First Electric van boat tail

[JohnForde]

By extreme good fortune and a little guile I I got my hands on the first built from the ground up Electric Van in the US. It's a GM/Brightdrop Zevo 600. I have had it since Sept 23. It's pretty fantastic. 178 kWh battery & AWD. Range is 275 miles in the winter (MN) and 360 summer.

I have been designing my aero tail for 2 years. I bought the aluminum & polycarbonate sheeting today and will start building soon. 2.5:1 slope doors extending 60" behind the body but only 48" behind the step bumper. The Zevo is 84" Wide. My doors should reduce the wake or 'aperture' to 24". Behind that will be roll out plastic 'feathers', eliminating the aperture entirely. Under hard braking the feathers will tilt then tumble back inside the doors. Ideally I will be able to post efficiency numbers later this summer. A 20% decrease in drag would be great!

I invite comments and suggestions.

[freebeard]

I would expect convex doors. How do you avoid separation at the break angle at the end of the box van? It might be possible with vortex generators or plasma actuators.

What do you use the van for?

[JohnForde]

It's just a personal vehicle/ camper van & daily driver.
The van has a 12" plastic cuff with a taper of about 20 degrees..

[aerohead]

Originally Posted by JohnForde

By extreme good fortune and a little guile I I got my hands on the first built from the ground up Electric Van in the US. It's a GM/Brightdrop Zevo 600. I have had it since Sept 23. It's pretty fantastic. 178 kWh battery & AWD. Range is 275 miles in the winter (MN) and 360 summer.

I have been designing my aero tail for 2 years. I bought the aluminum & polycarbonate sheeting today and will start building soon. 2.5:1 slope doors extending 60" behind the body but only 48" behind the step bumper. The Zevo is 84" Wide. My doors should reduce the wake or 'aperture' to 24". Behind that will be roll out plastic 'feathers', eliminating the aperture entirely. Under hard braking the feathers will tilt then tumble back inside the doors. Ideally I will be able to post efficiency numbers later this summer. A 20% decrease in drag would be great!

I invite comments and suggestions.

1) For a 'rigid' construction, the Highway Patrol is going to cite you for illegal length for anything beyond 48" of elongation.
2) If your swing-out 'stinger' ( which I like very much! ) were an inflated envelope, they'd let you have another 12", for a total of five feet of boat tail.
3) An inflated structure would also allow the aft portion of the stinger to project rearwards at the same, side elevation, 15-degree downslope angle, which would mitigate flow separation along the upper radius of the tilt-out, which is required to clear the opening, when deploying/ stowing. It would save you from some strong vortex-drag.
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4) Looking from above, in plan-view, holding a protractor to your image shows around 22-degrees of in-slope. If you had some lead-in curvature to get the flow moving around the sides ( like Walter Lay, and W.A. Mair employed ), you could get away with that much 'angle', but a 'sudden', 'abrupt' intersection as drawn, would introduce such an adverse pressure gradient, that you'd have flow separation there immediately, and with no chance of re-attachment. Bearman's modifications of the Ahmed Body, ended up with a 25-degree 'roof, 10-degree sides, and a 10-degree upswept diffuser for his lowest drag configuration. GM also used 10-degrees for sides and diffuser on their 'Optimum' Class-8 Semi-trailer boat tail. Kamm / Fachsenfeld used 12-degree sides, but also had lead-in softening.
5) Doctor Jeff Powell and associates are redoing some of the early boat-tailing research at Loughborough University, and basically, they're finding that drag is a linear function of wake size on their Windsor Body models.
You're sniffing up the right tree as far as drag reduction goes, you just need to be very careful not to allow separation to occur.

[JohnForde]

I appreciate your inputs very much.
I purchased ($200) a nearly ideally shaped inflatable but have decided against using it. It is pyramid shaped designed as an obstacle for paintball. The 12"W peak could be at the tail of air flow. The sides taper at 22 degrees to exactly 5'L. I would need to purchase a second one and stack the two of them.

I need to have easy access to load things in the van that is why I am building custom doors instead. I'll do another more in depth reply about the new door plan.

