Our friends up north in Canada have published a very informative paper by the National Research Council Canada in May 2012. You can see it here. I have highlighted in yellow, statements inside this copy as it pertains to this next discussion.
First item to keep in mind is that aerodynamics is 50% of our motor's power requirement at 50 mph. It is 70% at 70 mph. So if we want to go faster to pick up that extra half load by the end of the driving week without losing money to aerodynamic drag then we need to do something about improving our aerodynamics!
According to the National Research Canadian Report:
"The wind-averaged drag is the most important metric for evaluating vehicle drag because some drag-reduction technologies improve aerodynamic performance only for a subset of wind angles (typically centered on zero yaw)" - One technology that meets this statement is the skirt. It works well in zero yaw or straight-on, calm wind conditions but when hit with side-winds it captures air in that area at the rear of the skirt and before the trailer tires. Just like a parachute. But as the report states, "Only zero-yaw drag-coefficient results are required for reporting".
Also from the report:
"The primary recommendation of the NAS report in regards to aerodynamic drag reduction is a requirement for such technologies to be evaluated on a wind-averaged drag basis to take into account the effect of typical terrestrial winds."
These guys are NOT making a good case for our EPA, C.A.R.B. test protocols at all.
Winds are important when dealing with truck aerodynamics.
Another quote from NRC:
"Under such cross-wind conditions, the aerodynamic characteristics of a ground vehicle, especially tractor-trailer combinations, behave differently than when the ambient winds are either static or when they are in line with the vehicle motion".
"For moderate cross-winds the yaw angle can exceed 10º, and under these conditions the drag of a tractor trailer can exceed 50% of its zero-yaw drag load.
Therefore, the wind-averaged drag coefficient is the most criticalaerodynamic performance indicator for long heavy vehicles such as trucks or buses because it represents an average of the typical wind conditions under which these vehicles will operate."
Apparently turbulence also plays a large part in this game:
"A drag reduction device may show negligible sensitivity to turbulence when winds are aligned with the truck motion, but can show different levels of drag reduction in cross-flows depending on the turbulence environment. The characteristics of the turbulence environment are important for adequately assessing the performance of drag-reduction devices for road conditions."
So there you have it. Straight from the Ph.Ds, just what we have known for years here at Airman Inc
Geez, it's nice to finally get some validation!
The most important part of truck and trailer aerodynamics has to do with how we deal with side-winds and turbulence-not how they work going around a track in no wind conditions.
Of further interest.......
What about the roiling, unorganized flow that developes under the trailer and behind the rear-most drive tires?
Well, we studied this area using real trucks on real roads using smoke and cameras. You can watch a clip here. What we found is that in straight-on wind conditions, not only is that whole area just a swirling mess, it also moves along with the trailer with some wind components actually moving faster forward then the trailer - at times. Totally unexpected and we believe totally unknown to many in the industry. What this means is that whatever we place in this area under the trailer will not work as one might expect. Everyone is thinking that the air under the trailer is moving just as fast as the air along side the trailer - WRONG. Good forus to know and it explained a lot about the test results we were getting.
Our AirWedge units are designed to modify the airflow just before the trailer tire bogies and we have great success with that. The interesting part is that during our over-the-road testing campaigns we always got great improvement numbers when we were running at 70 mph into a side wind between 20 to 40 degrees off centerline direction. But, our EPA track tests come up short. So what is going on? We verified our ideas with the smoke tests. When running with an AirWedge it must be presented into an airflow in order for it to work. If it is aerodynamically hidden behind the tractor (as during a test on the track) then there is not enough airflow to generate large positive results. When running into a side-wind then the AirWedge has the airflow required to do it's job and reduce the trailer's air drag.
You might be thinking "OK, that sounds reasonable but side-winds only happen part of the time." Please do me a favor, during your run next week please keep track of the amount of time you run into some kind of side wind. I think you will be surprised as to how often that does occur. Here is a link to a website that shows wind conditions of America that you may find interesting.
In order to make the AirWedge work in all wind conditions we have designed forward mounted 'winglets' that attach near the outside edge of the trailer just behind the landing gear. These winglets remove the turbulence behind the drive tires and feeds the AirWedge when your truck is moving through straight-on airflow while not changing its side-wind performance. These two devices work together delivering the best aerodynamic solution out there.
Now we are getting reports of fuel mileage increases over 10% above baseline. How would you like to see your rig pull in 7.2 mpg while driving at 70 mph in a side-wind?
The rest of the story is livability with your aerodynamics. We have that covered also but on another page later on.................
Give us a call sometime to discuss questions you might have............
Reg Durham 480-593-0001
Bob Henderson 503-990-2069