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Porsche Aero

How come you criticize the 911's brakelight for separating the flow at the top of the rear windscreen, and then later in the same article conclude that the beetle would be improved by having a tiny spoiler to cause separation at exactly the same location? It seems to me that Porsche used their brake light to do exactly the aero mods that were needed.

Patrick McAuliffe
Australia

The Porsche needs airflow to the rear intercooler inlet.

LPG Oxy Welding

Your article on oxy/acetylene welding was excellent as usual, but I thought I'd suggest an alternative that I have been using for years - oxy/LPG. Oxy/acetylene can be a dangerous system to uses as you mentioned, especially if no anti-flashback devices are fitted (and for home/casual use they remain expensive).

I rent just the oxy bottle (so saving the cost of acetylene and the bottle rental, which is high for casual users) and use standard 4 kg or 9 kg LPG gas bottles - with my BBQ bottle always available too if I run low and LPG is available 24x7 in most places.

Easier to light, safer to use and the temperature difference is bugger all for most applications - and an advantage when using on thinner materials. Oxy/LPG is plenty good enough for cutting jobs too I have cut through large RSJ beams and 16mm steel plate with little effort, nearly as fast as with acetylene and with less distortion.

To use LPG you have to get a new nozzle for your cutting head (at around $40-50) and sometimes an adapter for the bottle so the regulator screws on correctly. The standard oxy/acetylene regulators work fine as do the standard flame nozzles. I have been using my CIG Comet 3 system for more than 15 years doing all sorts of work and have had no problems.

Steve Blume
Australia

Aero Testing

I was wondering if you guys could do some wool tuft testing on a car that has no spoiler and then again on the same car but with a spoiler. or if you guys can show a car that has an aggressive looking body kit and wool tuft test that car and then do the same car stock just to see if it really does affect the aero performance. i just think itll be cool to see. :D

Steve Dogg
Canada

We haven’t done this in AutoSpeed but we did exactly what you suggested for a previous magazine. After the kit had been fitted it looked better in the wool tufting but a professional motorsport driver could tell no difference on the track. Fuel economy? We don’t know.

Vortex Generator Inventor Comments

I just read your road test article on Airtab VGs. My compliments on your work and photos! It was an unusually good article. I am the original inventor of Airtabs (originally called Wheeler VGs or Wishbone VGs in various NASA WT reports) and sold the rights to some good friends I met while lecturing at U.S. Navy Test PilotSchool, who then formed Airtab back in the early 1990's. Prior to that, I had over 600 hours of wind tunnel occupancy hours working with them and can add some comments you might find interesting.

I have a couple of comments:

1) The slight "outward-pointing position" of the lower outboard tufts on the rear window of the Prius probably does not indicate ordinary flow separation.  It is more likely the case that the tufts are positioned along the edge of a large rotational flow caused when the side flow is pulled inboard as it exits the slanted C-pillar (due to the low static pressure on the rear window). Ordinarily a large, slow-turning vortex is formed there and it will be particularly strong if attached flow is created over the entire rear window. If the lower corner tuft fluttered wildly, then the flow is separated.  If it merely angles sharply outboard as shown in the photos, then it is being swept there by the slow-rotating C- pillar "vortex".

2)  I consulted to GM on their Aero 2002 and used my VGs along it's trailing edge just like you used them on your Insight, and we achieved the lowest "unpowered" Cd I know of on a driveable automobile. (I say "unpowered" because the Ford Probe-5 recorded a lower figure, but only with high-powered fans located in the trunk that sucked air in from the rear quarter panels then blew it out the rear. As far as I'm concerned, that doesn't count, because if enough power is supplied to the fans, the Probe-5 could record zero aero drag or even positive thrust!)

Regardless of that, to be honest, the gain on GM's Aeros 2002 was miniscule because of the small base area. Similarly, the Honda Insight is not a suitable vehicle to test the base pressure drag alleviation capabilities of the VGs. The base area of such cars is so small the parasitic drag of the devices will approximate the drag reduction achieved.  This application for VGs should be tested on a hatchback or van where the base area is a very large fraction of the car's frontal area. Then you should see some nice results. (If you Email me at my home I can provide some very convincing wind tunnel data proving Airtabs can reduce base pressure drag from both Lockheed Georgia WT on a full size 1982 Honda hatchback and 1984 McDonnell Douglas, where we spent a month evaluating them on a truncated external fuel tank for the F-15.)

Gary Wheeler
United States

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