Aero mods - Volvo wagon

[basjoos]

The rear spoiler should be better than your stock configuration, but is no substitute for a boattail. My civic came stock with a roofline extension rear spoiler at the top of the hatch, but still had a big eddy swirl behind the rear glass. My question is, if a Kammback is so good for low drag, why can't you find Kammbacks in nature? I can't think of a single bird, bat, fish, squid, or insect sporting a Kammback, they all have boattails. Likewise I can't think of any aircraft with a Kammback. If you really want to go low drag, you will need to have a boattail.

There are several possible building approaches. If your hatch opens upward, you could build the boattail on the back of your hatch, set up so it swings up when you open the hatch like the nose of a C5 cargo aircraft. If designed properly, it could be easily dismounted from the hatch when you don't need it. Get your lighting electricals from the trailer electrical plug. An alternative approach would be to plug a Reese hitch-mountable cargo tray on the back of your car and build the boattail on it, so the boattail can be removed with the tray. Of course the top part of the boattail will have to be clear for visibility and you will need to have a licence plate holder on the boattail to transfer your licence plate to when you use the boattail. You could install the boattail when you need it (high-speed interstate trips) and remove it when you are mostly doing local, slow-speed driving.

[MetroMPG]

I've read conflicting views about the effectiveness of the Kammback.

Wikipedia says they're actually better than a full teardrop boattail, because they achieve desired streamline characteristics without as much surface drag. For something moving through air at automotive speeds, I find this hard to believe.

I think I've also read that the Kammback is almost as good as a full teardrop, but not quite. Meaning it achieves nearly the same level of drag reduction with much less overall length.

[brucepick]

Originally Posted by basjoos

The rear spoiler should be better than your stock configuration, but is no substitute for a boattail...
If your hatch opens upward, you could build the boattail on the back of your hatch, set up so it swings up when you open the hatch like the nose of a C5 cargo aircraft. If designed properly, it could be easily dismounted from the hatch when you don't need it...

Dang. That's tempting. I could mount a boattail to the tailgate as you mentioned. It would extend sideways beyond the tailgate to meet the body sides - but would be attached to the tailgate.

I drive mostly highway. I could leave the boattail mounted steady-state and only remove if needed. I could build it from plywood and/or plexiglass/lexan and/or aluminum angle stock. There are some odd curves in the sides that would need some attention but I'd probably come up with something for that.

[VetteOwner]

plywood=heavy
wet plywood=very heavy
plexiglass=one rock and its toast and can get heavy
alumnum looks liek your best bet for a strong frame to start on. lexon would be a better choice for the rear window. and its relatively light.


aslo im sure ur state has some regulation about having objects sticking so far from your car it requires a red flag during the day and some type of warning light at night. but if you mount some tailights back there then you would be fine.

[brucepick]

Originally Posted by VetteOwner

plywood=heavy
wet plywood=very heavy
plexiglass=one rock and its toast and can get heavy
alumnum looks liek your best bet for a strong frame to start on. lexon would be a better choice for the rear window. and its relatively light.

aslo im sure ur state has some regulation about having objects sticking so far from your car it requires a red flag during the day and some type of warning light at night. but if you mount some tailights back there then you would be fine.

I'm pretty good with auto electrical work so I'm sure I can handle adding the needed lights. Locating nice bulb housings + lenses at a good price might be an interesting challenge.

Hmm. I always think wood since I'm experienced in working with it. I'll weigh my remaining 1/2 sheet of 4.5 mm. lauan plywood. Painted, it will stay dry. In any case the frame pieces against the body probably should be wood 1x1's as they have to flex to match body curves. Angle stock will just kink if I try to curve it. Plastic 1x1's might be OK if such exists, but could break as a surprise event under temperature + vibration stress.

Correct re. lexan vs. plexiglass, lexan wins.
All lexan = probably too pricey
I have to spend some time studying basjoos' pics to see that design.

[brucepick]

I can only find one pic of the rear aero work that basjoos did.
Can somebody post links to others please?

[red91sit]

Originally Posted by brucepick

I can only find one pic of the rear aero work that basjoos did.
Can somebody post links to others please?

Maybe this one helps?

[basjoos]

Plywood doesn't have to be heavy. Yes, home construction plywood is heavy, but the thinner thicknesses used in monocoque construction of aircraft and small boats can be used to make very light and strong structures. My boattail is a monocoque of 1/4"x1" aluminum bar stock and either coroplast or clear vinyl. Monocoque construction seems almost magical. You can take a flimsy sheet and a wobbly frame and bind them togather to create a very strong rigid structure.

Here are some photos of my boattail. I used the tail of a Questair Venture kitplane as a model for the shape. The Venture is a 350mph kitplane nicknamed "The flying egg" for its short fusilage. If its tailcone can retain attached airflows at over 300mph, its shape should have no problems keeping attached flows at highway speeds dispite its stubbiness. Since the boattail is going to be operating next to the ground, it ideally needs to be not as symmetrical as the Venture tailcone, but should have a flatter curve along the bottom where (because of the ground) the air has to come in the from the sides to fill the space under the boattail. I retained the symmetrical shape on my boattail because I have a steep approach angle on my driveway and didn't want any chance of my boattail dragging the ground.




An earlier version of my boattail that still allowed me to use my hatch


Questair Venture



More photos of my car are at:
http://smg.photobucket.com/albums/v4...Honda%20Civic/

[SL8Brick]

Hi brucepick. Here's a link with pics of my 245 hatch spoiler:

http://gallery.newbug.org/volvo

I don't think its along the lines of what you are trying for, but at least you know what I was refering to. As you can see, it is rather small and doesn't follow the full width of the car. I figured it be useful for accumulating high pressure on the center/rear roof section while still having 'dump spots' along the sides.

BTW - I'm working on deleting the d-pillar antenna in exchange for something more efficient.

[brucepick]

Your pics do give me the thought that I could attach whatever to the flat upper part of the liftgate instead of the rear surface. Gotta check my car; it has the older '89 tailgate.

Thanks!