Aircraft trick may give big rigs a gentle lift

In preparation for wind-tunnel tests, Robert J. Englar (left) and Graham M. Blaylock work on a model of a big rig equipped with air-blowing slits Gary Meek/Georgia Tech Research Corp.

Even as fuel costs soar, truckers’ loads could become less of a drag. Moreover, making that happen would be a breeze, say researchers at a Georgia Institute of Technology laboratory in Smyrna.

That breeze—sheets of pressurized air—would blow rearward from slits in an 18-wheeler’s roof, sides, and bottom at locations just forward of the truck’s tail. Last week, the team began wind-tunnel tests on a model truck fitted with slits.

In the model, artificial wind from compressed air slips over metal half-cylinders mounted along each edge of the trailer’s rear. By forcing air billowing off other truck surfaces to fall in with orderly currents, those slit-fed air streams may calm a truck’s typically stormy wake. That could cut drag on tractor-trailers by 35 percent or more, claims Georgia Tech engineer and team leader Robert J. Englar.

Computer simulations and earlier wind-tunnel tests of models by Englar’s group had shown that already-streamlined cars could shed 35 percent of their drag by blowing air through slits. For the boxy trucks on the road today, Englar would expect an even greater effect. Just a 35 percent loss of drag, however, would slice fuel use by 12 percent, he estimates. If all U.S. semitrailer trucks adopted the technology, they could thereby save 1.2 billion gallons of fuel per year.

Other benefits are possible when the pneumatic system is used to blow air through one slit more strongly than the others, Englar says. For instance, lift created by air coming only from the top slit could lessen the load on the wheels up to 15 percent, cutting fuel use even further. Conversely, negative lift from the bottom slit could push the load down, increasing traction under slippery conditions. Bursts through the side slits could counter swaying motions that can cause trucks to jackknife.

“The trucking people who are aware of [this research] are really enthused about it,” comments Victor A. Suski, an automotive engineer with the American Trucking Associations in Alexandria, Va. That organization helped drum up interest at the U.S. Department of Energy (DOE), which funds the current, 3-year study. Next year, the trucking group plans to help with tests on real trucks, Suski says.

Richard N. Wares, the project’s manager at DOE in Washington, D.C., concedes that the fuel-reduction estimates don’t weigh in any added fuel that the truck might require to supply air to the slits. That air could come from engine exhaust, from compressors already present in trucks, or from additional compressors or tanks. Even if supplying the air cancels drag-reduction gains, the technology “can still be a useful control mechanism,” Wares contends.

The technology takes advantage of an effect discovered by Romanian scientist Henri Coanda in the late 1930s. A jet or sheet of gas, if moving fast enough, will bend to move along an adjacent surface that angles away from the source of the gas. Engineers use the effect to steer a gas jet along a tilted or a curved surface, even around the bottom of a plane’s wing.

Englar helped develop the technology initially to increase lift for U.S. Navy warplanes. Although the approach was proven effective in flight tests in 1979, the Navy hasn’t embraced it, partly because of qualms about relying too much on compressed air that might shut off during engine failures, he says.

Nonetheless, a commercial helicopter currently uses the technique. Also, tests on Formula 1 racecars have yielded “very promising results,” Englar says. Most recently, automakers have contacted the team about improving the fuel economy of gas-guzzling sport-utility vehicles, he adds.

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