Quote:
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.
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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.