Article

Tractor-Trailer Aerodynamics

Streamlined power units are a given, and research on trailer aero improvers should make them common, too.

March 2006, TruckingInfo.com - Feature

by Tom Berg, Senior Equipment Editor

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The winds of change continue in the aerodynamics biz, as universities and manufacturers find ways to streamline tractors and trailers in order to reduce air drag and thus, save fuel.

Most road tractors today are built with smooth basic lines and carry fairings to ease air flow over and around cabs, and these sell well to operators serious about saving fuel money. But those same customers are almost totally disinterested in aero improvers for trailers, even though testing has shown that trailer aerodynamics can save as much fuel as those on a modern tractor.

Several entities are active in testing of trailer devices in wind tunnels and on public highways, and public funding is behind some of the projects. Among them is the Georgia Tech Research Institute in Atlanta, which has been experimenting with special fairings and a "blow" device to reduce drag at the rear of van trailers.

Clarkson University in Potsdam, N.Y., has developed box-like paneling for a van trailer's rear that also cuts drag and saves fuel. Volvo Trucks and Great Dane Trailers are involved in both projects.

Meanwhile, makers of trailer-smoothing products already on the market – Airtab vortex generators and the Nose Cone trailer fairing – say they're selling well, but not to the big fleets that stand to save the most in fuel money. Even with recent spikes in diesel fuel prices, fleet managers remain skeptical of claimed cost savings and cite maintenance and operating reasons for refusing to consider the devices.

Owner-operators, small fleets here and overseas, and owners of recreational vehicles are the ones who buy Airtabs and Nose Cones, company representatives say.

AIRTAB WISHBONE

Airtabs are wishbone-shaped fixtures invented by a former car racer. The wishbone shape, with the wide side leading, bunches air into a small tornado that leaves the flat surfaces with low resistance. This cuts drag, saves fuel and adds stability. Ron Davidson, operations vice president at Aeroserve Technologies Ltd. in Ottawa, Ont., and Jack Latimer, who sells Airtabs through Airtab LLC in Loveland, Colo., offer scores of testimonials from users who report fuel savings of 4 to 8 percent.

Airtabs are typically installed on the sides and top of a van-type trailer or truck body. They are put at the very rear, where air leaves those surfaces and would otherwise cause the turbulence and vacuum effect that dirties doors and lights. Airtabs eliminate much of that, improving vehicle handling and keeping the vehicle's rear clean. The best results come when Airtabs are also fitted to the trailing edges of tractor fairings. On the tractor, Airtabs work with a gap as long as 50 inches, Davidson says.

Three Airtabs per foot on the sides and top of a van trailer or tractor's trailing edges, within a few inches of the very rear, produce the vortex-generating effect. When mounted on a vehicle's side, an Airtab measures 4.75 inches long, 3.25 inches high and an inch thick. As an aero device, it is exempt from width restrictions, but of course it can be knocked off in a scraping incident. If so, it can be quickly replaced. Airtabs come in white or black plastic that's treated to resist ultraviolet rays, and are paintable. A $200 kit includes 80 of these plastic, adhesive-backed devices – enough for one vehicle – plus an installation template and instructions.

Although on-road experiences and some documented testing show that Airtabs do work, they don't test too well in wind tunnels. Thus Davidson has turned down an offer to test the devices in a Canadian government-owned facility because, he says, its chief seems to think that anything that doesn't show up well in the tunnel isn't worth consideration by the trucking industry.

Davidson's aversion to wind tunnels is not shared by others in the aero-improvement business, who believe that although the tests cannot duplicate all real-life conditions, they do show how design changes affect air flow and verify basic theories of how various devices work.

NOSE CONE EXAMPLE

Kathy Rose, marketing vice president at Nose Cone Industries in Murietta, Calif., has commissioned wind tunnel tests that show how her lightweight fiberglass products perform in both headwinds and crosswinds.

As an example, she cites the "leeward" side of a trailer moving through a crosswind. If the wind is from the left, the lee is on its right side, where a vacuum forms as the wind comes around the trailer's nose and from underneath.

