by yesplease » Sun 03 Aug 2008, 05:55:34
$this->bbcode_second_pass_quote('the48thronin', 'I') am the trucker "yoda" on the airtab web site ( sigh yes on XM radio as well). They reduce the drag but cannot eliminate it all together..
The vacume cone behind the trailer is reduced at speed to about 5 or 6 ft instead of the normal 35 ft... if you try to draft behind my trailer at 58 MPH you will have to get close enough to get the majority of your hood inside that cone...
It isn't so much reduced as evened out. Granted, you have seen significant gains, but if anything I'd guess that for the most part the VG's didn't change the overall draft seen by a vehicle behind you, just the distribution/location of it. I have some papers that were run at a NASA(?) facility from the creator, but I don't think I'm allowed to post them publicly so for now I'll just present you with
this paper by some Mitsubishi engineers regarding VG's on one of their cars. If you examine Fig.9 on Pg.4 you'll notice that the largest change comes from the distribution of pressure at the rear of the vehicle, not from an overall decrease, although there is one. This results in fewer abrupt transients, a more even change in the pressure behind the vehicle. This is why winds don't buffet the trailer as much, the air flowing perpendicular to the object doesn't encounter as large of a pressure drop in as small of an area, even though marring a slight increase the pressure drop overall is more or less the same, so if a big gust comes up from the side instead of adding a whole bunch of air to the relatively small area behind the cab/trailer, it'll add the same amount to a much large area and the truck won't jolt as much from the pressure differences. In terms of a car drafting you, while they will probably see much more pressure ~10' off your bumper compared to w/o the tabs, otoh as they go farther back they may see lower pressure than w/o the tabs due to the VG's lengthening the lower pressure areas farther back.
$this->bbcode_second_pass_quote('the48thronin', 'M')PH change tabs on tractor only.... +.2 and wind stops blowing tractor around in cross winds
a month later I got the tabs for the trailer... MPG up +.3 more
That ain't too shabby, but imagine what a streamlined combo could see. How much would a fleet average of 10-18mpg help the industry, not counting gravel hauling and the like?