5th wheel design

[KamperBob]

From Hucho 5-10% is a good target ratio for edge radius to section width. At 6" over 6+ feet you're right in the ball park. I think you're good. Rock on!

[skyking]

thanks guys, I do appreciate it. I have a 92" section, but the roof in the center rises up another 2" in a big radius. This will make the roof/side radius transition a little less harsh, whatever I choose.

[aerohead]

Originally Posted by skyking

Maybe Phil will stumble across this:
Are my proposed 6" longitudinal radii sufficient to reap the crosswind benefits?
I lean heavily toward those because it eliminates the need to put slideouts and doorways up into the radii, greatly simplifying the construction.

I recall that 150mm radii were mentioned more than once with respect to 'clean' wind tunnel 1:1 scale model tests.
Sounds like Kamper Bob had the 'book' in front of him,so I'm going to defer to his call.

[skyking]

very cool! I could go with larger radii in an ideal world where my time is free and I can clone myself as much as needed, but I don't live there either
Keeping doors, windows and slides out of the radii is priceless.

[KamperBob]

Originally Posted by aerohead

I recall that 150mm radii were mentioned more than once with respect to 'clean' wind tunnel 1:1 scale model tests.
Sounds like Kamper Bob had the 'book' in front of him,so I'm going to defer to his call.

Not hardcopy but I thought to scan some key pages during inter library loan last year.

[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?

[aerohead]

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!
------------------------------------------------------------------------
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).
-------------------------------------------------------------------------
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
-------------------------------------------------------------------------
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.
--------------------------------------------------------------------------
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.

[skyking]

Thanks Phil, I was just looking at the template again when you posted. The back of trailer and is about 63% or so. Essentially I'm going from a high point of 7.5' and following the template for about 16'.
I'd have a clean break there with a bumper recessed in out of flow, and the ability to hang a 3' boat tail for longer trips if it would do any good.

I have weights in mind. The combined weight should be between 11,500 and 12,000 pounds.
I have played with the calculator quite a bit. I figure I can get an average of .008 on the crr, using the best tires for each unit. I just discovered I can use the Bridgestone 381 with the really low crr on the trailer.
Just guessing from my results so far, I figure the Cummins 6BT at .29 thermal. The truck sucks hard at 34.5 ft and .44 cd. I think smoothing things up off the truck and finishing them off onto template with faired wheels should net the mid.2s for the combined rig.

[aerohead]

Originally Posted by skyking

Thanks Phil, I was just looking at the template again when you posted. The back of trailer and is about 63% or so. Essentially I'm going from a high point of 7.5' and following the template for about 16'.
I'd have a clean break there with a bumper recessed in out of flow, and the ability to hang a 3' boat tail for longer trips if it would do any good.

I have weights in mind. The combined weight should be between 11,500 and 12,000 pounds.
I have played with the calculator quite a bit. I figure I can get an average of .008 on the crr, using the best tires for each unit. I just discovered I can use the Bridgestone 381 with the really low crr on the trailer.
Just guessing from my results so far, I figure the Cummins 6BT at .29 thermal. The truck sucks hard at 34.5 ft and .44 cd. I think smoothing things up off the truck and finishing them off onto template with faired wheels should net the mid.2s for the combined rig.

I just now made some photo-enlargements from Hoerner's section on railroad vehicles.I haven't digested every word so far,but these materials may give us some insight into the trailer technology:
*Locomotives with Tenders are progressively streamlined starting at Cd 0.98
*a hemispherical nose and side fairings,and gap-fillers gets it to Cd 0.42
*raising the Tender roof up even with the Locomotive fetches Cd 0.37
*sloping the 137 sq ft Tender down to the 89 sq ft frontal area of the rail cars cuts drag to Cd 0.29 *************a 21.6% reduction in Cd based on the frontal area of the Locomotive.(10.8% better mpg @ 55 mph)
This suggests a historical precedent for your 5th-wheel technology as well as my Viking trailer.
It is saying that you can expect the drag reduction and offers a little quanta.
The Renault V.I.R.A.G.E.S. 10-wheel semi-trailer got to Cd 0.29 with the extended boat-tail.
53 sq ft,down to 17 sq ft is almost a 68% wake reduction.I'm not yet qualified to predict your mpg potential but I must believe that you'll be delighted.I think orbywan got 24% from his RV boat tail.I think you'll be harvesting massive low-hanging fruit.It's a tantalizing project!

[skyking]

I got hit by some TANSTAAFL when looking at the combined rig. I'd better use 55 sq ft as a combined frontal area, since the tires on truck and trailer will not track in a line and they are so far apart.
I think the real shocker is what I am coming from when I get this done. Current rig is a whopping 17,000 pounds combined, 90+ square foot frontal, tapering to a 70+ square foot tail over a complete crap-storm of junk on the roof, with side awning and legs. Not faired to truck.
I have all but abandoned any aero mods to it, as I think this will be replacing it in the next year or so. About the only effort that will translate from one to the other is the truck mounted fairing. I can build that to fit the new trailer and it would help the old one as well.