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Old 06-15-2011, 06:45 PM   #1 (permalink)
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Air Tabs

AIR TABS COAST DOWN
My friend and I played hookey from working on his front yard one night and took his van out to get some data. We didn't have much time to do data acquisition, so we only got a total of 5 bi-directional runs, 3 with Air-Tabs, 2 without, so it's not much data. We already knew from previous testing that the Air Tabs do alter flow patterns at the back of the van, bringing the flow more inward all around, so it gave us some confidence that we may see results. I ran the numbers, but don't have much confidence in them, since it's such a small data set and the lines are all over each other.
I assumed the rolling resistance would not change between runs, so I just set it the same, and solved for Cd.
JUST THE VAN
Direction A - RED (Cd~0.53)
Direction B - ORANGE (Cd~0.74)
VAN + AIR TABS
Direction A - BLUE (Cd~0.55) [~-2% Highway Fuel Economy]
Direction B - GREEN (Cd~0.80) [~-4% Highway Fuel Economy]
I think if we wanted a higher confidence we would need to do a larger set of coast downs in an area with less variations in directions (you can clearly see the Direction A vs. Direction B at their tail end, so the numbers above are split out). If the numbers are correct, and the AirTabs add drag, the difference seen here would equate to between 2% and 4% highway fuel economy.

AIR TABS WAKE TESTING
I attached a picture of the Van Wake Test Demonstration Instrument (VWTDI, my friend is a big VW Diesel fan). We did all flow tests at 40 miles per hour on calm days. My friend drove, and I rode in the back and watched what was happening through the rear window, then I took notes while he was setting up for the next run. Horizontal and vertical markings are on 6" centers, and the tufts on the "wing" are at 3" centers.

Differences in flow were seen with the use of Air Tabs. Flow came in at a sharper angle, but as can be seen above, that did not translate into drastic improvements in the coefficient of drag. Handling and stability were not tested.

I am guessing that since we have the Air Tabs we will be doing more testing with them placed in different configurations, but no guarantees.

Comments and questions are welcome!

Wyatt

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Old 06-16-2011, 04:09 PM   #2 (permalink)
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I would try doing only the top and only the sides and see what the wake looks like, also try only using half as many and maybe only start them above the line of the tires. It would be interesting to see what the difference would be if you turned them the other direction.
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Old 06-16-2011, 04:44 PM   #3 (permalink)
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Have you thought about turning them around (180) and using about half as many?
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Old 06-18-2011, 06:39 PM   #4 (permalink)
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Quote:
Originally Posted by Pendragon View Post
Have you thought about turning them around (180) and using about half as many?
Manufacturer of Airtabs specifically warns they are designed to work ONLY if the wide end is towards the front of the vehicle, not the other way around. I know it seems like the point should point forward. Yet another aero vodoo mystery. I have a set installed on my Mini Cooper hatchback.
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Old 06-18-2011, 09:52 PM   #5 (permalink)
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Quote:
Originally Posted by InsightfulRay View Post
Manufacturer of Airtabs specifically warns they are designed to work ONLY if the wide end is towards the front of the vehicle, not the other way around. I know it seems like the point should point forward. Yet another aero vodoo mystery. I have a set installed on my Mini Cooper hatchback.
Thaks for the clarification. That is interesting.
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Old 06-20-2011, 08:25 AM   #6 (permalink)
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Some of the air tabs don' look exactly straight in the picture, but I don't think that is enough to make much of an improvement. Also of note moving the position of the air tabs fore or aft will have an effect on their performance.

Do you have the air tabs spaced as recommended by the mfg?

Potentially a better test would to be marking a starting point on a hill someplace with some chalk and doing a distance coast test from the same point each time.
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Old 09-06-2011, 12:21 PM   #7 (permalink)
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Quote:
Originally Posted by ConnClark View Post
Some of the air tabs don' look exactly straight in the picture, but I don't think that is enough to make much of an improvement. Also of note moving the position of the air tabs fore or aft will have an effect on their performance.

Do you have the air tabs spaced as recommended by the mfg?

Potentially a better test would to be marking a starting point on a hill someplace with some chalk and doing a distance coast test from the same point each time.
Sorry, I somehow missed this post.
In response to their straightness, that was the side of the van my friend did .
Yes, installed per manufacturers instructions, both the "on center" and the "from the trailing edge" distances. The taper on the van actually follows the aerodynamic streamlining template very well, so intuitively we saw no advantage to moving them fore or aft, and did not pursue that testing.
There are potentially more and better tests that could be done, including getting a wind tunnel for a day... but we are time and budget limited, so a set of GPS-data-acquisition coast down tests is all we get! Plus, our main goal was to test for drag reduction, and a roll down test like you described would more likely be used to test the rolling resistance of the tires.
Thanks for the input!
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Old 09-06-2011, 01:33 PM   #8 (permalink)
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Quote:
Originally Posted by wyatt View Post
Plus, our main goal was to test for drag reduction, and a roll down test like you described would more likely be used to test the rolling resistance of the tires.
Thanks for the input!
As long as you get up to 45+ mph coasting down the hill in the test and keep the tires inflated the same the test results for drag reduction will be valid.
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Old 09-06-2011, 05:42 PM   #9 (permalink)
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There is a thread in the Unicorn Corral about these, I'm interested but doubtful.
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Old 09-06-2011, 06:10 PM   #10 (permalink)
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I really disagree about air tabs being put in the unicorn coral. They have been tested a proven to work on a car to show drag reduction that will result in measurable gains in FE. You do need a PHD and a wind tunnel to do it right though.

Just because they won't work on all cars and is beyond the abilities of anybody here to get it right does not mean it should be listed with shams like acetone and HHO.

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