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Go With the Flow
More and more trailers are “wearing skirts,” the aerodynamic devices seen hanging below the boxes, leading to more and more questions concerning aerodynamics and fuel economy. To help answer them accurately, I called on a noted authority on aerodynamics, Richard M. Wood.
Wood worked for NASA at the Langley Research Center for 30 years, where he ran the Wind Tunnel Facilities Branch. He formed his own company, SOLUS — Solutions and Technology LLC — in 2000.
The Navy veteran is a member of American Trucking Associations’ Engineering and Technology Policy Committee and concurrently chairs SAE’s Truck and Bus Aerodynamics and Fuel Economy Committee and several task forces. He has written 116 technical publications and holds 18 patents. Who better to answer your questions on this timely and important subject?
What do these trailer devices do?
Wood: To answer that question, we first need to understand a bit about aerodynamics. Simply put, it’s the science of controlling air flow. Air has mass, and when air pushes on a truck’s surfaces, it creates forces that must be overcome. That takes power, which comes from fuel. The greater the force, the poorer fuel economy will be.
Aerodynamic devices help manage air flow. They reduce drag, the retarding force on a body moving through air, by changing pressures pushing on surfaces — decreasing pressure on forward facing surfaces and increasing pressure on rearward facing surfaces like the rear doors. Pushing harder on the doors counteracts the air pressure pushing on the front of the tractor and trailer.
What kinds of devices work on trucks?
Wood: They typically fall into two categories, fairings and flow control devices. Fairings are surfaces that cover and shield non-aerodynamic shapes such as axles, suspensions, landing gear and even the front of a trailer. Flow control devices are surfaces that modify air flow passing over vehicles, like vortex generators. The skirts appearing on an increasing number of trucks are fairings that stop air traveling under the truck and pushing on the bogie system.
How effective are they? If EPA/SmartWay-approved, they are supposed to increase miles per gallon at least 5 percent.
Wood: Effectiveness varies with designs and locations of the devices. There are eight zones on a combination vehicle where drag is an issue. They are the tractor front, the area behind the tractor, the tractor undercarriage, the trailer front, the area behind the trailer, the trailer undercarriage, the wheels and irregularities such as roughness, sharp edges and protrusions.
Benefits vary. SmartWay does not test. It evaluates reports submitted by equipment suppliers. If tests are conducted according to procedures on the EPA website by an independent third-party facility and meet EPA goals, SmartWay will approve the device.
The problem is that tests are conducted under what could be considered ideal conditions. If you operate under similar conditions, you’ll get similar results. But conditions are rarely the same.
If the gap between tractor and trailer is different, results will vary. If a trailer has a different configuration, such as a sharp edge rain gutter at the trailer doors or a slider that varies the gap between a trailer skirt and the bogies, results will vary. For trucks operating in California, sliders may have to be fully forward to meet kingpin-to-tandem length regulations. Trailer skirts may be set to minimize the gap to the wheels. When the slider is positioned rearward for highway operations outside the state, a large gap is created. That compromises the skirts’ aerodynamic effectiveness. Skirts that tested to 5 to 6 percent improvement may yield only 2 to 5 percent in real world operations.
What other devices will be effective on a consistent basis?
Wood: For trailers, skirts can be very effective if they extend to the trailer wheels. If you don’t have sliders, you have fixed distance to the tandem. A properly sized skirt will be almost as effective as its SmartWay tests indicate.
Bogie fairings deflect air around the undercarriage, but they, too, need to be in the test position. Boattail or trailer extension tests show up to a 6 percent improvement. In operation, that can be 2 to 5 percent depending on weather, taper and placement.
Even a simple plate that runs across the top rear of the trailer to deflect air over the rain gutter can increase mpg up to 2 percent. Vortex generators can add 1 to 2 percent to mpg, depending on design and placement. Wheel covers on drive and trailer wheels minimize turbulence at the deep wells of the wheels. They can improve fuel economy 1 to 2 percent in both tests and real world use.
What effect do cross winds have on SmartWay devices?
Wood: Cross winds enter gaps between aerodynamic devices and vehicle structure. The greater the angle and the wind speed, the less efficient most devices will be. And as gap increases, so do the negative effects of cross winds.
Some devices may work better with other devices installed. For example, some boattails may be more effective if the trailer is equipped with vortex generators at the rear. Benefits of bogie fairings can diminish faster than skirts as cross winds increase.
The main thing to remember is that aerodynamic devices are tested under ideal weather, road and setup conditions. If those conditions vary, so will results. When choosing a device or combination of devices, operators should determine what test conditions were and ask the vendor how the device would operate in their world.