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Old 12-13-2016, 08:46 AM   #111 (permalink)
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New member here. I'm reviving this old post by @aerohead because it appears to explain a situation that I have been trying to understand in regards to towing two similar trailers; one a 17 ft 83" x 83" frontal area 1750 lb teardrop and the other a 17 ft 95.5" x 86" frontal area 1600 lb non-teardrop, very similar construction and materials and made by the same manufacturer. It has been reported to me by a reliable and knowledgeable source that the larger frontal area trailer takes less energy to tow at highway speeds. This is counterintuitive but is likely explained by @aerohead 's very informative post. The small weight difference between the trailers is certainly not the reason the larger trailer takes less energy to tow. Sorry but my information source for the towing energy difference (an RV dealership employee with many years experience designing and installing weight distribution hitches and very knowledgeable about travel trailers) could not provide me with quantitative data, he only said the energy savings towing the bigger trailer was noticeable and significant.
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Originally Posted by aerohead View Post
The FKFS (Koenig-Fachsenfeld/Kamm) tested something similar to the r-pod,except that it had inboard wheels.It was reported at Cd 0.45.
The r-pod,with exposed wheels and fenders would be higher.
The 'classic' teardrops would suffer the same attached longitudinal vortices as r-pod just as ChazInMT has illustrated above.
The rooflines are all too fast,the pressure builds over the aft roofline in advance of the side flow,we get separation up there,and the pressure differential causes higher pressure air from below and on the sides to race up there attempting to reach equilibrium.As the flow fields collide at different velocity and pressure,they coil up just like a mesocyclone,spinning into horizontal tornadoes.
The sharp edges rob about 16% of Cd potential.The gap sucks energy.If they don't have belly pans,there's some more loss.
A 'gutted' VW microbus 'trailer' would pull as well as a shrunken Airstream,at Cd 0.43 (free-air),lower in train behind a tow vehicle.It would have more usable interior volume than a 'teardrop.'

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Old 12-13-2016, 02:36 PM   #112 (permalink)
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Quote:
It has been reported to me by a reliable and knowledgeable source that the larger frontal area trailer takes less energy to tow at highway speeds.
It could very well be true for those two trailers (with specific tow vehicles); but untrue for similar others. Without pictures it's hard to comment; except to say the relevant factors are edge and corner radii, hitch length, &etc. As his quote says.
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Old 12-13-2016, 03:05 PM   #113 (permalink)
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Good point. Unfortunately because I am a new Ecomodder member and this is only my second post I cannot yet post images or links.
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Old 12-13-2016, 03:21 PM   #114 (permalink)
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Burn through them here, then come back:
Introductions - Fuel Economy, Hypermiling, EcoModding News and Forum - EcoModder.com
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Old 12-13-2016, 11:02 PM   #115 (permalink)
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Okay will do.
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Old 12-13-2016, 11:03 PM   #116 (permalink)
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I will be towing my trailer with an electric car so aerodynamics are very important to me.
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Old 12-13-2016, 11:05 PM   #117 (permalink)
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Others towing with electric cars report that their range between charges is cut in half or thereabouts.
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Old 12-13-2016, 11:06 PM   #118 (permalink)
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So towing with an EV requires careful planning and trailer selection.
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Old 12-13-2016, 11:07 PM   #119 (permalink)
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Here are the two trailers I am referring to.

This is the 83"x83"





This is the 86"x95.5"

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Old 12-14-2016, 03:39 AM   #120 (permalink)
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The teardrop falls off too fast. Same problem the VW Beetle has. As the curve of the roof surpasses a specific angle (which is endlessly debated here at Ecomodder) the flow separates. Everything abaft of that is 'in a negative pressure regime', IOW being sucked on. Probably just around the middle of the windows.

The taller trailer has a good roofline that the air can follow, and ends at a height not much greater that the max height of the teardop. So the effective hole it leaves for the air to reenter is bigger but differently shaped, lacking the inward-rolling vortexes the teardrop generates.

Construction details and the undercarriage are probably similar given the same manufacturer. It would take tuft testing, wind tunnel or CDF modeling to sort the details.

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