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Old 03-16-2012, 05:25 PM   #21 (permalink)
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Still wondering about the cabover length and will it affect the cd ?
I now have a copy of Hucho's book and I refer to his roof load info..Now it appears to me that if my camper overhangs the windshield even just a little as in #3 , it has a definite effect on the Cd. Click image for larger version

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So since I don't have the major panels fiberglassed together yet I thought I could move the side panels rearward and fill in the rectangles behind the cab and live with a slightly narrower bunk... and at the same time try to get a 5 degree roof angle as Yugomodder suggested. In blue below

Click image for larger version

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I may also incorporate a small molded-in 10 or 12 degree Kammback on the rear if the rear door permits ( I need a drip edge anyway )

On a side note since I hope more people use this glass/foam method . I am on my last big piece and I found you don't need any clamps at all ...just duct tape. It works great in shear. And I didn't even use the 3M stuff .What you do need is a dead flat table which you can make out of old recycled hollow core doors from Habitat Restore joined together with biscuits (no glue) Use gauge blocks or a long straight edge to keep them straight and winding sticks to remove all the twist. Mine was 13' long at one point and setup takes only minutes .Works like a charmClick image for larger version

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Advice, comments and suggestions always welcome and thanks to all those who replied so far


Last edited by turbothrush; 03-16-2012 at 05:49 PM..
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Old 03-17-2012, 04:07 PM   #22 (permalink)
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Turbo, on the cab-overhang have you considered rather than shortening the whole width instead radius the front edges in top view? Air splash creates a bow wave which increases drag. Hucho's section on trailers shows this clearly and Cd reduction as a result of edge radius. Food for thought?

Keep up the great work!
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Old 03-17-2012, 04:37 PM   #23 (permalink)
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overhang

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Originally Posted by turbothrush View Post
Still wondering about the cabover length and will it affect the cd ?
I now have a copy of Hucho's book and I refer to his roof load info..Now it appears to me that if my camper overhangs the windshield even just a little as in #3 , it has a definite effect on the Cd. Attachment 10530

So since I don't have the major panels fiberglassed together yet I thought I could move the side panels rearward and fill in the rectangles behind the cab and live with a slightly narrower bunk... and at the same time try to get a 5 degree roof angle as Yugomodder suggested. In blue below

Attachment 10528

I may also incorporate a small molded-in 10 or 12 degree Kammback on the rear if the rear door permits ( I need a drip edge anyway )

On a side note since I hope more people use this glass/foam method . I am on my last big piece and I found you don't need any clamps at all ...just duct tape. It works great in shear. And I didn't even use the 3M stuff .What you do need is a dead flat table which you can make out of old recycled hollow core doors from Habitat Restore joined together with biscuits (no glue) Use gauge blocks or a long straight edge to keep them straight and winding sticks to remove all the twist. Mine was 13' long at one point and setup takes only minutes .Works like a charmAttachment 10529

Advice, comments and suggestions always welcome and thanks to all those who replied so far
I'm very late to the party.Sorry!
With respect to the overhang,I suspect,that if you cantilever out beyond the windshield header,that the underside of the overhang will begin to act like an inverted splitter,directing some air under and to the sides rather than up the sloping nose of the camper.
Some of the centerline flow will displace far in advance of the shell/cab roof intersection and just go on up over the top.Streamlines below there will be directed sideways around the A-pillars and up into the void between roof and shell.
The hard edges as many members have mentioned,are sure to produce immediate separation.Hucho will have two formulas you could look at,one based on a single dimension (say width),the other,on the squre-root of the frontal area of the shell.
These would offer 'minimums' for attached flow at zero-yaw.You want as much radius as you can stand to build.In crosswinds,this is where you'll save big time.I realize it cuts into interior space so you're the boss of that.
If you do decide to push the camper out past the windshield,I would recommend that you 'soften' all the edges around the 'splitter' area.There's gonna be some wicked transverse flow,a lot of shear,a mixture of velocities attempting to marry one another,and it's a breeding ground for vorticity which may cause so much wind noise,the weather stripping and seals around windows will be overwhelmed,making a loud stressful driving environment.
If you absolutely must use the hard edges you might get some relief with VGs,used just to break macro eddies into micro eddies,altering the frequency to something you could live with inside the cab.
I used to drive a 35-ft moving van with cantilever box.You could clearly hear the sheet metal tin-canning as the turbulence was beating the s--- out of it above my head.Very stressful.
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Old 03-17-2012, 04:39 PM   #24 (permalink)
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My understanding of his plans was that he was always going to radius those leading edges.

