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.
Quote:
Originally Posted by aerohead
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.'
|