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.,

Looks good- get on it! :)

Not sure that there will be a benefit with Mom's Attic (the U-Haul name for same) as turns & crosswinds will take their toll. The increase in sail area is the concern. I'm happy to be wrong (not my project), but it's my first reaction to trying to smooth out the transition from one vehicle to another in this way.

Second concern is the occasional off-camber hitch up where the vehicles are at several "wrong" angles to each other.

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Skyking, since you are building a 5er from scratch I'd suggest scaling the template for trailer height. With only 12+ inches above the truck cab I'm wondering how comfortable the loft will be. But as slowmover says it's not my project. From there I would slide the template for convex transition after the windshield. This is hard to put into words...

that 12~14" inches above the cab is a cab mounted rounded fairing idea, to smooth the transition from the sloping windshield to the horizontal section. Nothing will go in there and it is fixed to the truck itself.
sorry if you guys misunderstood.
It was an attempt to slow down that air a bit. As Phil has mentioned from time to time, the surface velocity above a blocky cab is 90 MPH at 60 MPH forward speed. The whole purpose is to get that speed back down to 60 as it leaves the rig.
If I can mitigate that acceleration just a bit it will help.
As far as the sail area, the crosswind pulls you around, not pushes, and it pulls the hardest on a square edge.
That bump fairing will not add to frontal area of the combination, but it will have really gradual radiuses on the front and sides.

Thanks for the clarification, skyking.

As I was going to bed last night the Weather Hazard radio went off (again) and it reminded me of the storm we had on May 10th with sustained 60-mph winds. So the third concern in changing the trailer exterior envelope is how well it handles winds when parked (lift & tilt).

Strange weather (unseasonable or flat unusual) is no longer to be ignored. An uncoupled 5'er has a large area well above ground level for winds to play with.

Past the above (this, and other posts) sounds like the reason for the "extension" would play well.

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I'll have less up in the wind with the roof down than your airstream, @ 87" tall. Roof up, 108", about the same height. Much less weight though. I may have to lower lid and spend a night on the sofa in extreme weather.

Pic worth 1k words
 

Skyking, could you post something visual on this thread to help me get my head around your project? ;)

wind
 

The wind issue is a big concern.
Not too long ago,Schneider Trucking's Dallas,TX hub was struck by a tornado which sent 13,500-lb trailers flying hundreds of feet.A weather chopper was broadcasting live feed.
What was weird,is that after the twister passed,they scanned Schneider's entire parking area,and none of the tanker trucks had budged,even though they were adjacent to tractors with their hoods sucked off,and trailers which had flown away.
The Airstream/Argossy/Burro/Casita trailers with generous radii all around should be the most immune to wind.Of course there's a point where anything will become airborne.
I like to think that the radii skyking has planned for the 5'er will mitigate some of the hazard we can see on You-Tube,where caravan trailers are falling over like a drunk on a bender,out on the highway.
To me,that footage was like a salesman for Haulmark,Featherlite,and Wells Cargo.Anything with 'soft' sides.
And from some of the drag tables over in the boat-tailed -trailer thread,you can see that this rounding drops the Cd quite nicely.

I really like how it mitigates some of those pesky crosswind effects :thumbsup:

pesky
 

No kidding,Gilkison recently pulled his 22-ft APEX out to western New Mexico to observe the total annular solar eclipse.He had a 40-mph crosswind on I-40 and was afraid to attempt pulling at anything over 45 mph.His mpg dropped to 10.6 mpg,from an ave.12.5.He said it was pretty freaky.
My Karmann Ghia will change lanes when emerging from under an overpass in similar conditions.:p
Orbywan's Ford motorhome looks like a VW 1-liter car in comparison to the APEX.I think the 5'r will be friendly on the road.Diesel pumps will hate it!:)

So Phil, can you see in your mind's eye what I am about? I know bob wants graphics. I have some wrenching to do or I'd teach myself some:D

I will be going up 14" overall height on the combo, I just think a fairing on top of cab to roll things on a bit nicer, with a longer transition to horizontal and some nice side radii will be worth the troubles. The subsequent transition from fairings to actual trailer will have no increase in frontal. The trailer will widen about 6" per side over the first 6' or so.

14"
 

*I'm the anal retentive,so I'm always going to do the hardest to fabricate compound curvature.The empirical data indicates that these forms deliver the lowest drag.
*However,some will argue as to whether the 'ideal' is justified when something 'less' is statistically 'close' in performance.'I'm like the poor b---ard in Dirty Harry........"I just gots to know!"
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If we looked at your RAM in the wind tunnel we'd see that your cowl/windshield/A-pillars/and windshield header does a fine job of getting the air over and around the greenhouse of the truck.
*If you were to scale this area up to the larger dimensions of the trailer face,you could expect similar results in the wind tunnel.So this would be a 'guaranteed' minimum roof fairing.There's no way it wouldn't perform.(this is exactly what I'm doing for Gilkison's APEX,using a discarded rear hatch off John's former Highlander as the 'base.')
*If you wanted to go beyond this,according to Hucho's references,as well as R.G.S.White's investigations,you'd get to the drag minimum.
*You just need to play it out in your mind.Do I want to put in 300-hours and get 100%,or put in 40-hrs and get 90% (or somethin' like that).
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Personally,I would do as you and Sven7 have been talking about with the double 'Template'.But again,the Black Widow 'double-windshield' approach to fairings would be an okay 1st runner-up.And a lot easier to fabricate.
Your the boss of this,so it's your call.
For me,if I go 'all the way',then I never have to wonder what the 'extra' effort might have netted at the gas station.
And like Howard Hughes,I'll continue to go on and on,spending my (time) and money on.............. It's just what I do.:rolleyes:

Been outside installing the new injectors, and looking at that roof hard. It is a nice shape now, and it would be a work of art ( 50 hours minimum ) to graft on something tasteful, removable, that won't damage the paint job. I'm thinking the same shape, just starting at the aft edge of the cab will be less costly.