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Old 05-15-2020, 05:14 PM   #71 (permalink)
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Originally Posted by aardvarcus View Post
The project is still stalled out, mainly because I have other things to do that have taken priority. Plus I am still having issues nailing down a design...

Got a random thought today, what if instead of chopping the top, I left the top height alone and narrowed the rear as it went back in the cargo area? Basically sever the side walls (including the windows) of the cargo area from the roof and floor, and remount them at a modest (e.g. 5 degree) angle. (Results in a max width going from 79" down to 70ish inches over six feet.) Do a bit of compound curving over a few inches at the transition (so it isn't a sharp edge) but nonetheless still a reasonably abrupt transition. Remove a triangular section and reweld the roof and floor back together to the sides. Would need to modify or build a door, so probably a new door with a second foot or so section of full boat tail. Someday maybe even a hinged actuated extension...

The question being would the airflow stay attached to a vehicle side going from a zero to five degree angle with only a few inches of smoothing? The advantage of this approach is that I can reuse the rear cargo area windows and all the metal, since I am only repositioning the existing pieces versus building all new pieces from scratch. Yes, I am aware this is still a ton of work. Thoughts?
In the commercial vehicle section,Fig.8.63, of his 2nd edition,Hucho depicted a bus,which transitioned from nothing ,to 5-degrees of boat tail angle,top and sides,with no transition.Starting at Cd 0.88,all the mods,plus all the tail depicted, got the bus to Cd 0.239.

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Old 05-15-2020, 05:25 PM   #72 (permalink)
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Instead of triangles, cut bell-shapes, starting with a run of one saw-blade width. From black-and white pix in the 'little magazines'.
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Old 05-15-2020, 06:15 PM   #73 (permalink)
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Aerohead,

I am away from my copy of Hucho at the moment, but I remember the picture you are referencing. I am assuming based on your reply you think this may work.

Freebeard,

The reason for the triangle versus the curve or bell, is that I am trying to reuse the rather large (60" x 22") existing window in each side, which does not curve significantly from front to back, in order to save time and effort. Thus the concept of just angling the side at a fixed angle, verses a curve.
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Old 05-16-2020, 01:16 AM   #74 (permalink)
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I'd assumed flat sides and top with the gap taken up between them. I guess it depends on where the cuts are.

Cut a hole at the tip of the triangle to relieve stress.

Am I being too helpful?
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Old 05-16-2020, 07:57 AM   #75 (permalink)
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Freebeard,

Not trying to be dismissive, I appreciate all the help. I am not sure that I understand your bell curve suggestion. I may not be explaining myself well, I had intended on removing a 0" to 5" over 60+" triangle off either side of the roof and floor where it meets the side wall. Basically I would be severing the entire side, repositioning, and re welding, versus trying to curve/pull the side over.

Are you suggesting that I remove a bell shaped piece of roughly those dimensions, referring to making smaller bell cuts and trying to pull the side in without severing, or am I missing the point? Thanks!
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Old 05-16-2020, 12:49 PM   #76 (permalink)
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Quote:
Am I being too helpful?
Quote:
Not trying to be dismissive... I am not sure that I understand
I can't find an example picture. I'm not sure I understand where the cuts would be.

I'm thinking of a gentle non-linear curve, but it would bring the top down as well as the bringing the side in.

We can just let it go...
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Old 05-16-2020, 04:37 PM   #77 (permalink)
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Freebeard,

Attached are some example drawings of what I am considering. Step one would be to cut each side off (quarter panel, window, and structure) and remove it intact. Step two would be to cut a triangle out of the roof and floor metal and support channels. Step three would be to reweld the intact side at that angle, smooth the few inches around the transition, and build/modify a new door. The roofline would not drop in this suggestion. The future plan would be to add a kamm shell to the door or separate hinge to taper both the roof and sides.

I am weighing the downside of the fixed angle not curving given the tradeoff for not having to replace the window and window framing. The smoothing would be in the inches between the middle row door and the window starting.


---Edit: Added picture of future Kamm type shell
Attached Thumbnails
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Name:	Reweld and New Door.jpg
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Name:	Future Shell.jpg
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Last edited by aardvarcus; 05-16-2020 at 04:46 PM..
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Old 05-16-2020, 04:51 PM   #78 (permalink)
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If you leave the vertical cut attached at the top and bottom, and slit the middle so a gap opens up, would it preserve the panel alignment? It might bend outward first to facilitate the inner cuts?
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Old 05-16-2020, 05:10 PM   #79 (permalink)
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Freebeard,

Yes I could do that, I should have room to work anyway. I would have to pick equidistant points from the center-line. There is actually an "inner" quarter panel, that i believe is vertical, I could even leave that partially intact to act as a hinge. I attached a few representative shots of the sheet metal construction of the area. The side panel has sheet metal formed "tubes" to give it its strength. The roof and floor are sheet metal with stamped ribs going front to back reinforced with hat channels going across. Severing it and rewelding it right at the side wall to roof/floor seams would intuitively seem the most straightforward way to approach this.
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Name:	Cross Section Suburban GMT400 Cargo Area.jpg
Views:	6
Size:	24.9 KB
ID:	28197  
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Old 05-20-2020, 02:38 PM   #80 (permalink)
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think may

Quote:
Originally Posted by aardvarcus View Post
Aerohead,

I am away from my copy of Hucho at the moment, but I remember the picture you are referencing. I am assuming based on your reply you think this may work.

Freebeard,

The reason for the triangle versus the curve or bell, is that I am trying to reuse the rather large (60" x 22") existing window in each side, which does not curve significantly from front to back, in order to save time and effort. Thus the concept of just angling the side at a fixed angle, verses a curve.
Walter Lay achieved Cd 0.12 with his models at the University of Michigan using simple hard angles. Vortex-drag investigations with the Ahmed body demonstrated good effects using nothing but hard angles on top,sides,and diffuser. And then there's Hucho's bus schematic. My first 50-mpg trailer had hard angles.And the one-wheeler.Scott Funderburk got his Masters in Mechanical Engineering for his hard-angled boat-tail research,working around the General Motors Optimum Boat Tail, at Texas Tech.Scott did progressively softened corner intersections to mitigate attached vortices.NASA never softened anything with their Project Shoebox,and still got to Cd 0.238.
It's certainly within the realm of probability that you'd have success.Perhaps separation with re-attachment,dragging a modest,locked-vortex down the road with you,but the pressure regain from the boat-tail taper would outweigh the small vortex liability.I'll bet you coffee and an apple fritter!

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