What’s the point Tail Fin?

[scott449]

I have a 996 C4 and the cool tail fin that I can put up or down. wow how cool!

My question is.

Are there any performance stats on that tail fin? Do I get more down force? Better gas mileage? Cooler running temps? What is the point?

Edited April 22, 2009 by scott449

[Loren]

I have a 996 C4 and the cool tail fin that I can put up or down. wow how cool!

My question is.

Are there any performance stats on that tail fin? Do I get more down force? Better gas mileage? Cooler running temps? What is the point?

From the Porsche MY1999 (intro year) Service Technik Book (available here to our Contributing Members).

"Retractable Rear Spoiler

The spoiler stabilizes the air-flow at the rear-end of the vehicle by means of a defined burbling of the air-flow while simultaneously increasing the pressure around the front of the rear spoiler.

This reduces both drag and rear-axle lift. An additional side-effect of this increase in pressure is an increase in the volume of air drawn in by the engine as well as in the flow of air through the engine compartment ventilation system whose inlets are located at the rear lid.

At low speeds, aerodynamics play only a minor role in the composition of road resistance values. For this reason, the spoiler is only extended at approx. 75 mph and is retracted again at approx. 37 mph.As a result of detailed and intensive work on the aerodynamics, it has been possible to reduce the size of the spoiler on the new 911 Carrera compared to a"that of the old 911 Carrera."

[BruceP]

Just to guild the lilly on this: That thing in the back is a spoiler, not a wing. So it isn't meant to create downforce, which I guess is what you might mean by 'performance'. A spoiler is so-called because it disrupts airflow, in this case preventing the car from turning into a giant wing that lifts the back end off the ground.

[Speedobird]

Just to guild the lilly on this: That thing in the back is a spoiler, not a wing. So it isn't meant to create downforce, which I guess is what you might mean by 'performance'. A spoiler is so-called because it disrupts airflow, in this case preventing the car from turning into a giant wing that lifts the back end off the ground.

Actually, I was looking for a decorative tail on my 99 996 but this caught my eyes. I had to write into this stream to share. If you are interested in how auto makers develop aerodynamic design, please read on:

Road Adhesion: The rear spoiler reduces (spoils) the lift created by upper half of the body compared to lower half (under carriage), hence increasing the pressure/lift differential between the two and creates downward force. This increases road adhesion in conjunction with so called ground effect, which is a suction created by the tunnel effect between the road and undercarriage.

Drag control: As for the drag, this is a high speed phenomena. For most of us, it probably does not have significant impact because, of all drag components including road drag, mechanical drag and aerodynamic drag, aerodynamic drag should be the smallest in magnitude. But there is a need to control the source of aerodynamic drag, wake. Wake behavior behind the body determines aerodynamic performance. Smaller wake cross sectional area is desirable because, simply put, the body is more streamlined that way. (static pressure differential between two cross sections just in front of the car and behind determines momentum loss in air flow. This translates into aerodynamic drag.) Although I haven't seen this particular simulation or wind tunnel results, structure like this could be installed to "control" the flow structure. By disrupting the flow, wake becomes more manageable or predictable, like vortex generators on airplane wings. Spoiler controls the size of wake and ensures that it does not behave erratically like wild oscillation at certain speed and angle of attack, which leads to,

Stability: Auto makers, particularly Europeans, are concerned with aerodynamic stability in addition to drag and lift. The flow disruption by the spoiler makes the wake behave more regularly without wild oscillation at some high speed. Wake, although not visible, acts on the body just like when you have a strong cross-wind on high way. This becomes more pronounced at higher speed. Auto makers usually measures this by placing car body in a wind tunnel at an angle to measure torsional force.

Like I said earlier, if you know of good tail to install, please let me know.

Cheers,

[phillipj]

Speedobird,

Great info! Thanks for that. :clapping:

Edited May 21, 2009 by phillipj