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Folks, what effect would it have to stick a really large spoiler on the back of a ( sedan ) cars trunk/boot - but flip it upside down.
I'm not talking about those factory spoilers that are just for looks, but one that would actually be functional.
Rather than just having an extention off of the trunk/boot of the car, you could couple this with a spoiler about an inch or so above it. ( a biplane sort of thing )
The reason that you would have the top spoiler would be to grab the boundary layer air coming off the back window. The lower spoiler would clean up the flow coming off of the trunk/boot .
I know that you would increase lift on the back of the car with an upside-down spoiler. This could be remedied by having a duplicate set up on the bumper of the car - air from the underside of the car would be channeled upwards creating downforce.

Why attempt any of this stupidity ? Well what I am wondering is if it would be possible to 'squeeze' the air coming of the back of a car into a smaller wake, thereby reducing drag.

Speaking of wake size, how is it that a hatchback car can have a .Cd figure that is actually better than a sedan version of the same car ? The wake on the hatchback is twice the size of the sedan !

( My '93 Civic hatchback has a factory .Cd of .31 while the sedan version is at .32 )

( Why would I think up such a stupid question ???
Well .... I keep seeing Crown Victoria taxicabs with these signs mounted to their trunk on legs looking very much like a backwards spoiler.
The signs are tilted to where the guy driving behind the taxi can read them .
I guess I'm just really bored, because I begin to wonder what is happening to the airflow as it hits that 'spoiler'
I wonder if it actually decreases the cars wake. )

A hatchback can be (though is not necessarily) a Kamm back design which achieves much of the function of an idealized teardrop rear end shape, though is cut off so the taper is not complete. Well executed Kamm back cars usually get very low cd values, the Prius is one.

I suspect your Civic is an example of a Kamm back. A sedan with an angle where rear window meets rear deck will not likely be an example of a Kamm back. You can look it up in wikipedia for more information.

For a while, and possibly still, there was an "option" for
many minivans referred to as a rear spoiler, but it was an
inverted one that was supposed to bring air *down* off the
roof and merge it into a trailing airflow better. No idea
if it's really supposed to work that way. My Subie even
had a lame approximation of one, and I think I got about
1 mpg better [out of about 26] after ripping it offa there.
.
You would think that unweighting the wheels a little, if
it could be done without increased drag, would be a *good*
thing for MPG even if it's a bad thing for racecars.
.
_H*

I seriously doubt adding something like what you're suggesting to a car will help FE, it will probably make it worse. You'd need a wind tunnel and some aerodynamics engineers to make it the correct shape and size to do what you want it to, otherwise you'll just decrease the car's aerodynamics, as well as make it more dangerous to drive (drive it too fast and your rear tires could loose all grip).

If you're thinking on improving the aerodynamics of your car, I think the best thing would be to fit some flat panels on the underside of the car. Check out Wayne's review of the new Civic hatchback (in the Honda section), it has a really clean underside and excellent FE.

An example of how not to do it:

http://www.autoblog.com/2007/04/19/aerodynamic-dyslexia/

.

:purplebananna:

Wow tarabell, it looks like you came up with an exact example of someone else already doing it!

.