Sedan drag reduction with special spoiler

[Piwoslaw]

This is a section from the book I recently acquired:

An important parameter in researching the aerodynamics of sedan-style vehicles is the effective angle of slope, measured from the rear of the roof to the rear of the trunk.



The above drawing (according to Nouzawa et al.[2]) shows how Cx increases depending on the effective angle of slope. Notice the similarity to the angle of slope in a hatchback, which has a minimum around 12 degrees and a maximum around 28 deg. Two ways of improving Cx is to either extend or to raise the trunk lid.

Fukada et al. ([1]) noticed that Cx could be reduced by adding a deflector to the central section of the rear edge of the trunk lid. A spoiler on the whole width of the rear edge produces a large downforce, increasing drag, but a small (1/3 width of trunk) deflector gave half the downforce and a reduction of drag (compared to no spoiler at all). The reason for this is as follows: the body of a sedan produces a pair of trailing vortices and the small deflector produces vortices of the opposite direction of rotation, thus partially cancelling the first pair.



This lead to the idea of building a triangular-shaped deflector on the trunk lid to use as a vortex generator. The drawing below shows how drag Cx and lift Cz depend on the width of the deflector.



[1] Fukada H., Yanagimoto K., China H., Nakagawa K., Improvement of vehicle aerodynamics by wake control, JSAE Review 16 (1995), p.151-155.

[2] Nouzawa T., Hiasa K., Nakamura T., Kawamoto K., Sato H., Unsteady-wake analysis of the aerodynamic drag on a notchback model with crotical afterbody geometry, SAE SP-908, paper 920909, February 1992.

[tasdrouille]

The c6 z06 corvette has that kind of spoiler specifically to reduce drag.

[botsapper]

Calling for curious ecomodders,...for 'Nouzawa' mock up spoiler designs for coast-down testing, trailing edges & vortices tuft testing...(w/ video documentation, please.)
Instead of coroplast, try expanded PVC foam sheet material (Sintra) for a lightweight & durable modeling/fabrication material. Can be easily trimmed, die-cut, laminated, glued, routed, heat bent, welded & easily finished/painted.

[aerohead]

The center image,illustrating the trailing vortices does not share the same roof width,or tumblehome of the roof for the first and last image.------------ This would affect spoiler performance,as in the last image you can see that the width of the roof as measured at the base of the rear windshield is much narrower compared to the center image.----------- The airflow at the trunklid would be completely different and consequently,spoiler width parameters would also differ.-------- It's a subtle difference,but it is still "apples and oranges" to the air.------------- The trick for the spoiler,is to get top and side air to merge at similar velocity and pressure,so as not to spool up into the drag-producing vortice.

[Frank Lee]

no car has that spoiler

[shovel]

I've always been curious about the aero implications of the OEM spoiler on early 90's Dodge Stealths...

[aerohead]

Originally Posted by shovel

I've always been curious about the aero implications of the OEM spoiler on early 90's Dodge Stealths...

Mitsubishi,who builds the Stealth is sold in Europe where it may find itself on the Autobahn.Some of the sections are posted unlimited speed,and a motorist could legally run flat out.My guess,is that the spoiler was designed for that kind of application (true downforce at true high speed) and just happened to enter the US market only because of the cost to delete it.

[itsmedc]

Originally Posted by aerohead

Mitsubishi,who builds the Stealth is sold in Europe where it may find itself on the Autobahn.Some of the sections are posted unlimited speed,and a motorist could legally run flat out.My guess,is that the spoiler was designed for that kind of application (true downforce at true high speed) and just happened to enter the US market only because of the cost to delete it.

The Mitsubishi version (3000 GT) uses a more conventional spoiler location at the tail of the car compared to the one on the Dodge version (Stealth).
Link to the Mitsu 3000GT
http://www.swaqvalley.com/Blueprints...shi_3000GT.jpg

Although I have always wondered about the true Aero implications of the one on the Dodge version.

Especially condsidering the reported effectivness of the double spoiler on the Merkur XR4Ti.
http://cache.jalopnik.com/assets/res...Rr_RH_High.jpg

[groar]

The volvo S40, DRIVe edition, has a flat spoiler at the rear end of the trunk. From volvo's web site it participate to 15% of aero drag reduction (other is from grill partially closed, car lowered and better tires). I remember the HCH-II has such a spoiler.

In Huchoo's book, the trunk aero can be improved by lowering the angle, ie make the trunk longer and/or higher. He shows Audi improved the Cd by 0.025 between version II and III of the Audi 100 by making the trunk longer by 75 mm (3 in) and higher by 50 mm (2 in).

As I will never have enough time in following months to make my uglyback harder (and uglyback's coast down testing was a failure 'cause of traffic), I'm thinking about doing a flat spoiler in a quick and ugly way to the very rounded megane trunk.

Denis.

[Cd]