Foamie aerodynamics.

[bonnie]

Here's a question. How much depth is needed to make the air flow around a front and at the moment flat surface? Is the purpose to disrupt the airflow or to channel it along flat sides. With foam we have a lot of options for shapes and designs. I don't have much tongue length to lose. While I may eventually put a simple A frame under the BUB, right how I'm just working out the top design. The tilt of the from should be enough to blend into the airstream on the car, I'm just wanting to maximize the air flow I guess. I'm not looking at adding any volume to the trailer's usable space, nor was I looking for an integrated tongue storage box, though I am open to ideas.

Happy Saturday! I'm off to work.

[angib]

How much depth is needed to make the air flow around a front and at the moment flat surface?

Very little. What you have to understand is that it is the detail shape at the edges that really matter and not the overall shape or styling. All those slope nose or needle nose designs are just styling and are not needed for aerodynamics - at least not a road vehicle speeds. This NASA test truck body has a lower coefficient of drag than any production automobile, except for the GM EV1:



Those clear bits at the front are windows not openings.

So you need to go for the same priorities of well-rounded front edges, perfectly aligned with the sides, anywhere where the trailer may stick out as far as or beyond the tow vehicle - so there is no point getting all clever with the front-top edge of a trailer that is only half as high as the tow vehicle.

In between the rounded edges, the shape doesn't matter much, so a flat panel is fine. For that matter you can just add the rounding in front of the front panel - here is a graphic I did a while ago showing a box trailer with half-pipe added to the front edges. Something like 8"-12" plastic pipe cut in half lengthways and stuck on with mitred corners would do fine. This is as long as the outside edge of the half-pipe is perfectly flush (like within 1/8") with the sides - get it more than two inches away from the edge and it isn't worth bothering to do.

[GPW]

Angib , that’s Most interesting !!! That NASA truck looks like a Foamie in the works ... a simple translation ... Brilliant !!! Thanks !!!!

[loaderman]

A rounded front and tapered back is ideal. The tapered back should start right after the rounded front, butdoesn't even need to go all they way to a point (round). Did a bunch of reading on Kamback.
Basically the rounded front disrupts the air the least/best then as it goes along the trailer the taper actually causes the air to push the trailer forward.

[GPW]

Full taper ... apparently NASA went from square box to fully tapered ... http://spinoff.nasa.gov/Spinoff2008/t_3.html

[Ned B]

That's all styling without much aerodynamics involved in it, except the side skirts on the trailer, though even they aren't very well done - why don't they at least line up?

No attempt to match the tractor and trailer shapes, left the bad 'flip' spoiler all round the tractor - it's a dog's dinner of a job.

yeah, I wasn't at all impressed with their work. The ONLY thing I thought we'd enjoy was that they used foam to do the Buck for the nose... other than that they're blowing smoke and pretty well useless.

[Wobbly Wheels]

They lost me with this one:

...or a 21st. century design that travels up to 350mph on a bullet train...

Reynolds Number is pretty fundamental aerodynamic theory.
Model aircraft builders know that a perfectly scaled model has different aerodynamics than the real thing.
The viscosity of the air doesn't change, so the dynamics between hitting it at 80-100 km/h and hitting it at 350 km/h are completely different.
That thing might be a cool design experiment but from a practical perspective I think it's a disaster. Those skirts seem like they would be torn off pretty easily, especially coming over the hump on a sloped loading ramp. With its long nose it's going to be a beast to maneuver in tight quarters - you can't see the front corner from the driver's seat.
Just an armchair opinion though, maybe it's a dream to drive.

[loaderman]

GPW - I don't think it need the full taper to the point at the back, apparently your 1st pic with the Kamback is good.
The taper needs to start further up, right from the front.
The entire front should be rounded with no diameter or any arc being less then 2' according to my reading. No flat area as the winds off a tow vehicle are swirling, the flat spot would only work with a designed tow vehicle and trailer close enough so that when the winds come off it they would flow over the trailer.
So a rounded front with a sloping back/sides is what would be best.
The sloping sides/back doesn't need to come to a point, but can be truncated as in your first picture, the kamback idea.

[GPW]

All that rounding should be pretty easy to do in Foam ... I’m hoping the Foamstream works , big old whale like rounded nose ... but no taper .. may have to put some wings in the back .... maybe ??? maybe not worry about it , we don’t drive fast anyway ...

