Quote:
Originally Posted by Frank Lee
Radii don't help on the back.
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Crosswinds. Period. Trailers don't exist in a wind tunnel. Granted they may not "help" mpg, but any cross-section that will shed sudden hard wind loads is to the better.
Radii are part and parcel of good trailer design. It must get from Point A to Point B un-eventfully.
The trailer rear effectively becomes the front with overtaking big truck traffic.
The economics of big trucks, or of cars, in re fuel economy is not the same as with trailers. The first is restricted by what it does, the second is a relatively small vehicle with a low sail area also featuring a low center of gravity. The ratio of COG and sail area is tiny compared to a trailer. It is also self-powered.
The trailer is none of these. Any small penalty is irrelevant by comparison to control of the trailer by the driver. All edges rounded is key in this.
Vehicle control trumps all other considerations.
And, as long trailing exits compromise space (teardrop) the interior efficiency is lower and the sail area is raised, incommensurately. A trailer that widens and narrows makes a bit more sense, as does a humped back. but only to a point. Construction costs make them moot for the most part.
But, again, this is a travel trailer where most of it's time
and use is stationary. Interior space and overly long length compromised by aerodynamic concerns doesn't make sense at all. It's still a box. But is the box one with melted edges and corners, or sharp ones? Square construction is dirt cheap which is why you see it. There was but a short period where aero played a part in designing travel trailers, and only then in high end rigs.
The quotes below primarily describe commercial equipment, not RV's, and provide
some context. The quote I had in mind was more of description than prescription.
Aerodynamics Research Revolutionizes Truck Design
Rounding all four front edges yielded a 52-percent drag reduction, while sealing the bottom of the vehicle gained another 7 percent. The engineers estimated the potential gain in fuel economy to be between 15 and 25 percent at highway speeds.
Rounding the vertical corners on the front and rear of the cab reduced drag by 40 percent while decreasing internal volume by only 1.3 percent. Likewise, rounding the vertical and horizontal corners cut drag by 54 percent, with a 3-percent loss of internal volume.
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Standard semi-trailers have edges rounded with a 20mm radius. Research by DAF has shown that the ideal body or semi-trailer has roundings with a larger radius: the leading edges of the semi-trailer should be rounded with a minimum radius of 150mm, and the side edges with a minimum radius of 60mm.
Container Platform for Aerodynamic Road Transport
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The Aerodynamics of Heavy Vehicles: Trucks, Buses, and Trains, Vol. 1*
By Rose McCallen, Fred Browand, James Carl Ross
The aerodynamics of heavy vehicles ... - Google Books
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When designing the truck, the aim is therefore to increase the radius of the corners and to create more rounded shapes, thus avoiding the slipstream effect.
“There is a rule of thumb that says that the radius of the corner should correspond to 10 percent of the width. If we design the truck with these proportions, we will have fulfilled priority number one. Or rather, we will have avoided creating a poor aerodynamic design.”
Volvo Trucks Magazine
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The Aerodynamic Drag of Road Vehicles
Past, Present, and Future
by William H. Bettes
http://calteches.library.caltech.edu/528/2/Bettes.pdf
The back end of a typical 1980 car had fairly well~rounded trailing edges, which, ironically, is the wrong way to go here as opposed to the front end. Rounded trailing edges, by allowing the separation lines to fluctuate, produce a mean wake that is larger than one that would result if you fixed the separating streamlines at the lowest point. If hard edges are properly placed so as to minimize the size of the wake and prevent the fluctuation of the separation lines that occurs with rounded edges, the overall drag of the car would be reduced by about 5 percent.
On a straight truck, one with the van body attached to the cab, rounding the edges of the van with a 10-inch radius makes a difference in drag of almost 3 percent over one with sharp edges. Rounding the vertical edges with a 10-inch radius on a typical trailer pulled by a conventional tractor lowers the drag by almost 17 percent.
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These quotes differ in time. But none address forces other than those encountered in a wind tunnel, and, if so, only tangentially.
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