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Old 02-09-2011, 01:56 PM   #91 (permalink)
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Originally Posted by BamZipPow View Post
I would still put some sort of taper from the front to the rear. I have found out with mine that if I could just pull it in just by a few inches that it would be better in the long run...even if it's just the same width of the tailgate. This will be incorporated in my next version...
BZP, can you say more about what you found out regarding side taper? I am keenly interested in the basis for your view. What do you know that I don't?

I am concerned about discontinuity between bed and topper sides. Both start under positive pressure from displaced airflow. The bed sides don't change surface area or angle (much) over the bed length (ie, versus time in the flow field) so that pressure remains relatively consistent. An aerocap by contrast decreases surface area progressively from front to rear, which in turn reduces pressure. The resulting vertical pressure gradient encourages some air to flow upwards. In doing so it must roll over any discontinuities in vertical profile. Shoulders without sufficient relief radius create potential for flow separation. Especially towards the all important rear end of the vehicle where crisp detachment is desired in preferred directions to shape a converging wake. Does this compute, or am I all wet?

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Old 02-09-2011, 02:24 PM   #92 (permalink)
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Awesome work! I really like seeing how well the 1/4" foam works on compound curved surfaces.

You have got me thinking very hard about going this route with my CarBEN EV design.

Are you going to skin it with fiberglass? Is Gorilla Glue and this glue compatible with fiberglass? If so, what sort?
Hi Neil,

I can speak of Gorilla Glue use, since very small portions of this glue were used on the Tail.

While the Gorilla glue did react nicely with application of water, it continued to swell beyond what I originally expected. This swelling causes some minor damage in the area of the Tail in the side-lights area. I had to do some sanding after the glue stopped expanding. It did cause some concern as the curvature was altered because of the over-expansion.

The Gorilla Glue is compatible with foam, and there was no worry about chemical attack.

I really like how well the 1/4 inch foam curved as well. This could be useful in the future, but ensuring that there is total bonding between various layers could be an issue. Vacuum bagging could be used, but without a rigid mold to hold the overall shape, vacuum bagging can easily cause distortion of the shape.

One might have to go so far to put wood dowels across the layers to provide extra bonding protection against delamination.

Building a really light-weight structure from foam and fiberglass is quite a time intensive task.

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Old 02-11-2011, 06:34 PM   #93 (permalink)
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Originally Posted by NeilBlanchard View Post
I had fun doing it -- and I now know how I'm going to build my CarBEN EV prototype shell. The 1/4" polystyrene is a revelation for doing compound curves.

The 12 degree slope in the thesis was for a straight surface, and the slope was only on the top. The 3D cap shows how a curved surface can work better -- and there have been a lot of discussions here on EM with 15d slope being ideal, and even 18d works on cars like the EV1 because of the air flows in from the sides, as well.

I'll look around for the "ideal profile" image and see how it lines up with this cap? And it is no "recurve" like the prototype, so the main descending surface is much longer than previously.

[Edit: Here's the "ideal profile compared to the SU cap model:



So, the cap may be a little steeper than ideal -- I'll try a less steep version.]

The comment about the side mirror sight lines not being blocked by the cap is something I had not thought about.

[Edit 2: On the other hand, here's the EV1 compared to the ideal profile:



As you can see, it deviates as much or more than the cap. And here's the Dolphin:



I think the profile is a fine 2D guide, but is 3D, the air can flow in from the sides so the taper can be steeper *if* the sides are also tapered?

