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

[euromodder]

A more realistic 8 ft boattail (allowing it to fold on the rear), would angle in 1.7 ft over those 8 ft.

That'd reduce the wake area from 64 square ft to some 40 square feet (-37.5 %)

[skyking]

I'm getting there. I just found an old casio solar powered scientific calculator; I had to "borrow" my wife's sewing light to get it going
That seems a meager start to me. I was hoping to reduce that some more.
Will it work OK open-ended?
I am looking at folding it twice, that would get me out to 13' or so.
If I'm not building a long radius, 12 degrees is absolute the max I should go?

[skyking]

Originally Posted by euromodder

A more realistic 8 ft boattail (allowing it to fold on the rear), would angle in 1.7 ft over those 8 ft.

That'd reduce the wake area from 64 square ft to some 40 square feet (-37.5 %)

Hmm.
8 -1.7 = 6.3 high
8 - (2(1.7)) = 4.6 wide
I get 28.98, or a reduction of 55%

[euromodder]

Originally Posted by skyking

Hmm.
8 -1.7 = 6.3 high
8 - (2(1.7)) = 4.6 wide
I get 28.98, or a reduction of 55%

Oops. Now you got me !

I only counted the decrease from one side (i.e 6.3') and then squared that.
So it's indeed a reduction from 8' , to 8' - (2 x 1.7) = 4.6'

If you start from 8' x 8', and angle in both sides + top & bottom, you'll end up with a tail area of 4.6' by 4.6' - or only 21.2 square feet.
That's a serious reduction.

It will work open ended.
The aft edges need to be sharp / well defined so the air releases smoothly.

A 13 ft tail is going to stick out a lot, and will greatly increase the swept area aft while turning. Could be dangerous.


Seeing you might want to make the boattail folding twice, I wonder how much smooth length you'd need before you could do another angle change.
IIRC I've read somewhere that you can't get away with a 12° angle-in twice, so the second angle would have to be less.

Maybe aerohead can shed some light on that ?

[skyking]

I'm very familiar with the swept area behind axles. With a setup like that I can't see in the mirrors, I'd be using my camera in tight turns.
I suppose I could recess the tail lights in from the edge to get that clean break you speak of.

Edit: Let's try 10', and bring it in half the angle again. The first 8' @12 and the next 2' @ a total of 18 degrees.
1.6' + .65' = 2.25'
The starting width will be 7.67'
the end width would be 3.17'
3.17 squared is right about 10 square feet, a huge reduction.
Getting narrower does help the swept area problem during turns as well.

[aerohead]

Originally Posted by skyking

Which one first?



I'll post more detailed pictures when I get it hooked up again.

I just installed my wood bed sides/toolboxes, which allow me to tow the trailer at the proper, lower height.



Note that the sides of the boxes allow for an easy full wheel skirt.

I am covering the running boards with teak decking, and I am looking at a fillet fairing for that hard corner at the lower front of the toolbox/back of the cab area.

I am building new front and rear bumpers and grill. That is why I consider it a "clean slate" if you will.
The front bumper will be lower than stock and wider at the bottom to more effectively cover the front tire.

Back to the question: which to do first, a really tight articulating deflector in front of the trailer, going forward onto the truck cab roof, or a three piece folding boat tail?

I have to make the boat tail easy to fold, with a "door" I can open in front of that back window. Hinge the sides in, then lower the top down and open a door.
Advantage of the deflector: Don't have to fold it up to get parked. Don't have to worry about hitting things with it.
Advantage of the boat tail: Quicker build by far.

The tail is going to help but they don't operate properly if the air coming their way is trashed with turbulence.
The AC units and rooftop cargo box will ruin the airflow up there.If you could fashion better fairings ( boat-tails ) you could quiet down the storm up there.
Your running boards need a leading edge fairing ,even if it's just PVC pipe,ripped longways and bolted on.
Then,at the back,you need to soften the leading edge of your new box ( nice! ) where the running boards come up to meet it on the sides.
The wheel skirts will be great!
We need a look at the nose of the trailer itself.Any big gap that can be closed in is good.
Since you're starting fresh,you could do a cap over the box which acts like a flow integrator,smoothing the transition from the truck onto the trailer face,effectively having it 'draft' behind it.
The air has already collided with the front of the truck,you don't want it all to have to slam again into the trailer,so to speak.
The aluminum struts for the awning are catching some air.You could think about them.

