Boat tail on trailer vs. deflector in front: The beginnings of a clean slate project

[skyking]

I get that part. It was more a search for simple and foldable construction.
Now I am having second thoughts about the boat tail. The Ac unit is not a negotiable item in my household. As you said, it trashes things up there very badly, along with the rear kitchen vent.
I look at it and see a full and properly curved rear fairing starting at the high point of the roof, cleaning everything up and ending in a Kamm back about 1 foot aft of the current roof. If I play it right, it will be heading down at way more than a paltry 12 degrees and follow your recommended form. It would have a transverse bulkhead to separate the front and back of the AC unit, with a vent above for the inlet air to the condenser. I'd seal that area up in front and utilize the proper type of inlet that will let the air pass over it smoothly.
I'd need your help designing that.
The aft side of the bulkhead would vent out along the edge of roof.
It would provide the perfect point to mount a shorter and curved boat tail.
I am working on that geothermal system so I will be a few days before I can draw anything.
I think it would clean things up considerably.

[KamperBob]

Phil, thanks for posting the W.E.Lay data (1933 - pumpkin seed). I made a quick plot to see what the diminishing returns curve looks like.



I find the shape suggestive...

[aerohead]

Originally Posted by skyking

I get that part. It was more a search for simple and foldable construction.
Now I am having second thoughts about the boat tail. The Ac unit is not a negotiable item in my household. As you said, it trashes things up there very badly, along with the rear kitchen vent.
I look at it and see a full and properly curved rear fairing starting at the high point of the roof, cleaning everything up and ending in a Kamm back about 1 foot aft of the current roof. If I play it right, it will be heading down at way more than a paltry 12 degrees and follow your recommended form. It would have a transverse bulkhead to separate the front and back of the AC unit, with a vent above for the inlet air to the condenser. I'd seal that area up in front and utilize the proper type of inlet that will let the air pass over it smoothly.
I'd need your help designing that.
The aft side of the bulkhead would vent out along the edge of roof.
It would provide the perfect point to mount a shorter and curved boat tail.
I am working on that geothermal system so I will be a few days before I can draw anything.
I think it would clean things up considerably.

If you'll use the 'Template' to design the fairing you'll get Cd 0.09.It's the lowest drag form for a fairing.It's right out of Hoerner's book.She'll look like a scaled-up version of a teardrop clearance lamp lens.

[aerohead]

Originally Posted by KamperBob

Phil, thanks for posting the W.E.Lay data (1933 - pumpkin seed). I made a quick plot to see what the diminishing returns curve looks like.



I find the shape suggestive...

KB,I did want to mention that these data reflect no attempt to clean up the wheel drag itself.
In 'race' form.GM's Solaraycer had Cd 0.12,@ 89% 'Template.' In the wind tunnel,with the wheel skirts in place the car recorded Cd 0.089.
NUNA-3 has come in @ Cd 0.077.
So it looks like there's still food on the plate

[skyking]

I took a few pictures today on the way out to the beach, and gathered some numbers.
The last half tank of mixed driving(truck only), 60% highway got me an all time high of 21 MPG!
I took the rig across the scales and it is at about 17,800 GVW with full water and some tools. I aired up the tires before starting out, higher pressures than before.
I'll record the time the trailer is off the truck and re-figure that at 18 MPG per mile. It is all at 25~35 mph, less than ideal speeds.
My last trip like this was at 14.9 MPG. I hope to establish a good "B" mileage before starting the mods.

Profile: I see a fairing hitting the high point and continuing aft, parallel to a point I'll have to figure out.


It will start following the template at that point and just clear the edge of the roof at the back, so it will have an angle near the maximum at the back. It will go about 1 foot aft to form a Kammback. it will tie into sides that extend 1 foot aft with the recommended 12 degree angle.
I can then build the boat tail and keep it short, say 6 feet long, but will have good wake reduction due to skimming over the edge of the roof as planned. I can't bend the sides in any steeper but I can do quite a bit with the top this way.
The front view:
That forward vent monstrosity gets a new housing. This view illustrates the challenge of the front wing, with the truck considerably narrower than the trailer.
Don't even talk about the mirrors they are not a negotiable item



I do believe the wing/deflector up front will pay some since it is a good 3.5 feet from back of cab to front of trailer. I get some 'bonus' frontal there, I think.

