Worse than a fastback design?

[mikesheiman]

I am building a custom car with the following rear end shape:



When I showed this on one forum, a member claimed there was no way air could flow around the taper and said my design couldn't possibly hit under 0.30 CD regardless of the otherwise gently sloping shape (the hood slope joins all the way down to the floor, no bumper deadzone, no discontinuities between the hood/windshield/roof/tail).

Is the taper actually hurting my design vs. say, an untapered fast back shape or even a curved hatchback design like on a Chevrolet Sonic?

I know it's no boat tail, but I wonder how tapering could be, perhaps, worse than nothing.


Assuming I failed miserably at designing the rear shape, any crafty ways/places from which to duct air to fill any discontinuity vacuum in the rear?

I('d) really appreciate any help here.

[Frank Lee]

The front fender tops appear to me to slope too much and result in disturbed flow quite a way back. The rear appears to me to taper in too quickly in plan view.

I'm not convinced ducting is ever the best solution because ducts add surface area and thus surface drag vs a correct form.

I agree that sub .30 seems unlikely.

[mikesheiman]

Ok, so it tapers in too quickly...would it actually be better off as a hatchback e.g. no taper at all? I know even some hatchbacks get about 0.3 CD.

And I think I see what you mean about the sides. As in they push air up, only to curve into an area where it's quickly pulled down a bit, thus causing a vacuum/turbulence area? How much CD would you guess that issue alone costs and what's a good way to fix it (simply keep sloping upward and very gently start curving downward to meet the very side/tail area)?

[Frank Lee]

If it drives tuft test it.

[Grant-53]

The taper resembles some of the human powered vehicle shells. First tuft test the shape you have with and without a rear deck wing. Dr. Joseph Katz has some good books that are less expensive than Hucho's grand book. My suggestion is to use a shape based on a 5:1 airfoil with a Kamm tail at 0.75 the length.

[kach22i]

Aerodynamics by George Kachadoorian | Photobucket


A picture is worth a thousand words?

I included Frank's in-fill comment for the front wheel, that of course would be a rounded edge, just showing the concept here.

As originally presented, looks like one of those designs which would perform better backwards - sorry.

[mikesheiman]

Originally Posted by Grant-53

The taper resembles some of the human powered vehicle shells. First tuft test the shape you have with and without a rear deck wing. Dr. Joseph Katz has some good books that are less expensive than Hucho's grand book. My suggestion is to use a shape based on a 5:1 airfoil with a Kamm tail at 0.75 the length.

Will do and thank you!
The Kamm tail/desk wing seems to be an obvious mod to test. Will look into/read-up on Katz.

When you say 0.75 the length, do you mean 75% of a full teardrop shaped rear end (seems a bit long), or % of what, exactly?

The real mystery to me is how much should I taper the rear...given I'm starting with flat sides at about 70 inches width, what should my width be about, say, 3 feet back?

Or should I just try cutting off the back end taper entirely (straight down, forming a Kammback on the sides of the rear wheel) and just leave the rear wheel fairing sticking out of the back area with its further extended Kammback?

[mikesheiman]

Originally Posted by kach22i

Aerodynamics by George Kachadoorian | Photobucket


A picture is worth a thousand words?

I included Frank's in-fill comment for the front wheel, that of course would be a rounded edge, just showing the concept here.

As originally presented, looks like one of those designs which would perform better backwards - sorry.

Indeed...that just looks awesome and makes perfect sense...love the whole angled taper concept!

Far as the fill-in (actual fabrication), any recommendations on how to achieve that (materials, tools, method) without it looking like a really crude hack job and having to mess with redoing the fiberglass? I figure that's something I could knock out quickly given the right method, unlike the rear section redo.




And, agreed, it does look like it would perform better backward, but then how would/could I make it stable as a 3 wheeled vehicle and have enough headroom for the rear passengers? Not asking that as a rhetorical question...I may well (if only quite eventually) remake the car in such a body style eventually if it's possible.

[mikesheiman]

So I'm guessing 10-12 degrees is the suggested angle range for any and all tapers in the back (essentially along straight lines inward)?

[kach22i]

Originally Posted by mikesheiman

Far as the fill-in (actual fabrication), any recommendations on how to achieve that (materials, tools, method) without it looking like a really crude hack job and having to mess with redoing the fiberglass? I figure that's something I could knock out quickly given the right method, unlike the rear section redo.

This is what I did, and it lasted.........but some will cringe.

Bodywork on S10 Pick Up - Epoxy - Pelican Parts Technical BBS

Cab hole; This was repaired once before, opened it up and not a lot left to work with. I took a can of spray foam and let it form a backer (despite knowing that it may absorb moisture over time). I chopped up some fiberglass strands and epoxied them, using plastic wrap and tape to defeat gravity.

I'm thinking you could glue on some foam or spay foam on and shave off the excess, then glass it up.

My repair using foam lasted longer than the non-foam side, so my worries about the foam holding moisture turned out to be unwarranted.