fairing for trailer questions
 

The proposed trailer will be 12" to 14" taller than the truck. It seems that the air gets accelerated right at the top edge of the cab pretty rapidly. I think bringing the height change forward and fairing up on top of the cab might be worth the trouble. Thoughts?
The trailer will be ultimately wider than the roof of the truck but I can transition to that width from the back of the cab all the way to the back of the bed if I want to. I was thinking a fairing that had tumblehome on the side, and made the top of the cab more closely resemble an ideal shape.

As far as I can figure, lay a template over each. You want to manage the pressure buildup on the front so you can use it to your advantage at the rear if you have or will have a boat tail. Starting at the top of the windshield will not build the pressure as well as the template would.

http://i46.tinypic.com/2vv5o5v.jpg

I set this up as if there were two different sized vehicles drafting each other at negative distance.

lift the template up front so it follows the windshield and tops out at trailer height. That was what I was proposing. It will have about 17' of straight before it goes on back down the back side of the template. I was just thinking that top window edge is a bit of a storm right there, and since I am going up 14" or so, I could optimize the transition from window slope to long horizontal section.
I'd mount the aft portion of fairing on a headache rack at the back of the cab and a couple of suction cups on the roof forward. The back would have the leverage to hold it, the cups would just stabilize and minimize any possible flutter.
The back end of the trailer will be as ideal as possible, with tumblehome when possible and planwise taper as well as follow the template. Gently gently!!

Sounds like you already know what to do then ;) Just copy and paste that part forward and you'll have it- it's the same height either way and I neglected to save the Photoshop file.

Also, I'd be wary of suction cups unless they're those dinner plate size honkers you get at Harbor Freight.

Thanks man, I was just looking at bouncing the ideas around. I think there is a little bit of money there, smoothing that transition from sloped window to horizontal a bit more. Not often do you get to do that without adding frontal. OR getting out the welder, saw, and wallet. :D
Edit: I done did it, and named the file in your honor :P

Are you building or modifying the trailer yourself ?
How about rounding off the front of the trailer so it matches the cab for width and height ?
Any lost volume due to that could be taken up by increasing the length in a place where the cross section is far larger, and the space more useful.

You could use top and side plates on the rear of the cab to (almost ;) ) close the gap to the trailer, while not making the tow vehicle wider or higher.
It wouldn't impact the towing vehicle when used alone.

It is a custom built design I am cooking up. This roof fairing would come off quickly so no worries about that. The trailer is a fully faired 5th wheel with articulating panels to keep things tight but allow for turns.

what does the back of the trailer look like???

There is a LOT more opportunity for improvement at the back of the trailer than the front.

The back will be as close to perfect as I can make it and still use it. It will have plan taper, and radiused edges that will transition into a fully rounded aft section, following the template out to 65% or so. I will add a folding section to take is as far as I can aft when there are no length constraints.
frontal area of 53 sq. feet, tapered down to a 17 sq. foot tail.

that will be cool.

I am REALLY interested in learning what the measued Cd will end up being.,