Any Real Advantage To Profiling The Top?

[Angel And The Wolf]

If a boat tail extension's sides fit "The Profile", coming to a point at the back,
(Figure 1), with air pressure spilling off the upper edges as it departs the boat tail, is there any significant advantage to applying "The Profile" to the upper side of the boat tail (Figure 2), or can the top boat tail follow a horizontal line from the highest point of the car, (Figure 3) without any real drag penalty?

[Xist]

The guys on here have said that flat surfaces have surface friction. If you taper you reduce the surface area and the drag from the wake. Also, I am curious how leaving the top flat would affect aerodynamics. It would increase downforce?

[Angel And The Wolf]

It might decrease lift. It WOULD increase cargo space, and provide a tall tail light mounting surface. It would also be easier to put taller side windows, in line with the car's inside mirror, in the boat tail, and the departure face would be a vertical line, so rear drag would be zero, with only surface drag to consider. I'm wondering if the narrowing cross section would grant a similar reduction in drag as would applying the Profile horizontally to a non-narrowing cross section.

[Frank Lee]

Since vehicles are in cross-winds oh, about 100% of the time, the tall version increases frontal area.

[Angel And The Wolf]

Yes, it increases "Side-al Area", which could be a problem for tail wagging in a cross wind.

[euromodder]

Originally Posted by Angel And The Wolf

If a boat tail extension's sides fit "The Profile", coming to a point at the back,

The idea behind the profile, is to disturb the air as little as possible when splitting it @ the front, and then bring the disturbed flow back together @ the rear of the vehicle, with as little as possible extra disturbance added.

The profile is 3D, not 2D, and reduces the wake in all axis

So applying it in 2D, will still have some benefit, but make it less efficient than a 3D application.

(Figure 1), with air pressure spilling off the upper edges as it departs the boat tail, is there any significant advantage to applying "The Profile" to the upper side of the boat tail (Figure 2), or can the top boat tail follow a horizontal line from the highest point of the car, (Figure 3) without any real drag penalty?

If you do that, you're not bringing the displaced upper flow back down to where it was.
And you need to somehow fill the void made by your upturned underside - that'll come from the side, spill down from the top, and add drag.


What was Hucho's optimum angle for a rear diffuser again ? 2-4 ° upturn ?
With the drag penalty rising sharply from around 7°-ish ?
Your rear underside rake is far too steep, too abrupt.


That's not to say it can't be done though : you're essentially drawing a very crude Morelli shape, very close to the ground for a high proportion of its length

That needs to be done far, far smoother for it to work though.

[Angel And The Wolf]

Originally Posted by euromodder

Your rear underside rake is far too steep, too abrupt.

I would prefer a less abrupt angle, but if I decrease the angle, it will drag on steep driveways.

[freebeard]

See the Tropfenwagen. The added area is behind the center of pressure, it will act like a tail.



You're departure angle is suitable for off road; not driveways.

I did this study to see about rearward visibility. The cone is the shape of the VW back window* projected from the rear-view mirror out to the boat tail. You can see the unreflected tip ahead of the windshield.



*the big back window, even though the model has an oval/baby window.