> what do you think about the spoilers mounted on the rear window?
> I'd like to install one on my Audi A3 (for the aesthetics effect) but
> I'm insecure about his effect on boundary layer.
> Do you think it can raise the drag?
Maybe, maybe not. The devil is in the details.
If the purpose of the spoiler is to create downforce (a lift force) then
there is induced drag created.
Some cars inherantly have a lift force at speed because of the body
shape and interaction with the road surface. A spoiler that merely
reduces this lift could be drag free (or even reduce drag). However,
such a spoiler is a difficult design and must be done with a wind tunnel.
