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Old 07-20-2012, 12:31 PM   #431 (permalink)
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This is some EXCELLENT work!

I'm sorry if this has been asked and/or answered already (there are a lot of pages), but with the foam...

1. They hold together well with simple wood glue?
2. The foam doesn't melt with the fiberglass resin?
3. Body filler sticks to it without issue?

Thanx!

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Old 07-20-2012, 01:25 PM   #432 (permalink)
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Quote:
Originally Posted by NoD~ View Post
This is some EXCELLENT work!

I'm sorry if this has been asked and/or answered already (there are a lot of pages), but with the foam...

1. They hold together well with simple wood glue?
2. The foam doesn't melt with the fiberglass resin?
3. Body filler sticks to it without issue?

Thanx!
I don't about wood glue. But as to latter two questions: if you let the foam cure a long time, fiberglass resin should not melt it. I have had both experiences. For my mirror delete, I foamed the hole, let it cure--actually abandoned the project for weeks--and then glassed it, then bondo for a smoother, better-rounded finish.
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See my car's mod & maintenance thread and my electric bicycle's thread for ongoing projects. I will rebuild Black and Green over decades as parts die, until it becomes a different car of roughly the same shape and color. My minimum fuel economy goal is 55 mpg while averaging posted speed limits. I generally top 60 mpg. See also my Honda manual transmission specs thread.



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Old 07-20-2012, 01:44 PM   #433 (permalink)
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Quote:
Originally Posted by Varn View Post
That is a great way to look at the shape. It is nearly spherical.

Do you think that if the front is not as good as the ideal image that we might need to be more conservative and use a 6 or 7 d shape?

I am thinking of my 86 jetta with it basically being a box with very little radii on the front.

Did I miss you tuft testing your tailbox?
As for using a 6 or 7d shape, I will leave that question for AeroHead, but my inclination is that this shape is the most efficient as is.

You did not miss my tuft testing, as I have not done it yet.

I really plan on doing this yet before it gets cold out!

Jim.
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Old 07-20-2012, 01:51 PM   #434 (permalink)
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Quote:
Originally Posted by NoD~ View Post
This is some EXCELLENT work!

I'm sorry if this has been asked and/or answered already (there are a lot of pages), but with the foam...

1. They hold together well with simple wood glue?
2. The foam doesn't melt with the fiberglass resin?
3. Body filler sticks to it without issue?

Thanx!
The foam pieces are initially held together with Elmers Wood Glue.

The epoxy used to hold the fiberglass to the foam, is designed not to attack the foam itself. Polyester resin on the other hand does attack the foam, so be careful if you use that.

I no longer use a product like Bondo body filler on fiberglass, as in my experience, it does not stick to the fiberglass epoxy very well.

Just recently, the underbody fiberglass panels were off the car for maintenance, and just about all the Bondo is falling off from the fiberglass. I would not recommend it.

The epoxy/micro-balloons however work great, stick well, and allow one to sand down the extra thickness, so I would recommend that product.

Jim.
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Old 07-20-2012, 05:57 PM   #435 (permalink)
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duplicate

Quote:
Originally Posted by 3-Wheeler View Post
I still am not completely satisfied that we have adequately addressed how to duplicate the Streamlining Template, especially for those of us who need to render this shape for tails on our cars.

To help determine how to duplicate this shape, I downloaded the "C" version, straightened the "road surface" in a photo editor, then placed that copy in a 2D CAD program, and started working.

The first picture shows the "C" template with some geometry applied. You can see that a radius of 5.64 times the height of the car body provides a very good curve fit of the aft curvature down to 21° slope.



This is a closeup of the curve fit to AeroHead's rendering. Very close indeed.







I would have to say, overall this method gives a very close fit to AeroHead's standard.

Jim.
Jim,the 'original' architecture for the 'Template' is in Hucho's book.It's in one of his tables,depicting a 2.5:1 streamline body of revolution of Cd -.04 in free flight.I don't have that with me today.I'll bring it in the morning.
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I did come up with a 4-step function for approximating a 'duplicate' of the curvature.And it's done exactly as you have begun.only in 4,rather than 2-steps.I'll dig that out and bring in the morning also.
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Old 07-20-2012, 06:17 PM   #436 (permalink)
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front

Quote:
Originally Posted by Varn View Post
That is a great way to look at the shape. It is nearly spherical.

Do you think that if the front is not as good as the ideal image that we might need to be more conservative and use a 6 or 7 d shape?

I am thinking of my 86 jetta with it basically being a box with very little radii on the front.

Did I miss you tuft testing your tailbox?
All my texts recommend a nose based on a convex hemisphere for sub-transonic flow.When we consider the 115-degrees of radius rule for the point of turbulent boundary layer separation,the convex hemisphere is the only shape which best satisfies that condition for crosswinds.
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With respect to the Jetta,Hucho was personally involved with streamlining of the 1st-gen Golf/Jetta.You can see smoke flow images of their noses in the 'Flow-Images' thread.While I would say that the plan-view rounded nose has an advantage in crosswind,Hucho has said that it's attached flow leads us to go straight to the rear as JethroBodine has done.
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If we go' longer' than the 'Template',it's okay in that there won't be flow separation,but the structure will be weaker structurally as we move away from the 'tighter' ovoid form,and it will of course be heavier,which won't really show on the highway,but would impact urban driving mpg.
Each of us must weigh out their specific needs and mod accordingly.Nothing's set in cement.
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Old 07-20-2012, 11:14 PM   #437 (permalink)
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Thanks for the responses.

