book thing.They must all meet template limitations mandated by the governing body.
*they'd also be faster and corner better than the NASCAR 'cars',which would too embarrassing.
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Old 05-13-2017, 01:51 PM   #23 (permalink)
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angle 'optimums' are very contextual in nature

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I'd be interested to see an A-B-A test of the taller, more gently curved topper that follows that damn template exactly versus the 12 average slope version suggested by that one engineering thesis. I may or may not get around to that. It would be kinda neat if OP and I went with somewhat opposing designs to compare. Definitely slope the rear backwards not forwards though, I agree, and leave an overhang to get that hard edge around the convex section for the fluid tail like aerohead always talks about.

How critical is it to round the joint between the sides and top? It makes sense if the goal is to create an elliptical curve on the backside to induce the "Morelli's Fluid Tail" smoke-ring effect, but it doesn't apply as much if you intend to get the rear edge height close enough to the height of the tailgate. But I also wonder if a hard edge on those "shoulders" to ease construction and purposefully induce flow separation there like the kammbacks' rears (except on top) would be somewhat helpful too. I think it's likely there's a minimum radius for those shoulders below which you're better off with a hard edge. Though as always I'd love for someone to question and point out if/how I'm wrong. Crosswinds may play into it too. (I asked aerohead but he's only on his computer on Saturday mornings haha.)

Attached images are of a quick model from a 3d sketch I made a couple weeks ago in Autodesk Inventor (which for this I prefer over Solidworks). This represents the laziest possible geometry for the aerocap: I lofted the curve of the rear window down a straight 12 slope then trimmed it up. That would require zero heat-forming of the polycarb, just cutting shapes out of a single 4'x8' sheet and solvent/heat welding them together, and bolting them down such that the top section has that longitudinal curve that follows the window curve (which, if I choose to follow it, means I can't have any transverse curvature). Frame discussion in other posts. Sorry for the derail. Again, I'm very curious about your other planned mods!
Research has demonstrated that 'optimum' -drag aft-body angles can only be appreciated within the context of a particular vehicle body architecture.
*Type of roof
*Length of roof with respect to overall car length
*Camber
*Tumblehome
*C-pillar design
*Boot,or lack thereof
*etc.
all impact the optimum angle








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Old 05-13-2017, 03:13 PM   #24 (permalink)
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Quote:
*It's just a rule book thing.They must all meet template limitations mandated by the governing body.
According to the Golden Rule, Camping World makes the rules. They could mandate a slant-chopped canopy with the rear height equal to the those spoiler they use instead.

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*they'd also be faster and corner better than the NASCAR 'cars',which would too embarrassing.
That's probably it, can't have that.
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Old 05-13-2017, 05:48 PM   #25 (permalink)
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Thank you for the information! I remember riding in my parent's car in the 1950's (remember vent windows) and sticking my hand outside and trying the exact experiment that you described! What fun that was! My thought about the rear end of a pick-up was based upon my observation of semi-trucks in the rain, e.g., the "burbling effect" of rolling air; hence, trucks have taken to using large bubbles and or "trailer tails" to reduce drag. But your are right: a pickup is not a semi, and the drag issues are different. I will publish a video of my smoke test when done.

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