Update! Are there any magic numbers? Updated!
 

I am doing a rear window extension, defuser, rear window spoiler. Seems like many names are being used for the same part.

Anyway, is there a magic number for the length or just what works or looks good?

I know the degrees from the flat roof line is 15 other than that I am just winging it.

Any contributions would be greatly appreciated. Comments work too but money is better.

Experimentation and tuft testing may help in figuring out the size and angling, though an oft-cited resource is Hucho's book on aerodynamics. Search for it on Amazon.com. It's massive (both price and size) but it has those "magic numbers" you're looking for.

If you're lucky maybe your local free library has one....

The 15 degrees came from Hucho's book originally but was posted here in this forum.

I am guess I am going to have to wing it I don't have access to such a book. I figure that anything is better on a sedan or coupe anyway. I think I will go with 6-7" range. It won't hurt to try. If it doesn't help, I'll take it off and sell it. It's going to look cool after I am done. Somebodies gonna want it.

Oh well back to work. Got one side of it done already. The other side will be faster, I know what I am doing now. :)

Not sure what you are building
if it is a Kamm back - just make it gently curved
metroMPG has a good thread going on this right now
no magic - just the longer the better

if you are building a spoiler and not a Kamm back
you might find this interesting

about a third of the way into the infomercial they show a back view of this concept SAAB
looks like it has active spoilers that extend behind the hatch and under the bumper - but they seem to indicate they are for low drag


YouTube - Efficiency at Work

been thinking of something similar myself
interested to see what you are building

Here's a pic of what I am working on.

This is just the one end cap. It ended up being three pieces.

Two end caps and the center visor part. Going to get welded together when it's done. The two ends are done and the center is done just have to put it together and make a couple of supports for the center.

It basically snaps into and around the rear window molding. I made it out of steel because that's what I had and know how to work with.

It is 15 degrees from the flat of the roof. and is 7" in depth and 2" from the rear lip to the rear window.

I also have in the plans a trunk lip extension.

I would have finished it if it hadn't been for that damn weed wacker I had to fix. Got it going too.

angles
 

Lots of pages in the aero forum now.If you can find my installment on "Rooflines",it may answer your questions.It's not a matter of "angles",its a matter of curvature,and it all depends on where the Accord's roofline begins it's downward slope.Do the research before you jump in.At 15-degrees,I think your headed for trouble and disappointment.

justpassntime,if you'll go to the top of the forum page,at ecomodder search,Advanced,and type in, mods-data-% change or Cd change ( installment # 5- roofline data ),that should get you some material to chew on.

It’s always a tradeoff between minimizing wake area and having an excessively steep slope. My bed cover does a nice job of minimizing wake area – a 47% wake area reduction – but does so at the price of having a seventeen degree slope.

It is time for me to revise my wake cover to make it handier to use. What I am thinking of is to reduce the slope to 11 degrees (1-in-5). This will mean my wake area reduction will be only 27%.

Going to a 1-in-4 slope (14 degrees) would result in a 37% reduction in wake area.

Does Hucho have a graph showing the effects of wake area reduction on drag?

I can’t do a full boattail. That would result in a thirteen foot long “tail” if one stuck to the one-in-five slope I’m planning. The truck is enough of a bear to park as is. The other extreme is simply a flush commercial cap. Zero degree angle but zero wake area reduction.

Nice job on the metal work, Looks good. :thumbup:

yep, it is going to look cool either way
drag wise it looks pretty clean too
but I'm with Aerohead if you are 15 degrees below the roof line
who knows, at this point tack it together and do some tuff test
you will figure out if it is a drag reducer pretty quick

interested in the outcome
looking at this for my truck - but only virtually for now :)
YouTube - s10 aero sim - air dam 1 & foils 1
YouTube - s10 aero sim - re 5,000,000 & Kamm backs
need to get my wheel covers and skirt done first :turtle:

Big Dave - like your bed cover
spent my money on a cheap flat one before I knew any better :o