Quote:
Originally Posted by MetroMPG
Woohoo!
After seeing the latest pictures, I'm a little concerned about this transition:
Hopefully it's not too abrupt, going from horizontal on top to the "template" angle. (If it is, do you have the ability to make a bigger radius/smoother transition there?)
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Me too.
If the horizontal surface was just a deck lid it would have an abrupt lip, but if it were an aero-fish-scale (from the roof-line to the template) it should be rounded. I wasn't sure which to do in planning, now I'm thinking I guessed wrong in both cases.
It might work fine the way it is if I do a rear-window treatment (full or half? ...) to get separation right along the template and keep the air flow away from the trunk.
When I tuft test, I'm expecting to see the roof-to-rear-windshield radius pull the airflow down, a little turbulence at the base of the windshield followed by a lot of horizontal air-flow along the trunk line to the lid transition, where it will fall apart at the sudden ~19° drop.
The alternate solution, if I wanted just an Aero Box with no rear windshield treatment, should have been a very gradual curve from the horizontal trunk line back to the template - with a radius similar to the roof-to-rear-windshield. (This is what I think
now.)
EDIT: what I think I'm trying to say - case 1 no detachment before trunk: need a large radius to move horizontal airstream back to the template - also I should arguably head to 12° instead of the template. Case 2 detachment somewhere on the template: need a small radius (~4") to keep reattachment stable. (I'm not really sure what I've got here, a 4" radius from 0 to 19° doesn't look like much visually.)
If I have to make a big change to that transition, the lid will get reborn in aluminum with snipped-out-fiberglassed-forward-corners, just like the rest of the box. (Hands down my favorite method between the two, so much so that I don't want to bother editing my somewhat bungled glass-over-foam version.) The lid would need compound edges but they would be well within the limits of tuck shrinking. That would set me back to the tail end of phase 2, basically, most likely needing to edit the upper lip of the box, but both items ought to go pretty fast. Cross that bridge when I get there though...
EDIT: Between the two methods, I'd expect the rear-window-treatment to work the best aerodynamically, because it is the closest to ideal profile in the final product, whereas relying on a large curve to bend the flow off the trunk results in not one but two regions of non-ideal flow. The big question of course is how big the split is and thus which is favored in cost-benefit. (If only I had CFD...) My guess is that the split is significant, considering the prevalence of Kammbacks in the eco (hybrid) market, also the more extreme Kammback-like sedans (Elantra, Cruze) ever since Congress 2007 wrote the CAFE hike.