[loaderman]

Here is my take on an larger aerodynamic trailer, not necessarily to scale.
Rounded front mostly to top takes more air to the top then pushes down as it goes over the taper to the rear.
The front should also be rounded where it meets the sides, I did not know how to do it in sketchup.
The sides are also tapered to the back.
So the trailer it narrower in height and width at the back.
the undersides are smooth, tapered down to the center of the axle then back up, not sure on this may be better to drop the front end as low as the axle height then taper up all the way from the front to the back as I did on the top.
Axle is wider then the trailer for stability with fender skirts.

http://s1224.beta.photobucket.com/user/mae-ling/media/trailershapeaerodynamic_zpsd1123b9e.jpg.html?sort=3&o=0

Couldn't get pic on here so here is the link. If someone else can please do

[DriverOne]

Wobbly Wheels, you're correct about the nose on that thing. Those people were so focused on MPG's, they forgot about tangible maneuverability and real-world impact recovery. They also insulted every major truck manufacturer out there before they built a single production-line unit! It's stuff like this that bothers me as a driver. Nobody in that company is thinking about the frustrations of piloting the thing in a NYC downtown area, or having to worry about hitting a deer and destroying the high-dollar superfan in a matter of seconds. The flat nose on the front of my snub-nose International is so I can have a much smaller turning radius while ensuring I can get my truck inches from an immobile object without colliding with it. That thing is an intra-warehouse highway queen. I would be surprised too to see it parked in a Pilot truck stop... Those are notorious for being hard to maneuver.

[mezmo]

Hi Bonnie,

Does the BUB still have the front 'cowling' that Coal Burner' put on the
front of the top section ? If I recall right, he said that helped immensely
with mpg. Reason being, I believe, is that it thus made the front of the
trailer shape/area coincide/equal to the cross-sectional shape/area
of his tow vehicle, a VW Golf then I believe, when he did that. Do that again,
as that is still close enough for your Nissan Sedan's cross-sectional shape/area.
If you cut that off when you trimmed the top, and still have the pieces,
just put them back on, cutting them to fit/allow for the moving top and static
bottom, but looking and acting like one piece when the top is down in
the travel position. If you don't have the pieces yet, just make some new
ones your self. The car has already 'punched the hole' through the air
for it and the trailer, Trying to keep the airflow around the car go smoothly
onto the trailer is the objective. No gap between Tow Vehicle and trailer
is the ideal [see angib's posts at the pottom of page 3 of this thread], but
practical considerations like them not touching when backing up or going
over dips and up rises have to be dealt with too. So closing the distance
between the two is worthwhile but within reason and practicality and
necessity.

Another thing to try, would be to make a fairing/spoiler for the back of
the car, it starting out level/even with the roof and the sides and gradually
flaring out to the width and height of BUB's cross-section [especially with the
front narrowing and lowering cowling]. That would aid the air flow in
straight-line travel. But It'd probably be more work than major aero
results, although every little bit helps.

The dead or turbulent air between the tow vehicle and trailer will more-or-less
stay 'in place' if contained within the smoothed airflow between the tow vehicle
and trailer if you can get them lined up and flowing together, generally speaking
of course. Details matter, and one of the most important is smooth, i.e. smoothly
'radiused', edges and gradual change in planes/surfaces [within 15 degrees or
less it's said].

[This is all from what I've read or seen on the topic - no personal experience
yet though.]

Cheers,
Norm/mezmo

[GPW]

With our sized trailers , I don’t think we have to worry so much about Drag .... You haven’t really experienced Drag until you’ve towed a big commercial RV trailer ... Big square boxes ... It’s like towing a Parachute ... a very Heavy parachute ...
The limited towing of the smaller trailers felt like there was nothing there , in comparison ... I found myself looking back frequently to just see if it was still there ... Sure , we can do things to help reduce drag , but then it shouldn’t interfere with “Practical considerations” ... Do what we “can” (practically) , enjoy the rest ...

[bonnie]

this looks like a winner. I do have the pieces I have removed from the top. I'd have to rework everything. Got something to ponder now. Thanks everyone.

[angib]

It might be worth giving the drag coefficients that NASA measured for that test truck as it may show some effects that folk don't expect:

1) All edges square, front and rear - 0.89
2) Horizontal edges rounded/vertical edges square, front and rear - 0.54
3) All edges rounded, front and rear - 0.41
4) All front edges rounded, all rear edges square - 0.365
5) Rounded front, partial boat-tail rear (as my photo above) - 0.242
6) Rounded front, full boat-tail rear to point (as GPW's photo above) - 0.238

Remember these are data for an independent vehicle, not a trailer, so don't read too much into them. But one thing to look at is the comparison between 3) and 4) - the one with the square back end has less drag than the rounded back end.

Oh, and here's the full NASA report: A Reassessment of Heavy-Duty Truck Aerodynamic Design Features and Priorities