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've been studying trailers and commercial vehicles since early October and don't yet have any confidence in predicting Cds.
From what I've looked at so far,the theme is:
* match the tug height and width to trailer as close as possible
*do a full 'oval' nose on tug (elevation and plan)
*kill the gap in between
*turn the 'van' back into a 'fast back'(elevation and plan)
------------------------------------------------------------------------ So you're definitely sniffing at the right tree!
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In SAE Paper # 860093,by Carr and Stapleford,Fig.8,something interesting showed up.
They took the same vehicle model and tested with three different backs.
*Van back had highest drag.Cd 0.327
* Notch back had next highest.Cd0.285
*Fast back had lowest,Cd 0.237 (just as Hucho promotes).
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In a crosswind:
*Notch back sees up to a 33% drag increase @ yaw = 18-degrees
*Van sees up to a 12% drag increase @ yaw = 18-degrees
*Fast back sees up to a 14% drag increase @ yaw = 20 degrees
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So the 'fast back' 5th-wheel might see a little higher drag increase than the van back in a crosswind,but in light or no wind she should return the best mpg.
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If we could have a guestimate of the all-up weight for the rig,that would give us something to chew on.
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Your mpg is going to track your Cd just like a car.At 55 mph,any 10% drag reduction is going to mean a 5% improvement in economy.
If your flow is 'clean' down to that 17-sq-ft exit area,compared with a 53-sq-ft frontal area,that suggests some tasty numbers!
If you were working it in the opposite direction,from 17,to 53,all else being equal,that would be a 3.11X increase in drag,and should mean a 1.55X mpg penalty.
If the composite Cd is lower (as it should),then the figures are even more liberal.