Do you guys consider the sides to be similarly shaped. with a radius something like 1/2 the width X 5.6. At least for the first part.
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Old 07-21-2012, 12:54 PM   #438 (permalink)
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'duplicate'

I found what I was thinking about.It was drawn over Mair's boat tail 'template',not the 'Template.'
Mair's wind tunnel model,if the rectilinear flow portion is removed,and only nose and boat tail combined,forms a 'template' with a fineness ratio of 2.52:1.Very similar to the 'Template.'
----------------------------------------------------------------------------
To duplicate a 'Template'esgue form :
*drop 3.35-H from max camber point and revolve a line for 12.5-degrees,stop.
*drop to 5.294-H and revolve 12-degrees,stop.
*drop to 6.588-H and revolve out 7-degrees,stop.
*drop to 7.96-H and revolve until you strike the ground plane.
This image is very similar to the 'Template' and shaves a little length off Mair's overall length,as his 'template' has a constant 22-degree line of convergence once that angle is achieved.
--------------------------------------------------------------------------
As to the 'Template',it is modeled from 'Streamlined body l/D=2,5,Cd 0.04,which appears in Table 2.1 Drag coefficients for different bodies-----------,Hucho,2nd Edition,Page 61.
--------------------------------------------------------------------------
The drag curve for Streamline bodies of revolution is asymptotic at around L/D= 2.1 but the 2.5:1 is the 'shortest' body which just fulfills Mair's 22-degree maximum boat tail slope angle for protecting boundary layer attachment.Rolf Buchheim thought 23-degrees would be the limit.I'm just playin' safe with 22.
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Old 07-21-2012, 02:13 PM   #439 (permalink)
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sides

Quote:
Originally Posted by Varn View Post
Thanks for the responses.

Do you guys consider the sides to be similarly shaped. with a radius something like 1/2 the width X 5.6. At least for the first part.
I've meant to do a thread for plan-taper 'n haven't.
Since you bring up the topic I'll do something quick and dirty.
----------------------------------------------------------------------------
Looking over some vehicles which have represented plan views,it appears that designers have taken a cautious approach with the sides of the vehicle.
Since the air along the sides is slower than over top it would have less kinetic energy and it makes sense that we'd handle it with kidd gloves.
--------------------------------------------------------------------------
Low-drag vehicle sides,viewed in plan view, appear to have been designed as a symmetrical wing/strut section.
From Goldstein's research it was found that the lowest drag is found with a length-to-thickness ratio of 3.84:1.
At this ratio the aft-body of the section forms a 'tail' which is 2.4X thickness.
*VW's Cd 0.14 'long-tail' Flow body of 1981 uses this 'tail.'
*Walter Lay's Cd 0.12 car models use this 'tail.'
*CALSTART's Showcase EV used this 'tail.'
*Moto Guzzi's world speed record motorcycle of 1955 went a little longer with 2.56:1.
*the Cd 0.12 MG EX 181 of 1957 went longer,with 3.13:1.
*Paul Jaray's Cd 0.20 'small' kombination car of 1922 Patent application used 2.14:1.
*Paul Jaray's Cd 0.13 'pumpkin seed' of 1921 used 1.88:1
*the OLDS AEROTECH of 1987 used 1.82:1.
*LEXUS' 1994 LS 400 used 1.892:1.
*The VW 2000 of 1980 had 2.169:1.
*Mercedes-Benz Cd 0.178 C111 III used 2.08:1
*The 1995 TNE-3,Cd 0.10,solar racer would be around 2.4:1
--------------------------------------------------------------------------
So far,my take on this,is to just go with the (2.4 times body width) aft section of the 3.84:1 streamline section as the basis for body side curvature near the ground plane.
If your project allows,you can be adding tumblehome higher on the body as you move aft,blending into the greenhouse.
This form is already working on low-drag,high mpg vehicles and land speed record cars,so it looks like good off-the-shelf technology.Yes?
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Old 07-22-2012, 10:40 PM   #440 (permalink)
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More Pictures

I gave the car a nice bath, and then drove to a park close by for some higher quality pictures.



















This last picture is one of the most telling, since it shows what happens when water based products are used on a harsh automotive application, with intense sunlight and high temperatures.

Note the three black arrows, as they are pointing to several areas that have bulged from water based spackling expanding under the high summer temperatures. After these areas settle out, they will need to be trimmed from the surface, filled with micro-balloons, sanded down and then primed and painted. Again!

I have learned an important lesson during this build. *Use high quality automotive products when building this type of stuff* !!



Oh, almost forgot. The gray primer is starting to degrade from the ultra-violet light, so the upper part of the tail will need to be painted red soon.

Jim.

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