Aero Villager (I hope)

[ShadeTreeMech]

Well, here's my impossible dream. Since my wife has her car, I have the ok to play with the van.

Besides doing some grille blocking and belly panning I want to do a boat tail. It may be a bit extreme for the sake of economy, but considering the skulls of my children would be in the crumple zone in case of a rear end collision, I want a bit more material for the offending vehicle to push through. And the extra storage capacity would be a welcome relief.

My abilities for building a frame are promising but untested. I'm debating on wood (cheap to free for me, and I can get oak for free) steel (I can weld it) or aluminum (never messed with the stuff frankly) or some combination thereof.

It would be about 1.3 meters in length, with tapers that are yet to be determined. I intend on making it strong and light as feasible. I am thinking a tin or coroplast exterior, or possible fiberglass on a cloth exterior. I have a friend who can work fiberglass, so I may take advantage of it. I would be removing the tailgate entirely, and either removing the bumper, or attaching the bottom of the structure to the bumper mounting points. Of course I'd have to move the lights and plate to the rear, and I would prefer to have rearward visibility. I am considering reusing the rear glass, since it is the type that lifts without the taligate. But i suspect that at the angle it would be, it wouldn't be long enough if oriented as currently installed.

If I could make it look like the rear of the Aero Civic (except for difference in scale) I'd be happy. But strength is also important to me. Any thought, suggestions, hints would be appreciated, as I'm in limbo as to how to proceed. As this van has a smallish v-6 I think 30 mpg is a resonable goal.

[RobertSmalls]

Have a look at the streamlining template.



Thinking big is good, and you may want to think bigger. The distance from where the curved roofline begins to where it meets the ground (roughly the length of the boattail), is 1.78*D, where D is twice the height of the vehicle. In your case, that's 6.34m! That's obviously not going to happen.

But if you line up the template with the widest part of the TOP view of your minivan, you find that D is the width of your vehicle, 1.90m, and 1.78*D is 3.39m. That's still a little long, but 70% of that length (i.e. 2.37m) gives you almost as good of a result. See this thread for drag reduction estimates for shorter boat tails. Even a 50% chop, 1.69m, looks to be good for a very large reduction in Cd.

I like the way it looks, and you'd probably have the least thirsty seven-passenger vehicle on this site. Very cool, and I hope you do it.

[ShadeTreeMech]

I think going right up to 2M in length would be acceptable.

I looked over the thread you mentioned which indicated I could halve the Cd with just a 30%, which comes in at 1.9 M. 50% would be 3.15 M, which is really pushing the zones of practicality. Without additions I'm looking at almost 5M, so adding another 3 would make me need to get a bus license! But seriously, I would still like to be able to park this rig without extreme measures or too much interest from the boys in blue.

I noticed the bottom of the boattail was flat; is that the optimum shape? I suppose I would need a bit of an agling upwards to keep from scraping the rear of the tail off.

I was planning on straight lines, but the templates call for slight arcs. Is there much of a difference? straight lines are a bit easier to build around....

Does choice of material matter much?

[RobertSmalls]

A flat underside offers the best drag, but it's not practical, especially when you don't have the freedom to move the rear wheels (inwards and rearwards would be ideal). For best aero, you probably want to go with the shallowest departure angle you can get away with.

You will want a curved shape. Airflow may not be able to make the transition around a corner, which would result in high drag. It's gotta be smooth. But if you want to test a hybrid design, you could build it in cardboard and tuft test it to see if flow remains attached.

As for materials, whatever gets the job done is fine. As long as it's got the right shape, there's no aerodynamic difference between materials.

[ShadeTreeMech]

thank you Robert for the thoughts. It's easier to me to bounce ideas off someone than to just ponder it alone.

How do I go about making a good curve? Only curves I'm familiar with involves my lovely wife, but I don't know how to reproduce them. Unless you count my 5 daughters

[RobertSmalls]

Identifying the curve is the easy part. Fit a parabola to the streamlining template above to create an equation for its curvature. Place the origin at the point of max. roof camber where x=0, y=0, y'=0, and knowing the point (1.78d,-.5d), the curve becomes y(x)=-.157808*x/d.

Since you're in Arkansas, I'll assume you have been unable to find a metric tape measure, so here's the equation for your sides (d=74.8in) in inches: y=.00211x²
and for the top (d=140.2in): y=.001126x²

Plug in x, the number of inches past the start of the curvature, and it spits out y, the number of inches inward your curvature has moved. I chose to plot points every few inches and cut out the curve in cardboard. Both the negative and the positive cardboard templates are useful for checking your curvature while building the boat-tail.

Now, how are you going to manufacture this beast? People have created body bucks in wire, which is easy enough to bend into a parabola.



I've been working in styrofoam, which has its merits and drawbacks. You could also do a quick approximation of the ideal curvature using flat sheets of coroplast or sheet metal, screwed or riveted to a metal frame. This method here is pretty attractive:

[ShadeTreeMech]

Dear sweet Jesus I think i have some research to do. I think I just went in over my head.

Appreciate the sentiment, but I found my metric tape measure. trust me I hate SAE.

I was thinking I could bend a parabola out of oak lumber I can get very cheaply. Assuming I do it carefully I can get a fairly predictable curve from a good hardwood board, then simply put in the screws. there's just the question of how to make a prescribed curve.....

[aerohead]

Originally Posted by ShadeTreeMech

thank you Robert for the thoughts. It's easier to me to bounce ideas off someone than to just ponder it alone.

How do I go about making a good curve? Only curves I'm familiar with involves my lovely wife, but I don't know how to reproduce them. Unless you count my 5 daughters

Shade,once you lock in the angle you're going to use,you can tape a stick of 1-inch schedule-40 PVC pipe to the roof of the van,extending over the back,and by pulling the pipe down to your rear bulkhead position create a nice gentle curve,something the air will appreciate.
The tail I built for the VW bus was made with western red cedar formers and stringers connected with steel angle brackets and screws.
Scribing off the 1-inch pipe gave me the curved lines later cut with a saber-saw.
After the skeleton was built,I clad it with aluminum sheet.
The most difficult areas are the radiussed intersections descending down and inward from the roof to which the back and sides are connected,as they are all compound curves.
I cheated on mine with a lot of relief cuts to very thin aluminum sheet,the smoothing the nasty parts with aluminum duct tape.An English Wheel would have been handy,this is the sort of thing they're designed to create.
Also,foam composite can make perfect sections.
bondo has us all beat,as he is a master clay modeller and he'll make a perfect clay master from which to create the tooling for a perfect composite part.
Do really smooth curves if you can,it's what the air wants.

[ShadeTreeMech]

I had actually thought about building the skeleton of the boattail from PVC piping due to its high value, practically indestructable, light, and strong (and impervious to rot.)

I've been having a rethink, and realized if I built the boattail on a trailer, and had attachments to link it to the van, I could go full blown with the thing quite easily. I doubt i would go the full 6.34 meters, but going with a 8 foot trailer with a 3 foot tongue would give me half of that quite easily, besides keeping the police happier (it's just a trailer, officer, not a part of the car.) It would still fulfill my goals and i would be able to have a flat bottom all the way from front to back as the aero template calls for.

Aerohead, I heard through the grapevine that you had built just such a trailer, and would treasure any thoughts you have on the subject.

[RobertSmalls]

Originally Posted by ShadeTreeMech

I doubt i would go the full 6.34 meters...

I just want to note that "full blown" is only 3.39m, when the sides taper inward and meet at the centerline.