Aft view, illustrating the curvature of the roof.


Here is a high res of the profile, in case somebody with photoshop skills want to take pity on me
http://i810.photobucket.com/albums/z...rucking004.jpg

[KamperBob]

Sky, nice rig. Here's a quick & dirty template overlay.


I didn't aim for perfect just good enough for first pass side check. Suggestions to consider:

* Reverse roof AC front to rear. I suspect flow will reattach then for sure - if it's not already. (I'm planning the same mod to my rig once I get a round tuit.)

* Move hitch as far forward on truck as possible. A slider mechanism can retain low-speed tight maneuvers when needed.

* I agree a cab roof deflector could help a little. (Mine would also be a solar panel, ideally.)

* A vertical divider over the pin on the loft nose can reduce gap cross flow in cross winds. (My front-mount generator doubles for that purpose.)

Your loft nose is already raked, rounded on top, and somewhat convex side to side, so I don't see huge room for improvement there. If the side edge radii are less than 10% of the trailer width then per Waters 1969 (ref. p259 fig. 4.128) those might be increased with nose work. OTOH aiming for clean side wash is complicated by awning rails, for example, so I wouldn't focus much up front.

Another RV benefit of an overhanging rear boattail is awning over the rear window. Ideally, the top part could be a solar panel.

PS- That custom box is neat!

[skyking]

Thanks Bob. What I had in mind was starting with the high point of the template at a point somewhere above the AC unit. If it was scaled up right it should just pass over the back edge of the roof heading down at the 22 degree angle. In front of that it would be straight, connecting with the high point of the existing roof. The whole thing would be curved to fit the roof curvature left to right.
This fairing would take care of the too steep angle as the roof descends aft, cover up the AC, cover up the kitchen vent in back, the fridge, the TV antenna. I would make doors or slots as needed to keep all those functioning properly. I would put a large forward opening hatch over the AC that deploys with a spring and is retracted by a simple rope at the back of the rig. It would be smooth as a baby's derriere and shed water as well.
I can remake the pin box on the trailer and move it forward about 4". I cannot do a slider hitch.
Hidden under all that woodwork is a custom made 5th wheel air ride suspension system that is not adjustable that way.

[KamperBob]

Originally Posted by skyking

Thanks Bob. What I had in mind was starting with the high point of the template at a point somewhere above the AC unit. If it was scaled up right it should just pass over the back edge of the roof heading down at the 22 degree angle. In front of that it would be straight, connecting with the high point of the existing roof. The whole thing would be curved to fit the roof curvature left to right.
This fairing would take care of the too steep angle as the roof descends aft, cover up the AC, cover up the kitchen vent in back, the fridge, the TV antenna. I would make doors or slots as needed to keep all those functioning properly. I would put a large forward opening hatch over the AC that deploys with a spring and is retracted by a simple rope at the back of the rig. It would be smooth as a baby's derriere and shed water as well.
I can remake the pin box on the trailer and move it forward about 4". I cannot do a slider hitch.
Hidden under all that woodwork is a custom made 5th wheel air ride suspension system that is not adjustable that way.

I look forward to your rooftop solution. Compound curvature makes the puzzle fun. Your access vent ideas are intriguing too.

Now I understand your hitch constraint. I thought someone made cushions compatible with sliders but maybe not, or too expensive, or depressing to scrap a DIY accomplishment (I know). When you said pin box it clicked that you have an extender. I don't know how much cab clearance you need but that seems like another immutable requirement.

Rock on!

[skyking]

I took some measurements and I can move the trailer forward no more than about 3" or 4" before it starts making problems for me.
I was hoping for something in the 14's but I got a solid 13 MPG on this trip. 21 empty to 13 loaded, I see opportunities to excel here!

[skyking]

Here is a crude paint rendition: just imagine the template on there instead of scribbled on.


I have many questions on there.

Here is a front view:



Should I flare out the front deflector like that to accommodate the difference it width, or just make it cab width to eliminate the bad wake of the cab itself?
How close to trailer? How high?