What do you folks think?]
Neil,I've been looking over Hucho'sfastback and boat tailing work at Volkswagen.He published SAE paper 760185 along with Janssen and Emmelmann and this work is in his 1st English version book.
First off,the fastback lowered the 'squareback' drag from Cd 0.44 to Cd 0.34,and this was with straight sides,no plan taper.
--------------------------------------------------------------------------
In his Fig.23,pg 14 ( Paper )/ Fig.4.43,pg.142 ( book ) Hucho lays out different boat tail forms for a car like a Jetta with a graph of the associated Cds.
The length of car body concerned is about the last 1,010mm ( 40.4" ).
--------------------------------------------------------------------------
The body was pulled 'in' at the back in 50mm increments,up to a total of 200mm.
The tangent angles for each increment,measured at 'breakaway' ranged from 7-degrees, to 25-degrees.
-------------------------------------------------------------------------- The greatest drag reduction occurred with a contraction of 125mm@ a tangent angle of 16.5-degrees ( at the 40.4-inch span ).
Increasing the contraction beyond 16.5-degrees provided zero additional drag reduction.
-------------------------------------------------------------------------- On the 200mm contraction the tangent angle reaches beyond 22-degrees at a position equal to the beginning of the rear bumper.
In this same paper,Hucho describes 23-degrees tangent angle as the extreme limit.
W.A.Mair's boat tail research limited boat tail angles to a maximum of 22-degrees,and you couldn't get to that angle in anything less than about 1.3 vehicle heights distance down the body from where it was 'flat.'
-------------------------------------------------------------------------- My T-100 has a 100-inch bed.The maximum contraction at 100-inches is 11-inches at each side.Tuft testing shows clean flow all the way to the end.The 'curve' of the sides start of course at zero,and grow progressively steeper just as in the Template.
--------------------------------------------------------------------------
If we were to plot a curve which intersected the 40.4-inch point and 5-inch (125mm) contraction,and 100-inch @ 11-inches ( approx. 275mm )contraction,this curve would be a 'sure thing.'
Depending on bed length,any member could just go to any position down the bed rail and measure the contraction distance directly,all the way to the tailgate.There should be absolutely zero possibility for flow separation if a cap were built to this 'template.
-------------------------------------------------------------------------
And VARN mentioned the fillets and I totally agree.You'll notice that all high performance aircraft have wing root fillets to smooth pressure spikes at these locations.Also,Ford's Probe-V roof structure is so well blended into the body,that if you didn't have paint and glass to distinguish the greenhouse from the body,you'd never know where one began and the other left off.The air loves this sort of thing and I like this part very much about the Thesis.
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Old 03-13-2011, 11:34 PM   #94 (permalink)
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Started work on the version 2 aerocap tonight. So far, I cut one rib, and figured out how to integrate the cap onto the existing tonneau cover framework.

I will be using two inch thick FoamulaR for the ribs, and I am going to cut out the inside of the ribs to provide more internal cargo space. I am also going to use a much stronger anchoring design that will use hinges with removable hinge pins, for ease of removal and installation.

There will still be no windows or hinged opening options for this design. The increased cargo capacity (compared to version 1) should handle almost any load I may think to put into the bed, and the cap itself will easily be removed and set aside for any really large item (like a washing machine or refrigerator).
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Old 03-14-2011, 11:25 AM   #95 (permalink)
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Would you consider putting in a rear camera then?
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Old 03-15-2011, 01:19 PM   #96 (permalink)
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Old 03-15-2011, 02:08 PM   #97 (permalink)
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Would you consider putting in a rear camera then?
I would! ( ref )
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Old 03-16-2011, 10:17 PM   #98 (permalink)
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V1 looked great, can't wait to see how V2 comes out.
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Old 04-08-2011, 03:16 AM   #99 (permalink)
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Still no progress on the V2 cap.

However, I did get to examine rainfall patterns on my tonneau cover from my commute home tonight. Amazingly, they angle back from the cab to the tailgate almost exactly as much as what I have planned for the side tapers.

There is something to be said for side tapers, I think.
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Old 04-16-2011, 05:58 PM   #100 (permalink)
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plan-taper = 12 % drag reduction

I gleaned an article about Volvo's 3CC concept coupe from the Feb.2005 Popular Science,p. 17.
This car has a roofline of generous plan-taper which Volvo credits with a 12 % drag reduction.
Since my cover has this taper and gave a 13% mpg improvement,the article helps explain the difference between Ford's 10 % straight-sided aero-shell of 1988.

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