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Old 05-15-2019, 10:20 AM   #11 (permalink)
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Freebeard,
Thanks for the tip. I have been sanding the shredded surface with an orbital sander as a work around, I don’t really do much foam shaping in the grand scheme of things or I would be more motivated to make/buy additional tools.

Everyone,
Speaking of foam shaping, I got a few of my pieces roughed out of foam last night. I am trying to get the foam blank closer to the final shape to reduce the amount of shaping I have to do, which is taking much longer than expected versus making a large oversized block.

Edit: I forgot to add I got two tanks in for a preliminary baseline (with tire swap and trim removal) showing 23.6 MPG for short trips and 25.6 MPG mostly highway.

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Last edited by aardvarcus; 05-15-2019 at 10:37 AM.. Reason: Added Baseline Info
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Old 05-17-2019, 09:01 AM   #12 (permalink)
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Trying to keep this moving forward. Got the glueups for the sides done. I am going to add another layer to the inside of these pieces to make it thicker at the front and add some extensions on the back to get to my desired distance. I am hoping to have it shaped and start the fiberglass this weekend.

Question:
When following the AST profiles (AST-II for the top, AST for the sides), if I encounter a bump (e.g. roughly 1/8" bump from trim on the rear of the side windows) do I need to slow up the profile immediately after that (simulating that the angle of that section is less due to the obstruction) or can I press forward following curves as if the bump did not exist? Basically I am calculating the angle of the rear of the vehicle and intercepting the templates based on this angle. If I use the glass/sheet metal the angle is different than if I use the trim.
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Old 05-17-2019, 01:09 PM   #13 (permalink)
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Dirt doesn't affect drag but rear-view mirrors do. There is going to be some scale, depending on the thickness of the attached flow, below which there's no difference.

It might depend on the angle of the flow to lengthwise of the trim. And it might be possible to over-reach for the optimum.
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Old 05-17-2019, 02:03 PM   #14 (permalink)
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Freebeard,

For sure I don't want to over reach, but I don't want to under reach either. As the tail is fixed, it is limited in length due to practical reasons. Obviously like everyone I want it to be as effective as it can be in that length.

I am not quite sure I am following "angle of the flow to lengthwise of the trim," could you please reiterate?

What I think I know from Hucho's book was the necessity of window flushness decreased the further back you went, stating that a 5mm inset glass did not increase drag at the rearmost window. That led me to believe the boundary layer was built up enough to accommodate the slight bump. However I don't know if parallel glass and parallel panels with a transition translates to following a taper with a bump.
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Old 05-17-2019, 03:28 PM   #15 (permalink)
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Quote:
I am not quite sure I am following "angle of the flow to lengthwise of the trim," could you please reiterate?
[speculation]
  • A ridge parallel to flow would have negligible effect.
  • A ridge athwart the flow would act like a Gurney flap; given enough height, promoting reattachment.
  • A ridge at a 45į angle could trigger a vortex on the lee side.
[/speculation]
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Old 05-21-2019, 07:55 AM   #16 (permalink)
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Got the rest of the foam pieces glued together to form a blank. Transferred AST-II/AST tapers to the blank via shaping to form a few contour lines. Shaped the remainder of the block freehand by eye to form the overall shape. Did lots of test fits to get the overall lines close to right. Shaped out some of the insides of the block to improve rear sight lines. Laid layers of epoxy/fiberglass on the outside and inside of the block.

Current status is that it needs a few patches of fiberglass around some corners and other tricky areas and it will be ready for sanding, prep, and paint.
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Old 05-22-2019, 11:10 AM   #17 (permalink)
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bump

Quote:
Originally Posted by aardvarcus View Post
Trying to keep this moving forward. Got the glueups for the sides done. I am going to add another layer to the inside of these pieces to make it thicker at the front and add some extensions on the back to get to my desired distance. I am hoping to have it shaped and start the fiberglass this weekend.

Question:
When following the AST profiles (AST-II for the top, AST for the sides), if I encounter a bump (e.g. roughly 1/8" bump from trim on the rear of the side windows) do I need to slow up the profile immediately after that (simulating that the angle of that section is less due to the obstruction) or can I press forward following curves as if the bump did not exist? Basically I am calculating the angle of the rear of the vehicle and intercepting the templates based on this angle. If I use the glass/sheet metal the angle is different than if I use the trim.
Without seeing it,it may be that it merely functions as a trip-strip,actually energizing the boundary layer as vortex generators would.
Volkswagen (Hucho/Buchheim) found that the rear hatch gap of the Rabbit reduced drag,compared to filling it in flush (taping it over) with the body contour.
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Old 05-22-2019, 11:15 AM   #18 (permalink)
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foam pieces glued..............

Oh yeah! Nice work.I don't think you'll have any problem with attached vortices with those edge radii.
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Old 05-23-2019, 10:04 AM   #19 (permalink)
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Quote:
Originally Posted by aardvarcus View Post
Current status is that it needs a few patches of fiberglass around some corners and other tricky areas and it will be ready for sanding, prep, and paint.


This is going along nicely.
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Old 05-28-2019, 08:42 AM   #20 (permalink)
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So I got the shell built, I am calling this version 1.0 for now. The topside looks good from a few feet away, the underside is rough as I didnít attempt to bondo or do major sanding to the bottom. The transition from the hatch to the shell is not as smooth as I would have liked, it has a rather large step. The overall lines are aligned back to the bodywork so it is placed where it needs to be for AST tapers as if the gap did not exist. It would have been more ideal to taper this down to nothing right at the front edge of the hatch, however the way I was building this made that very difficult. If I do not get the results I expect I will probably go back and try to fill this gap in.

I havenít run a tank with the shell on yet, just a partial tank that was half and half. My driving throughout the week is fairly consistent and mostly highway, however the weekends are short trips, different routes, more AC use with the family, etcetera. My plan for testing MPG change is to compare my commuting tanks from pre and post changes. I will fill up at the beginning and end of each week at the same pump to enable that separation. While it is not ABA, I feel it is a reasonable method for me to estimate percentage changes. (I have no time for ABA.) The weekend tanks are far to varied to give any sort of defensible indication of changes.

My mental dyno tells me the drag coefficient of the vehicle hasnít changed drastically, however I can tell the rear end feels much more planted in turns which I attribute to reduced rear lift. Assuming this mod shows a reasonable positive change indicating it is working as designed, I will turn my attention to the underside and tire spats.

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