Tweaking trailer tails

by Carroll McCormick

When researchers learn more about what affects the amount by which trailer tails, aka boat tails, reduce drag – typically said to be around 5% – manufacturers can improve their designs. Fleet operators need not be passive bystanders, however: recently published results from wind tunnel tests by the National Research Council Canada (NRC) include shiny nuggets that may help inform their purchasing decisions and increase the likelihood of lowering fuel consumption.

About a year ago, NRC tested 30 different scale model trailer tail designs and configurations on its 30% tractor-trailer in its 9.1x 9.1x 22.9-meter wind tunnel in Ottawa. It was an NRC-funded follow-up to work done a year earlier for Transport Canada’s ecoTechnology for Vehicles program.

The researchers looked at the effects on drag of trailer tail length, panel design and angle and the vertical placement of the whole trailer tail, and the combined effect of trailer tails and side skirts in reducing drag.

The results are reported in Aerodynamic Performance of Flat-Panel Boat-Tails and Their Interactive benefits with Side-Skirts, authored by Dr. Brian McAuliffe, NRC’s thrust lead for the Enhanced Aerodynamics Performance thrust of the Fleet Forward 2020 program, and one of his colleagues, Dr. Alanna Wall.

The paper is for sale by SAE International, and will otherwise be made publicly available in early 2017.

NRC tested four trailer tail lengths, with full-scale equivalent lengths of 0.6m (2 ft), 0.9m (3 ft), 1.2m (4 ft), and 1.5m (5 ft). In general, the longer the trailer tail, the less the drag.

However, the angle of the panels had an effect.

“Short boat tails demonstrate improved drag-reduction at larger panel angles,” McAuliffe writes. And in general, he writes, “All of the boat tail configurations demonstrate an improvement in performance when the lower panel is added.”

As for those panel angles, which the researchers variously set between 10 and 15 degrees, although they did affect drag, they are set by the manufacturers. Operators are expected to keep their hands off.

“You wouldn’t even think about (changing the panel angles),” says Gerry Spachman, trailer shop foreman with the Erb Group.

The vertical location of the trailer tail had quite an effect on drag, although it did depend on the panel angles.

“A 13-degree top will lose performance more quickly than an 11-degree top, as it is dropped,” McAuliffe notes.

The NRC research showed that the ideal placement for a trailer tail is with its top edge flush with the trailer roof. Unfortunately, however, putting it there would hide the marker lights, in violation of regulations.

That’s too bad, because, as McAuliffe explains, “When the panel is dropped about three inches, you increase the drag of the vehicle by about 2%, which is sizable. What I have noticed with a lot of designs is that the vertical position is often left up to the installer … you want to install the boat tail as high as possible.”

He adds, “In some European countries they don’t have the marker lights at the top of the trailers. The boat tails are always installed flush with the top.”
With the coming of the GHG Phase II rules, perhaps North American trailer manufacturers will do something with those marker lights so the vertical position can be optimized.
Trailer tail side panels come in different shapes and lengths; ie., straight bottom, angled bottom, partial- and full-height, and some with a cut-out, depending on the manufacturer. NRC tested four panel shapes and lengths. In general, full-height side panels, even with the cut-outs, reduced drag more than partial-height side panels.

Adding a horizontal panel on the bottom of the tail (at least one manufacturer offers models with and without a lower panel) further reduced drag.

NRC also investigated the combined effect of running both a trailer tail and side skirts. The results weren’t cut and dried (“Everything is dependent on everything else,” as McAuliffe puts it). But as a broad-brush statement, NRC found that the drag reduction obtained from both devices in combination exceeded by 1-3% the simple sum of the drag reduction of each device.

The size of this effect depended on the side skirt design (the inset design was the least helpful) and the trailer tail design and panel angle, but scattered throughout the mixed bag of designs being run out there, some of the trailers running trailer tails and side skirts in combination are surely overachievers in the drag reduction department.

Looking ahead, manufacturers may take some of the guesswork out of all this. McAuliffe speculates, “I’m hopeful that we’re going to see some more integrated design features.

Rather than being add-on technologies, trailer manufacturers will design into their whole structures the aerodynamic shapes we see; for example, maybe a different boat tail will give an improved performance with a tridem than with a tandem.”

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  • How much good do they do when they have 6 in. of snow built up on them and you can’t get them closed to open the doors.