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Each Airtab® produces two counter rotating solid body vortices that rotate at approximately 3000 rpm. These vortices are about 4 to 5 inches in diameter and trail behind several feet. They help to reduce turbulence and suction drag at the tractor-trailer gap and at the rear of the trailer.".
Aerodynamics of Trucks
Until recently, commercial vehicle streamlining efforts were mostly confined to the front of the vehicle in the form of deflectors or fairings.
Airtabs™ are unique because they work in concert with side extenders to protect the tractor-trailer gap and reduce the base pressure or suction drag at the rear of trailers, RVs, buses or straight trucks.
The Tractor-Trailer Gap
The gap size is an important aspect of fuel economy
in trucking operations and should be minimized as much
as loads allow. If the gap between the trailing edges
of the tractor fairing and the front of the trailer is
greater than about 18 inches (1/2 meter) increased
drag in this region begins to reduce fuel economy.
For every additional 10 inches of gap size above about
30 inches, aero drag will increase 2% and fuel economy
will decrease by 1%. The operator can reduce this gap
with side extenders and/or moving the 5th wheel forward.
However, moving the 5th wheel forward can reduce
maneuverability and can place more weight on the
steering axle increasing steering gear and tire wear.
Through the use of side extenders and Airtabs,
operators will have the flexibility of setting the
5th wheel more precisely for the load, not add weight
to the steering axel and avoid a reduction in fuel
economy by using Airtabs to reduce gap turbulence.
The Back of the Trailer
Airtabs™ reduce drag by changing the dominant vehicle trailing edge turbulent flow mechanisms from large vertical eddies to a horizontal array of small vigorous solid body vortices. This reduces vehicle sway, improves stability in gusty cross wind conditions and increases fuel economy. By altering the airflow in this manner, Airtabs™ artificially taper the trailer and make the airflow behave as if the trailer was slightly smaller than it actually is. This results in drag reduction and an increase in fuel economy.
Look closely at the images. The animation seen below is a very accurate depiction of the airflow behavior surrounding a moving trailer without Airtabs™ as viewed from above. Notice how large vertical eddys of air (like small tornadoes) alternately form and “shed” making room for another to form on the opposite side. Although not visible here, there is an additional eddies that rolls off the top of the trailer as well. These eddys tumble into the partial vacuum formed by the moving vehicle.
A lack of trailer streamlining causes a partial vacuum to form at the rear of the trailer at highway speeds. This causes turbulent air in the form of large vertical eddies to tumble into the partial vacuum and the large alternating eddies cause the trailer to sway. Disturbances from passing vehicles or obstacles tend to amplify these eddies and increase the trailer’s tendency to sway, increasing driver work load and fatigue, increasing drag and lowering fuel economy. Streamlining the trailer by tapering would be the best solution but is not practical due to a necessary reduction in trailer capacity and massive changes to docking and loading facilities.
The large square area at the trailer rear causes a partial vacuum to form as the vehicle moves through the air at speed. These are usually devices that physically alter the trailer rear by using inflatable sails, fiberglass boat tails or rigid metal panels that extend to taper the trailer rear. However, these devices can be cumbersome, expensive, heavy and prone to collision damage. Physically tapering the trailer or load would be the best solution but is not feasible due to trailer capacity and loading operations
Wind Tunnel Test Photos
The images below were taken at a low speed wind tunnel facility looking down on a simulated tractor trailer gap. Smoke generators were used to visually assess flow patterns. The left image shows the airflow and drag regions around an unfitted unit. The right image shows the unit fitted with Airtabs(TM). Notice the marked reduction in the amount of smoke entering the gap. This equates to drag vehicle drag reduction."
Spray Reduction and Stability
Drivers report improved mirror visibility in rain or snow conditions yielding safer lane changes and better visibility for other road users. Improved stability and handling in windy conditions is reported by all drivers. This reduces driver workload, stress and fatigue
Road Grime Reduction
Because Airtabs™ alter the airflow at the rear of the vehicle, the partial vacuum there is reduced. There is less accumulation of road grime or snow. Less road grime means advertising remains more visible and the company’s image is enhanced. Less snow build up means less weight to haul and conspicuity tape, tail, running and brake lights, especially low temp LEDs, stay visible longer.