[JohnForde]

At 48" behind the vehicle my 'whaletail' (WT) still has an aperture of 24" wide.
The 'feathers' eliminate the aperture. They will be hand rolled out before each highway trip. They will extend another 33-36 inches behind the lights. At the rearmost point they will rub against each other and can flex either way for the wind.
They are made of twin wall polycarbonate. They slide thru a 1/2" slot between two 1"x3" aluminum tubes that are the rear door frame.
They can be quickly retracted by hard braking. I can adjust this with the weight of the metal 'stop', that keeps the feather from tumbling out.
It has no top after the lights, just sides. I am thinking about how to improve this.
If the police pull me over I briefly (1/2 second) hard brake causing the feathers to tumble in and conceal themselves.
"Officer, I believe I am up to code in every way". And I am.

[aerohead]

Originally Posted by JohnForde

I appreciate your inputs very much.
I purchased ($200) a nearly ideally shaped inflatable but have decided against using it. It is pyramid shaped designed as an obstacle for paintball. The 12"W peak could be at the tail of air flow. The sides taper at 22 degrees to exactly 5'L. I would need to purchase a second one and stack the two of them.

I need to have easy access to load things in the van that is why I am building custom doors instead. I'll do another more in depth reply about the new door plan.

Qualitatively, you're rubbing noses with the Golden Goose!
At EAA's Oshkosh, 1997, at the International Space Station exhibit, a sign company had created an inflated, scale Space Shuttle, standing about 25-feet tall, of very accurate geometric exactness.
These aerospace-grade fabrics, like the bounce-house are constructed from, are perfect for creating a very 'aerodynamic' boat-tail.

[aerohead]

Originally Posted by JohnForde

At 48" behind the vehicle my 'whaletail' (WT) still has an aperture of 24" wide.
The 'feathers' eliminate the aperture. They will be hand rolled out before each highway trip. They will extend another 33-36 inches behind the lights. At the rearmost point they will rub against each other and can flex either way for the wind.
They are made of twin wall polycarbonate. They slide thru a 1/2" slot between two 1"x3" aluminum tubes that are the rear door frame.
They can be quickly retracted by hard braking. I can adjust this with the weight of the metal 'stop', that keeps the feather from tumbling out.
It has no top after the lights, just sides. I am thinking about how to improve this.
If the police pull me over I briefly (1/2 second) hard brake causing the feathers to tumble in and conceal themselves.
"Officer, I believe I am up to code in every way". And I am.

It's an elegant design, something Kelly Johnson would probably have given the nod to, had he been around to see it.
With the 'open' top, the sides are still there to aid in sequestering dead-air 'inside' the footprint of the tail, like flying buttresses behind the cab of a Lotus, Lamborghini, Ferrari, Porsche, etc..
I like the idea of the weight-biased 'stops'.

[Piotrsko]

Having BTDT, I disagree about the officer not citing you on the retracted feathers. If you get pulled over for having them deployed, you will be still be cited and it is then up to the licensing dept to decide if they stay or not. Generally either way you're fined for having the oportunity to otherwise occupy the officer's time.

[aerohead]

I guestimated:
Height ( H ) = 103.25"
Ground Clearance ( GC ) = 10.5" at the front
'Box' height ( h ) = 87.5"
We know the width ( W ) = 84.0"
Tires , around 285mm ?
Box upper side radius, about 12"
Tire wake, about 1.636 sq-ft
Box wake, about 50.6118 sq-ft
Cd, about 0.34
CdA, about 17.764 sq-ft
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PhD Jeff Howell et al., at Loughborough University, suggest that lowering drag by some percentage, is achieved simply by lowering the wake size of the 'body' by the same percentage ( not the total wake including the tires, just the body wake ).
* If the Zevo's body presently has a 50.6118 sq-ft wake, then, theoretically, you'd hit your 20% drag reduction goal when the wake was reduced to 40.489 sq-ft ( call it 40.5 ).
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In 1995, Texas Tech University's engineering magazine published an article about their 'Tailwind Project,' honcho'd by Professor Carver.
The cover photo included their 1/12-scale, 18-wheeler in the wind tunnel, with the General Motors' 'GM OPTIMUM Tail .'
This tail was designed for the upper edge radii, as on Zevo, and embodied a 20-degree rear top downslope, 10-degree side body in-slope, and a 10-degree up-sloped diffuser.
For 'non-curved' boat-tail technology, it's likely that GM's tail represents the 'best' design extant.
All of it's edges are 'softened', including the vertical trailing edges. It's worth seeing, if you don't have it in your library.