The leeward vacuum effect is visible on a curtainsided trailer moving down a highway, Rose observes. The curtain bulges out, as if air from inside is pushing against the fabric. But it's actually being pulled by the vacuum. If the trailer has a Nose Cone, air moving around the front end turns smoothly and moves away from the leeward side. This cuts drag and saves fuel, as customers have testified. A Nose Cone also pushes air out of the tractor-trailer gap, eliminating turbulence and the vacuum otherwise formed inside the gap. Tests show that this becomes more important if the gap is longer than 18 inches, as it is on many real-life rigs.

Owner-operators who've installed Nose Cones on their own trailers claim they gain 0.6 to 1.3 mpg in everyday operations. Rose says that a gain of 0.5 mpg is more typical. One o-o who bought a Nose Cone and Side Burns – rounded panels fitted to the forward edges of a trailer's vertical front corners – says he's saving $500 a month on fuel. At that rate, he paid off the $1,400 cost of the two devices in two and a half months.

Nose Cone also offers the Eyebrow, a 10-inch-high rounded panel that covers the top leading edge of a trailer. This edge is often exposed even if the tractor has complete fairings because it is probably shorter than 13-foot, 6-inch-high trailers. In such cases, an Eyebrow can save an additional 5 percent in fuel, Rose claims.

Another product is the Tail Cone, a boattail structure that smooths the flow of air as it leaves the trailer. Car haulers using van-type trailers buy Tail Cones, which fold upward for loading and unloading. Because it's a non-load-bearing aero device, a Tail Cone's rearward protrusion is exempt from length laws by federal regulations. But she says it's not practical for use by most cargo vans and reefers that back up to docks.

Like others in the business, Rose laments the fact that fleet managers won't believe customer testimonials or test results. Trailer builders say that few customers buy aero devices, citing the risk of damage, added weight and sometimes the fact that owner-operators, not company tractors, pull the trailers, so there's no gain for the company. Also, many trailers spend much of their lives sitting in yards and at docks, where aero devices do no good. The common wisdom among fleet managers and their bosses is, they've already spent money on tractor aerodynamics, so tractors can take care of any air-drag problems.

FREIGHT WING'S SUBSIDY

Managers' spending objections might be muted by the offer of a government subsidy.

Freight Wing Inc., a manufacturer of belly and gap fairings in Sammamish, Wash., has obtained $250,000 from the federal Department of Energy to subsidize customers who purchase its products. Some of the money will help further the development of the devices, says Sean Graham, the company's president. Most of the funds will let him cut the prices of his fairings in half. A Belly Fairing lists at $1,600 and the Gap Fairing usually costs $800. An operator can buy one or both at half price, as long as the grant money lasts.

Freight Wing originally got $75,000 from DOE to help develop the products, Graham says. How do you get this money?

"You write a grant application," he explains, adding that his request was preceded by two years of research and development. Grants are available from DOE's Inventions and Innovations program. Congress tasked DOE to encourage energy-saving ideas, which it does through several channels, including the granting of real money.

The initial grant helped Graham test his fairings at the Transportation Research Center in Ohio. TMC/SAE Type II tests on TRC's track showed that by reducing underbody turbulence, the Freight Wing Belly Fairing saves 4 percent in fuel. A Gap Fairing saves another 2 percent by smoothing air flow across the gap and around a trailer's nose. The Type II procedures are credible to fleet managers who know about them, and Graham says the results have helped him place 30 units with five fleets for in-service trials.

The first 20 sets of Belly Fairings were installed on refrigerated trailers owned by Whole Foods Markets, a retailer of natural and organic groceries, as part of a demonstration program. Fuel savings from the fairing-equipped trailers, running out of the company's distribution center in Munster, Ind., began showing up immediately, says Steve Burse, a Whole Foods team leader. The fleet expects to save 16,900 gallons of fuel over a year's operation, which equates to $42,200 at $2.50 a gallon. In that time, it will also prevent about 180 tons of greenhouse gases from entering the atmosphere, according to DOE.