There is another way to fix the flow on those leading edges and that is to attach curved edges of the appropriate radius to the sides, that stand proud of the front face. Aerohead, you remember those?
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Old 03-17-2012, 04:54 PM   #25 (permalink)
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attached edges

Quote:
Originally Posted by Frank Lee View Post
My understanding of his plans was that he was always going to radius those leading edges.

There is another way to fix the flow on those leading edges and that is to attach curved edges of the appropriate radius to the sides, that stand proud of the front face. Aerohead, you remember those?
Frank,I do remember.Dr.Paul MacCready got a US Patent on such a thing.
Hucho has a depiction of such devices back in the commercial vehicle chapter on the face of a trailer van.
The fella with the dump truck was doing this with PVC pipe sections.
It would be way out ahead of nothing,and maintain some interior space otherwise lost.Especially where the jackstands are going.
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Old 03-20-2012, 07:27 PM   #26 (permalink)
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Sorry guys I was away for a few days and didn't get to respond until now
Quote:
Originally Posted by aerohead View Post
I'm very late to the party.Sorry!
With respect to the overhang,I suspect,that if you cantilever out beyond the windshield header,that the underside of the overhang will begin to act like an inverted splitter,directing some air under and to the sides rather than up the sloping nose of the camper.
Some of the centerline flow will displace far in advance of the shell/cab roof intersection and just go on up over the top.Streamlines below there will be directed sideways around the A-pillars and up into the void between roof and shell.
The hard edges as many members have mentioned,are sure to produce immediate separation.Hucho will have two formulas you could look at,one based on a single dimension (say width),the other,on the squre-root of the frontal area of the shell.
These would offer 'minimums' for attached flow at zero-yaw.You want as much radius as you can stand to build.In crosswinds,this is where you'll save big time.I realize it cuts into interior space so you're the boss of that.
If you do decide to push the camper out past the windshield,I would recommend that you 'soften' all the edges around the 'splitter' area.There's gonna be some wicked transverse flow,a lot of shear,a mixture of velocities attempting to marry one another,and it's a breeding ground for vorticity which may cause so much wind noise,the weather stripping and seals around windows will be overwhelmed,making a loud stressful driving environment.
If you absolutely must use the hard edges you might get some relief with VGs,used just to break macro eddies into micro eddies,altering the frequency to something you could live with inside the cab.
I used to drive a 35-ft moving van with cantilever box.You could clearly hear the sheet metal tin-canning as the turbulence was beating the s--- out of it above my head.Very stressful.
Thanks Aerohead and Kamperbob. OK no overhang for me .. Like Frank Lee mentioned I'll for sure radius the edges ( sides to roof and sides to front ) I am planning a nice 6 " radius in those areas and will do a mock up and post a picture later to show how I did it in case someone else wants to give it a try for their own project.
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Old 03-20-2012, 09:20 PM   #27 (permalink)
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Quote:
Originally Posted by Frank Lee View Post
My understanding of his plans was that he was always going to radius those leading edges.

There is another way to fix the flow on those leading edges and that is to attach curved edges of the appropriate radius to the sides, that stand proud of the front face. Aerohead, you remember those?
Quote:
Originally Posted by aerohead View Post
Frank,I do remember.Dr.Paul MacCready got a US Patent on such a thing.
Hucho has a depiction of such devices back in the commercial vehicle chapter on the face of a trailer van.
The fella with the dump truck was doing this with PVC pipe sections.
It would be way out ahead of nothing,and maintain some interior space otherwise lost.Especially where the jackstands are going.
Are you guys talking about these?

On the front corners at the same height as the Mercedes badge?
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Old 03-20-2012, 09:38 PM   #28 (permalink)
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Nope.
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Old 03-21-2012, 06:08 PM   #29 (permalink)
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these?

No.MacCready came up with a half-cylinder device which attached to A-pillars and the windshield header,providing some radius where none existed.Dead air would pool inside the the inverted basin with the outer flow spilling over the new edge without separating.Way our ahead of nothing!
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Old 09-17-2013, 09:18 PM   #30 (permalink)
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Hey Turbo, what's new w/ your project? Any updates?

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