First Electric van boat tail

[JSH]

Originally Posted by JohnForde

I did a 117 mile test this morning. It is right at the outer reaches of efficiency - 1.96 mi/kWh. But it is only point zero one better than three other runs including my first outlier run on July 13.

Conditions were good & It was a two way out and back run. Maybe the others were not.

I am within 2% of getting to two.

1.96 / 1.85 = 1.0595. Almost 6% better than baseline. Excellent progress.

[JohnForde]

Originally Posted by JSH

1.96 / 1.85 = 1.0595. Almost 6% better than baseline. Excellent progress.

I'm not really sure where baseline is. It could be 1.75. Getting to 2 is gonna be success.

It's probably possible but I am hoping to need to experiment with door angles and pressure testing.

JF

[aerohead]

Originally Posted by Piwoslaw

Superb photoshipist's rounded upper corners:

General Motors 'IDEAL FAIRING' ( semi-trailer boat tail ) of 1980, had generous, 'ALL' edge-radii corner intersections, both horizontal & vertical.
And Professor Carver & Scott Funderburk et al., of 'Tailwind Project', Texas Tech University, 1995, commented that these 'softened' edges were basically capable of eliminating the formation of any high-drag, longitudinally-attached vortices, regardless of orientation.
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To 'include' these features to John's tail would 'magnify' build man-hours by an untold amount.
It could be remain a 'Plan-B', but John's the 'Captain' on this one, so, I'll wait until asked about it.

[aerohead]

The OEM exterior surfaces on ZEVO would constitute a 'Class-A' finish, rendering a 'wetted surface-friction drag coefficient ( Cf ) of 0.003-pound/square-foot of area.
It's not beyond the realm of belief that, the increased surface roughness of the mods, and especially the plywood tail surfaces themselves, could allow an 'unnatural' turbulent boundary-layer thickening towards the tail's trailing edges, which would reduce momentum transfer from the inviscid flow, 'down' to the tail's surface; compromising the pressure recovery, compared to what might exist, should the tail comprise a class-A finish.
Which is a long way of saying that, you may have already reached your 'target' drag reduction as it stands, however we can't 'see it' because of 'smoothness' issues.
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My 'gut' feeling is, that, if you had a trailer light kit on the tail, and went ahead and closed off the existing fenestation which allows observers behind ZEVO to 'see' the OEM taillight/turn signal units underneath the 'floor,' closing off that entire area with a second 'floor', would get you 'beyond' your target.
The lack of the rear-most section of belly pan, plus the lack of diffuser, on a passenger car, would be costing you something on the order of Cd 0.035.
With that, and 'smoothness', you might 'hit one out of the park' drag-wise.'

[JohnForde]

Thank you everybody!
I appreciate the 'rounding' idea, but I need access to the door.
Here is an idea. Increase door angles to 16 degrees. That leaves an aperture or wake area 36" wide. Tuft test.
Then fabricate a detachable cone as shown. 100% polycarbonate Twinwall. Under 10 lbs.
Length is now 108" behind bumper but I can move the tail lights easily to within 48" of the rear tip. Aperture is rounded and now 12" W & 36" Tall.
Fine for highway. Very tricky in a parking lot.
Still square to 66" behind bumper but rounding the final 44" or so.

[Piwoslaw]

Originally Posted by JohnForde

Looking at how your boattail keeps getting longer, then maybe you should have a mechanism to shorten and extend it while in motion, like old collapseable cups:


There were a few threads about such ideas.

Aerohead could share experience on inflateable extensions.

[JohnForde]

Detachable rounded 48" cone. Will the square corners at the attachment point hurt efficiency badly? Making the entire length rounded seems insurmountable.

[ennored]

NASA got pretty darn good results (on the order of .347 to .242 adding a tail to a rounded box) using a square truncated tail. I see no reason to think rounding your tail is going to make it suddenly "work" where a square one does not.

[aerohead]

Originally Posted by JohnForde

Thank you everybody!
I appreciate the 'rounding' idea, but I need access to the door.
Here is an idea. Increase door angles to 16 degrees. That leaves an aperture or wake area 36" wide. Tuft test.
Then fabricate a detachable cone as shown. 100% polycarbonate Twinwall. Under 10 lbs.
Length is now 108" behind bumper but I can move the tail lights easily to within 48" of the rear tip. Aperture is rounded and now 12" W & 36" Tall.
Fine for highway. Very tricky in a parking lot.
Still square to 66" behind bumper but rounding the final 44" or so.

* On a simple, prismatic, 'saloon/ sedan' fastback test model, Rolf Buchheim et al., of Volkswagen, went as steep as 16-degrees top slope, on a car with an aft-body length equal to around 20% of total length.
* With your 70-inch tail, you were at 19.4% body length. But Zevo isn't a 'saloon' body. It's 'taller' than it is 'wide', requiring a different solution.
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* We have no empirical test data for any vehicle which is analogous to Zevo.
* No investigator has tested a 'rounded-roof' vehicle, with a 'sharp-edged' tail extension.
* So we're in un-charted waters, data wise.
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* When Dr. Hermann Burst designed the 'duckbill' rear spoiler for the Porsche 911 Carrera RS 2.7, his original 'solution' was to add a Kammback, following the exact contour of what the aerodynamic streamlining template, part-c has. 'Porsche' forbid it, and after surrendering to the Paris Dressmakers, he ended up constructing the rear spoiler, of which, it's upper termination point ( it's tearing edge ) is 'exactly' where the 'Kammback' contour would have been.
* When you examine the truncated boat tail on NASA's Ford Econoline-based 'Project Shoebox', you see them doing basically the same thing.
* Same for Lawrence Livermore's 11-degree, 'straight' tail. That tail constituted 5.8% of total body length.
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If you've got the stomach for it, run a range of angles, record tuft orientations, and get your pressure coefficients recorded.
I'm inclined to think that the 'smaller' angles will be your 'friend.'
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The 'value' of the rear 'aperture' area, is completely predicated upon fully-attached flow, and it's attendant pressure recovery, for the entire length of whatever you build.
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Transitioning, from square-edges, back to rounded edges, while within an environment of increasing pressure may provide little probability for mended flow.
Paul Jaray would transition from 'round' to 'square', and then back to 'round, but he'd never do it 'in' the aft-body. His would be at the 'beginning' ( the 'cuff' ) of the aft-body, and increase, incrementally, with distance as it grew nearer and nearer to the trailing edges.
And, if a 'radius' is too daunting, a 45-degree 'chamfer' is the next best thing.

[aerohead]

Originally Posted by ennored

NASA got pretty darn good results (on the order of .347 to .242 adding a tail to a rounded box) using a square truncated tail. I see no reason to think rounding your tail is going to make it suddenly "work" where a square one does not.

I would want to interject some qualifiers into NASA's tail 'solution'.
1) The body that they constructed 'over' the original Ford Econoline van, had a complete belly pan
2) Their tail spanned the entire vertical body height.
3) And the tail incorporated a curved diffuser, which integrated seamlessly into the belly pan.
4) We'd want to note the 'length' of their tail, as a percentage of the bodies total length, 25.48%.