Made of 0.60-inch aluminum, a pair of Belly Fairings adds about 150 pounds to a trailer's weight. Its lower edge hangs 15 to 16 inches off the pavement, enough to clear driveway aprons, railroad crossings and most other obstacles, Graham says. The aluminum is flexible enough to "give" when it encounters something, and spring back when it's clear of the object.

BLOWING AWAY DRAG

Georgia Tech Research Institute's work, funded by grants from DOE, included wind tunnel testing of a scale model and full-scale testing on highways and the TRC track in Ohio, says Robert Englar, a principal research engineer who's leading the project. Tunnel testing showed that a trailer fitted with special fairings and blow fans would get significant fuel savings, and subsequent high-speed runs at TRC last September verified them.

The track test included an unmodified tractor-trailer for comparison. The vehicles were supplied by Volvo and Great Dane, and both were run at 65 and 75 mph. The TRC test showed that gains totaling as much as 12 percent are possible, Englar says. Work included the rounding of aft trailer corners, installing longer cab side fairings on the tractor, and boxing in the tandem and rear underride guard, all of which smooth air flow over a box-type trailer. These generate fuel savings of as much as 7 percent.

An additional 5 percent gain comes from fans that blow air from slots at the rear corners of the trailer to destroy the vacuum there. Operating pressure is a half to one psi. Fans on the test trailer were powered with a dedicated engine, but a reefer unit's engine, or a chain drive off the trailer's wheels, would also work, Englar thinks. He notes that any fuel used by an auxiliary engine would have to be factored into overall fuel savings. He further suggests that air might be bled from the tractor's turbocharger when it's not working hard and piped to the trailer's vents. Sending air in pulses would lower power requirements, and the fans could be shut off at low speeds and while the vehicle is stopped.

A mechanical engineer would have to figure out the cost of such modifications to a trailer, Englar says. He also understands that the fully faired tandem and underride guard might not be practical in daily operations. DOE funding over four fiscal years, including FY '06, totaled $800,000, and additional money was granted to start the program eight years ago. The project's main backer at DOE, Dr. Sid Diamond, passed away, and Englar says he's not sure how this will affect the project's future.

In the meantime, he says he's received a lot of inquiries from major fleets who are looking to save fuel money.

BOX PANELS

A somewhat simpler solution to rear-end drag is being developed by Ken Visser, an aeronautical engineer who says he has worked at Boeing and NASA and is now a professor at Clarkson University in upstate New York.

Visser designed a box-like structure that mimics a boattail and is attached to a trailer's rear; panels angle inward to catch air streaming from the trailer's sides and help the air detach smoothly from the trailer. "We tested 150 different geometries" on a 1/15 scale tractor-trailer in a wind tunnel and on a full-size trailer before settling on the current design, he says.

Working panels were built at Composite Factory Inc. in Plattsburgh, N.Y. The panels, made of foam-core fiberglass, together weigh about 25 pounds, adding next to nothing to tare weight, says Ed Marin, an aeronautical engineer formerly with Northrop Grumman, who heads the project at the company. The panels extend rearward about four feet. That may or may not be the optimum length, and further testing will determine if it should be closer to three feet or five.

The panels fold easily onto the trailer's swing doors, he says. The panels are hinged to the trailer's door frame; aluminum support struts double as levers, allowing a driver to fold them. When the panels are freed, molded-in springs push them away from the doors for easy deployment. Marin says he and Visser have thought up designs that would also work with roll-up doors.

Anyone can see that the panel box – for now it has no marketing name – will sooner or later be crushed as a trailer is backed against a dock or warehouse door's cushion by a driver who's forgotten they were there. Not so, says Visser. "We thought of that, but every time drivers back into a dock they have to get out first and open the doors. So they'll see it."

And the device does save fuel, he says. A graduate student who's also a truck driver arranged on-road testing that captured design data, and also registered actual economy gains. A tractor pulling a 48-foot trailer fitted with the panel box got a 0.5 to 0.6 mile-per-gallon improvement over a plain trailer. A local owner-operator pulled the trailer to the West Coast and back on a regular run and reported a 0.5 mpg improvement over his usual average.

