Quote:
manu said:
any spoiler always increases drag, but otherwise decreases lift and the flow what come from the roof will find a grate and faster exit if the spoiler is down, cause the rear turbulences will decrease and this is even more important in terms of aerodynamics than getting into air and in this items the 911 has the best possible silhouette

greets



I'm sorry but no. If the slope is too much, it creates more drag. Did you ever notice those little lips they put on the back of sedans? I've read a few articles that discuss how those small lips actually have important effects that include reduction of lift and drag. It turns out that the soft edge of the usual audi is not the way to go. The same thing is true with the 911. The following quote is made by fellow by the name of Bill Verburg.

"there are 3 different aero aids that are often confused w/ each other.

spoiler - decreases lift and increases drag, only has flow on 1 side
wing - decreases lift and increases drag, has flow on 2 sides
appendage - decrease lift, does not increase drag, and can reduce drag

The aero aids on the early 911, like the ducktail, are more properly called appendages.

Another good source of aero info besides Frere is Dr. Robert White's article in the Nov '77 issue of Pano. He is an engineer that worked at Weissach, and was often a presenter at various PCA tech sessions. A very interesting guy.

this is one of the graphs he presented, it was from the original research at Weissach where it was discovered by accident that as the lid is raised both lift and drag are reduced."

So they've known since the 70s that the slope of the 911 is not the best for drag. As one more example, take the Toyota Prius of all things. It is one of the most aerodynamic cars out there and only has a very slight slope(much less than a 911) to the roof leading to an abrupt drop off. The abrupt drop off actually reduces drag although in a counterintuitive way.