Quote:
Originally Posted by skyking
I'm about settled down on dimensions again (  ).
This will have a frontal area of 53 sq. ft., and will follow the aft template faithfully till about the first 22 degree point. The sides will taper in a couple of small angle changes.
The rocker angle aft of the wheels needs to be a little steeper than ideal at 4.6 degrees.
The area at the back will be ~17 sq. ft.
What kind of Cd should I expect out of that, considering it will be faired smooth and tight to the tow rig and have above average fairness in general? |
I spent some time with Hucho's chapter on 'Commercial vehicles'.In his 2nd Edition,page 332,Figure 8.63,'Influence of bus rear-end design on air drag',in the far right is a depiction of a boat tail and it's Cd reduction.
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*The bus has a Cd 0.341 with no tail at all.
*With the #2 configuration,the section behind the rear wheels of the bus is sloped down and in @ 5-degrees leading to Cd 0.274.
*With the #4 configuration,the boat tail is extended out to 41.7% of the 'Template' aft-body length,yielding Cd 0.218.
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The boat tail depicted in Fig. 8.63 indicates a 5-degree slope,but it is drawn with a 14.5-degree slope.
I redrew the bus @ 5-degrees and evaluated the cross-sectional area at the rear truncations using the frontal area of the 'Front Roof and Vertical Edges Rounded' bus.
*At position #2 the frontal area is reduced 18.1%/drag is reduced 19.64%
*At position #4 the frontal area is reduced 39.2%/drag is reduced 36%
*For the 1st 41.7% of aft-body,each 10% reduction in cross-section yields a 10.05% drag reduction.
*If this relationship held,then if the tail was extended back to 70%,as has been recommended with the 'Aerodynamic Streamlining Template',the the drag coefficient would fall to Cd 0.13.
*If wheel drag is removed,@ Cd 0.041,this leaves a ground-proximity Cd 0.09 for the boat-tailed bus body, which is in close agreement with the
'Template.'
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Using this relationship as a 'loose' rule-of-thumb,then we might guestimate that if you start at 53 sq ft of fully attached frontal projected area,and then streamline the body,separation-free,down to a body truncation of only 17 sq ft at the rear,the 32% of remaining base area equates to a 67.9% reduction in area.
At 10.5%/10,the drag would be reduced 68.23%.
At a steady 55 mph (88.5 km/h) you might see a 34% mpg increase,minus any penalty for added weight/R-R if applicable.
Pretty tasty!