Volvo and Great Dane have promised to do a TMC/SAE Type II fuel economy test to document the savings, Visser says. And a Canadian government-owned wind tunnel will also test the device in full scale to validate his design. A grant from a state economic commission has paid for some of the work and for the building of two prototypes. State money will also pay for 50 sets of the production version, when its design is ready.

Visser says he has applied for patents that will be shared by the university and Composite Factory. He hopes Marin and the company will take it to market. Its aimed-for retail price is about $2,000, which the half-mile-per-gallon fuel economy improvement should pay for quickly.

INFLATABLE BOATTAIL

Two thousand dollars is also the expected price for the AeroTail, an inflatable boattail now in the final stages of development by of AeroVolution Corp. in Olympia, Wash. On-highway testing using TMC/SAE procedures showed a fuel economy gain of 3.5 percent at an average speed of 58.5 mph, says Lee Telnack, the company's president and designer of the AeroTail.

The test was run by Kenworth drivers using a pair of KW's T2000 tractors pulling Utility vans, one with the device and one without.

At higher speeds commonly sustained by most over-the-road rigs, the fuel economy gain would be more like 5 percent, Telnack estimates. With today's fuel prices, that improvement would allow the device to pay for itself in less than 100,000 miles, the distance covered by certain trailers in one year, he contends. "This is for the long-hauler," he says, noting that trailers that don't see high miles wouldn't be candidates for the AeroTail. Owner-operators who pull their own trailers would see the economic benefits of buying the product. He hopes to have it ready for sale by May.

Boattails usually interfere with the loading and unloading of cargo. AeroTail's collapsible feature should overcome that problem. It is made of woven, recycled polypropylene with chambers inflated by the rig's compressed air system. With a working pressure of 8 psi, it inflates in about 30 seconds to a bulbous shape and extends to just under 5 feet, the length allowance set by the National Highway Traffic Safety Administration for aerodynamic devices. It deflates quickly to a near-flat shape and lays against a trailer's doors, which then can be opened and secured.

The material has a silicone coating that should shed ice and snow, and during road testing has encountered sleet with no problems, Telnack says. The production version will weigh under 100 pounds. An AeroTail should last four years if used every day.

COVERED UNDERSIDE

The bottom of most trailers is an aerodynamic mess, with landing gear, crossmembers and main beams grabbing at the air and causing troublesome turbulence. Karl Schwartz, an owner-operator in Grove City, Ohio, figured there'd be a lot to gain by covering it all up, and he fashioned aluminum sheets to do it. The encapsulation works so well that he gave it a name, AeroFlow, and formed a company called Black Aeroworks to manufacture and sell it.

The sheeting covers the entire underside, front to back and side to side, including the landing gear. It weighs 300 pounds, which Schwartz realizes is too heavy. So he's planning to switch to plastic, which would weigh only about 100 pounds. He'd have built a plastic version by now but has yet to book an order, so has held off buying the materials.

AeroFlow's projected price is $2,600 installed, which Schwartz admits is a little stiff. But he has documented savings that would pay that off quickly. A husband-wife o-o team has AeroFlow sheeting on their Utility dry van trailer and has been running it for more than a year. They do a New York-California trip every week and seldom stop, and have covered more than 400,000 miles since it's been installed. They say AeroFlow has improved their fuel economy by 0.8 mpg, raising their average to 8 mpg.

The two drivers also report that AeroFlow has improved lane control in cross winds, enabled the rig to climb hills faster, reduced road spray, and cut corrosion on the trailer's under-carriage and wiring caused by salt spray. Because the rig handles better, they experience significantly less fatigue. Believe it or not.

Fleet managers won't believe such claims because they come from drivers, who in most managers' eyes lack business sense, are prone to exaggeration and are, in general, unreliable sources of information. As we've noted, managers and executives do have legitimate economic concerns about add-on devices, and prefer to invest money in power units. Here's another worry, legit or not: What if the devices fall off and hurt somebody? That spells liability. It's no wonder this remains a hard-sell business.

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