Blowing the Vortex, Part 3

This article was first published in 2006.

In this series (see Blowing the Vortex Part 1 and Blowing the Vortex Part 2) we’ve already covered how vortex generators work and what they do on the Mitsubishi Evo Lancer. But what about fitting them on your own car? The first step is to source – or make – some vortex generators...

Making Your Own

Vortex generators are often pretty simple shapes. The ones trialled by Mitsubishi on the Evo were categorized as either bumps...

....or deltas. The dimensions are given as multiples of the height (h) with 25mm high ones typically proving to be the most effective on the Lancer. For testing purposes, these vortex generators can be made of anything that can easily shaped – wood, clay, even papier mache.

This Décor brand plastic spoon closely resembles the bump-shaped vortex generators trialled on the Evo. With the handles cut off....

...it’s not hard to quickly make a bunch of vortex generators very similar in shape to the Mitsubishi ones and see how well they work.

Aircraft vortex generators used on wings are often just flat pieces of aluminium slightly angled to the flow....

...which are quite easy to make using aluminium extrusion.

Commercially Available

However, while it might be worth making your own simple ones for initial test purposes, producing effective and durable vortex generators is a much more difficult ask and may well be more trouble than it’s worth. That’s especially the case when there are at least two different design commercial vortex generators available quite cheaply, both designed for road vehicles.

www.fuelsavers.com.au is an Australian company offers that vortex generators folded from sheet aluminium. A car kit that comprises nine vortex generators, cleaning wipes, a template and instructions costs AUD$110 including (Australian) delivery.

Airtabs are made in the US and are widely available. They can be bought directly from www.buyairtab.com at a cost of US$2.50 each and are available in black or white. In Australia, the Kenworth ALLRig dealer network stocks clear Airtabs at a cost of AUD$5.50 each – see www.trucknbus.com.au for the nearest dealer.

Airtabs are made from ABS plastic and are quite differently shaped from the vortex generators we’ve so far examined, although the shape is used in some wing applications. They come with a pressure-sensitive adhesive tape on their base. The main Airtab website is at www.airtab.com.

Commercial Claims

Both the manufacturers of Fuelsavers and Airtab vortex generators make major claims for the effectiveness of their products.

Fuelsavers claim fuel economy improvements of “up to 11 per cent” and have testimonials claiming 11 per cent fuel economy improvements on 5 and 8-tonne pantechnicon trucks, 12.5 per cent on a 1990 Holden Commodore, and 9.5 per cent on a Subaru Outback.

However, is this technically likely? The Bosch Automotive Handbook (6^th edition, P 890) shows changes in fuel consumption with changes in aerodynamic drag. For a medium sized car with a drag coefficient of 0.30, a reduction in drag by 0.04 (ie a 13 per cent decrease) reduces fuel consumption at a constant 120 km/h by 7 per cent. In the European highway cycle it reduces fuel consumption by 2 per cent and in the city cycle, by 0 per cent.

Based on this data it is difficult to believe that the fitting of vortex generators would reduce drag to the extent that fuel consumption improvements of over 10 per cent are realised in normal use.

Both vortex generator manufacturers suggest using lots of vortex generators and purchasers are advised to position them at the end of the vehicle as well as at body transitions (eg from roof to rear window). However, even given their more numerous use, it needs to be kept in mind that when Mitsubishi placed vortex generators at the end of the Evo Lancer’s roof, the drag reduction was less than 2 per cent (assuming a 0.35 drag coefficient). (See Blowing the Vortex Part 2).

Airtab’s fuel economy claims are more modest, with a 4-6 per cent improvement cited. The devices have been fitted to many trucks and semi-trailers and it is for these vehicles that the claims are made. Airtab’s testimonials page at www.airtab.com is very interesting, with higher stability, better rear visibility in wet conditions, and improved fuel economy all claimed by users.

Incidentally, near the bottom of the Airtab testimonials page is this incredible pic of a truck racing at Pikes Peak. The truck is fitted with Airtabs across the rear of the roof. The website says:

The amazing photo shows how the 16 Airtabs on the roof smoothly bend the airflow down into the area behind the cab. The photo shows the Airtabs create an array of vortices that smoothes airflow behind the cab so it can closely follow the wing's contour to create more downforce and traction. The bending of the air coming off the cab allows the wing to operate in clean, undisturbed air, improving the wing's effectiveness. The effect of the wing can be clearly seen as it lofts the air upwards. Rare weather conditions make the vortices look like a layer of white fog in this remarkable photograph.

However, without another photo showing the foggy airflow without the Airtabs in position, not a lot of conclusions can be drawn. But it is a fascinating shot!

Conclusion

For our money, Airtabs are the vortex generators to buy. The company producing them makes claims which are more in keeping with technically feasible outcomes, the devices themselves are arguably better looking than folded aluminium designs (and certainly are much less dangerous protrusions to have mounted on vehicles) and are cheaper.

But do they do anything for cars? Next week we’ll find out.

Interested in do-it-yourself car aerodynamics? You’re sure then to be interested in the Amateur Car Aerodynamics Sourcebook, available now.

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