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Old 07-07-2024, 09:03 AM   #21 (permalink)
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Quote:
Originally Posted by freebeard View Post
Will you, or how will you, cap the top and bottom?
1/2" Marine plywood with alum tubing perimeter. Bottom open.

The pic shows my lthinking on ength. MN State patrol approved the 48" then lights then 48" format.

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Old 07-07-2024, 09:11 AM   #22 (permalink)
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WT Mock up

Before I cut the very expensive aluminum and polycarbonate I decided to do a plywood prototype. I am glad I did. It helped me think thru top corner intersections and attachment mechanisms.

Here are 5 pics. The angle on these is 28 degrees. I am aiming for 22.
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Old 07-07-2024, 12:39 PM   #23 (permalink)
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22 degrees is the maximum angle you can use without boundary separation in perfect conditions like a wind tunnel.

You can't have perfect conditions as you are not coming out to the edge of the van but instead leaving the stock rear trims that looks to have an angle greater than 22 degrees to start. You will have boundary separation at the rear of your van and your tail is just keeping the vortex from wrapping around as far.

Any particular reason you aren't going all the way to the edge of your van with your tail?

It looks like you are basically trying to make a trailer tail like those used on tractor trailers. You will find lots of info on them here:

https://nacfe.org/wp-content/uploads...Report2020.pdf


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Old 07-07-2024, 01:40 PM   #24 (permalink)
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Thanks JSH!
I do plan on keeping the boundary flow smooth. The width at the hinges is ~72"W. The hinges are 8 " behind the cuff which is 78"W. I will tape coroplast to the van's body to maintain this edge across the gap.
The hinges need to be 8" further aft to allow sight line to the tail lights.
The van's black plastic cuff has a 3" "rise" over 10" "run" for a slope of 16.7 degrees.
The red line in the photo is an even steeper angle to meet the orange line. The orange line is 3" below the cuff but ideal because it is in plane with the vans back up camera.
Photo in my previous post shows a lot more wood in the field of vision than is necessary.
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Old 07-07-2024, 02:05 PM   #25 (permalink)
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Thanks JSH! I have been looking for this 60" rule and it's formal citation for about 2 years. Thanks again!
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Old 07-08-2024, 11:00 AM   #26 (permalink)
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plan-view angle

Quote:
Originally Posted by JohnForde View Post
My latest drawing. Maximalist. Probable realistic length is 47" to lights and 89 - 91" total.
If you construct the tail as shown, there's an extremely high confidence that the flow will separate almost immediately and never reattach.
You'll be partially 'stuffing the wake', as NASA did with the Cd 0.238 stinger, added behind their 'Project Shoebox' Ford Econoline's initial, Cd 0.242, 'curved-contour' truncated boat tail.
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Old 07-08-2024, 04:12 PM   #27 (permalink)
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Quote:
Originally Posted by JohnForde View Post
Thanks JSH!
I do plan on keeping the boundary flow smooth…..
As Aerohead noted - your current design will have boundary separation. It will happen at the red line. Every one of your angles needs to be less than 22 degrees and more realistically more like 15 - 17 degrees to realistically have the flow stay attached in the real world.

The simplest solution is to just continue the 16.7 degree angle from the stock trim with your extension and then cut it off when you reach your desired extension length. It doesn't do any good to continue your extension to a full cone if the flow separates at the back of the van.

I would also plan to add an extra set of lights at the rear of your extension just to make everyone happy – you don’t want any question about whether or not your taillights are visible. I would just use a set of trailer lights on a simple extension.

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Originally Posted by JohnForde View Post
Thanks JSH! I have been looking for this 60" rule and it's formal citation for about 2 years. Thanks again!
You're welcome. Just remember that is specific to commercial vehicles – It doesn’t apply to RVs or personal vehicles.
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Old 07-08-2024, 05:55 PM   #28 (permalink)
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I would like no aperture. I think I should cut the top board for 3.33:1 / 16.7 deg & do that first, then cut the top board and try 3.0:1 /18.4 deg.

I can always buy another piece of marine plywood.

Thoughts?
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Old 07-08-2024, 06:03 PM   #29 (permalink)
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At what point does the Bowlus Road Chief trailer lose flow attachment?

Is speed a factor in max angle? I always drive 63 mph.
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Old 07-08-2024, 06:38 PM   #30 (permalink)
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Quote:
Originally Posted by JohnForde View Post
I would like no aperture. I think I should cut the top board for 3.33:1 / 16.7 deg & do that first, then cut the top board and try 3.0:1 /18.4 deg.

I can always buy another piece of marine plywood.

Thoughts?
I would most definitely mock this up with something cheap like 1/4 plywood or even coroplast. With those beefy supports and hinges you shouldn't have much issue with support. Then you can use the old-fashion tuff test to see if you are getting separation.


With a 96 inch extension you WILL need another set of lights on the end

(I'm doing something similar with my RV project - mocking things up and testing with wood before a start cutting expensive aluminum.)


Quote:
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At what point does the Bowlus Road Chief trailer lose flow attachment?

Is speed a factor in max angle? I always drive 63 mph.
I have no idea how that particular trailer does. I will say that many things that look aero aren't actually in real life. The wind tunnel tells the tale. (Even engineers that deal with aerodynamics for a living can be surprised after testing)

I am surprised that an almost $265K trailer has a cheap looking plywood interior like a budget van build. (Well after visiting an RV show I'm not that surprised - most commercial RVs are junk)

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