[skyking]

What you see in the picture, front to back:
a "maxx" air fan and housing over the bedroom. Max disturbed air, that is!
Maxxair Vent Corporation
I can take the fan out of it, and build a bubble housing that will have a lower profile and much less drag. A wintertime project.
There is a smaller fan over the shower and a skylight that are much cleaner.
Next is that huge AC unit. I was thinking of a lightweight razorback cover transitioning from the trailing edge of the upper roof, over the AC unit, all the way to the trailing edge. I'd ventilate it for the fridge, AC, and that last Maxxair cover over the kitchen vent.
That would smooth things out considerably, but as with the boat tail it would have to be stout to withstand the gusts.
I called my wood supplier and got some prices. The least expensive 4MM marine ply was hydrotek, at $28 per sheet. 18.5 pounds per sheet. Okoume was down to 13.5 pounds but right around $60 per sheet.
That is too much.
If I build an aluminum frame, will 4 or 6 MM Coroplast hold up? Has anybody glued it, or epoxy?
On the truck bed/running board:
I will be building a fillet fairing out of wood for the crash there at the front of the box.
Imagine a triangle with the point almost to the back edge of the door inboard, angled out to eventually match the box profile.
I started covering the running boards with wood today. They are not ideal aerodynamically in any way, but they are not going to be changed.
It will be very easy for me to put a 3/4 belly pan on it and tie it all the way to the new bumper and air dam.

As to the front of the trailer I had a vision of an upside down and backwards NACA inlet shape, starting mid cab roof and tapering back and up and out, going halfway up the face of the trailer and getting very close to it. I'd articulate the outward parts to allow for the trailer turning, but keep the gap around 6" or so.
I'm hoping ( dreaming?) that higher pressure air under the wing that enters around the cab will "blow" that slot and cause some laminar flow across the front of the trailer.

[KamperBob]

If you haven't already, consider the MaxxFan (from same folks). It's also rainproof but unlike the quick and dirty (literally aero wise) cover it folds flatter for travel. They are also built much sturdier than regular roof vents. Instead of one flimsy crank on one side only so the lid is prone to rattling in the breeze (even parked) the MaxxFan mechanism is two sided plus it locks it any position so you can (if you want) leave it up (however little or much you want) going down the road without damage. I put one in my trailer and love, love, love it. Not cheap (a bit more than Fantastic Fan for powered vent) but worth it to me. YMMV

[skyking]

Originally Posted by KamperBob

If you haven't already, consider the MaxxFan (from same folks). It's also rainproof but unlike the quick and dirty (literally aero wise) cover it folds flatter for travel. They are also built much sturdier than regular roof vents. Instead of one flimsy crank on one side only so the lid is prone to rattling in the breeze (even parked) the MaxxFan mechanism is two sided plus it locks it any position so you can (if you want) leave it up (however little or much you want) going down the road without damage. I put one in my trailer and love, love, love it. Not cheap (a bit more than Fantastic Fan for powered vent) but worth it to me. YMMV

Thanks Bob. I have a plan for that fan. Since I already have the switches and high dollar fan, I will take them out and make my own low profile housing.

[aerohead]

Originally Posted by skyking

I'm getting there. I just found an old casio solar powered scientific calculator; I had to "borrow" my wife's sewing light to get it going
That seems a meager start to me. I was hoping to reduce that some more.
Will it work OK open-ended?
I am looking at folding it twice, that would get me out to 13' or so.
If I'm not building a long radius, 12 degrees is absolute the max I should go?

skyking,it's more about degree of curvature than it is angles.That's why I put the 'Template' together.
If you can't do curves,then consider Walter E.Lay's research of 1933.
He did a 'pumpkin seed car(like Jaray,MG EX-181,GM Solaraycer ),only he did no curves on most of his models.
He had an 18-degree angle for the roofline,and the sides angled in @ 12-degrees.This looks to be about what GM has done with its 'OPTIMUM' boat-tail.
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Lay's models achieved the following drag coefficients as a function of boat-tail length:
* 0% = Cd 0.326
* 10%= Cd 0.325
* 20%= Cd 0.305
* 30%= Cd 0.235
* 40%= Cd 0.187
* 50%= Cd 0.157
* 60%= Cd 0.140
* 70%= Cd 0.127
* 80%= Cd 0.122
* 82.4%= Cd 0.12 ( his 100%template only goes to 82.4% 'Template"
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If you have to do a simple construction this may give you a sense of what's been done with good effect.