Whale Watching: The Aerodynamics of Economical RVing
January/February 2009 Edition
Aerodynamics are back in the forefront these days, and what's old is new again. The curved taillights of a Flex mimic those on 1970s Mercedes-Benzes, which were grooved to help keep the lamps clean; closing grilles have been on semis and Porsche's 928; add-on vortex generators made the last Evolution MR, box trailers, and motorhomes. I'm glad they came back. It's about time.
At highway speeds, aerodynamic drag accounts for the majority of fuel consumption, which increases exponentially at the square of the speed, and most recreational towing is done at highway speeds. As readers questioned, what's the best kind of RV relative to fuel economy?
There is no simple answer. Do you base your decision strictly on fuel consumption or develop a methodology to value it against floorspace, weight, garage length, or any of 100 other variables?
The sleekest modern pickups have a coefficient of drag around 0.42, and Cd multiplied by frontal area gives an indication of total aerodynamic drag. A good car is in the 0.25-0.30 range and a box is about 1.0; those big, flat clear windscreens at proving grounds are the worst-case scenario with perfectly flat, squared leading and trailing edges.
In 2008, the SAE developed a standard (J2807) for tow ratings that includes frontal area and trailer loading, based on box trailers because there are too many variables with RVs or boats. The standard includes minimum frontal area that "accounts for entire trailer frontal silhouette to the ground plane" of 12 square feet (for a trailer less than 1500 pounds) to 75 square feet for fifth-wheel or goosenecks; all conventional trailers above 7700 pounds are assigned a 60-square-foot minimum.
A cabover camper will add a lot of drag and disrupt airflow around the truck, and the square, flat back is not aero efficient. A typical cabover will add at least the 20 square feet of frontal area that the SAE assigns to a 2000-pound trailer. On the downside, campers don't have much floorspace and frequently cost more per unit area than conventional trailers, while on the upside you don't need a bed cover and you can still tow a boat or dirt bikes.
Conventional trailers don't disrupt over-truck airflow at the windshield nor have the height of a fifth-wheel, but most are box-shaped. They offer decent floorspace and pricing, and the truck bed is free for cargo, but tankage and payload don't approach fifth-wheel capacities. A sleek trailer rounded off at both ends like an Airstream could easily reduce drag by 15 percent, perhaps cutting some overhead cabinet space or headroom, but you could make that up with a longer trailer, which would have virtually no added drag. A teardrop trailer with a sloping tail is among the most efficient--look at a raindrop, Porsche Cayman, or the Goodyear blimp--but they are all quite compact. Adding a fairing on the truck's roof usually creates more drag, because they generally have to be within a yard of the trailer's leading edge to be effective.
Pop-up trailers like the TrailManor and Hi-Lo give similar floorspace with limited overhead cabinetry, but present an aero profile that more closely shadows the tow vehicle; the sides are cleaner because much of the clutter is enclosed, and they have a low center of gravity.
Fifth-wheels have the advantage of being closer to the truck, but the airflow around the hitch and leading edge of the belly is disturbed and they are the tallest. Many have a roof that slopes down to the rear, but it's still a square end. Fifth-wheels typically have good weight capacity, tankage, and floor area and are the first to overload a truck's rear axle.
The most obvious thing to improve economy through aero drag reduction is to slow down, but that somewhat defeats the purpose since you spend more time driving and less time enjoying recreation. All other things being equal, a longer box is more aerodynamic (but it weighs more), and except for Prevost-based coaches and expedition vehicles like the Earthroamer and Unicat, bigger doesn't necessarily mean better.
Look for rounded, angled, or sloping edges and a tail cross-section that's smaller than the snout. Search for ones with enclosed undersides, since slideout rails, tanks, lines, and plumbing could generate 20 percent of the drag, and only the brakes and wheel bearings need cooling airflow in motion. Cleaner sides help, too, as hinges, protruding panels, locks, steps, handles, and lights add drag, and don't forget all that antenna, vent, and panel stuff on the roof. Finally, lower ride height is better as it limits air traveling underneath--think race cars and "extra fuel economy" pickups--but this must be balanced with ground clearance and breakover angle.
The NoseCone company claims 10-percent mpg gains in headwinds and 12-percent in a 10-degree crosswind, noting tractor fairings work in headwinds but can add drag in crosswinds. Tail cones, like those used on Waggoner's enclosed car haulers, also lessen drag and are exempt from federal length restrictions because they don't carry weight. Few RVs back up to loading docks, and a lift-up cone over a drop ramp would be ideal for a toy box.
Then consider additions. People have had success by taping over hinges, using low-profile door/panel fittings, and enclosing frames with plastic panels. Adding vortex generators along trailing edges of a truck or trailer could add to economy. Users say they add stability.
If you want to roughly compare different RVs for aero drag, you merely need to hook each up, find a deserted road to reach highway speed, and time or measure how long it takes to coast down to a set speed around 40, averaged each way for wind. True, it's not aerodynamicist-grade testing and it doesn't take into account rolling resistance changes among tires or wheel bearings, but it will give you a good idea which unit takes the most fuel to pull.