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Old 01-24-2013, 08:06 PM   #417 (permalink)
Tesla
Master EcoModder
 
Join Date: Feb 2012
Location: Australia
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The Truck - '02 Nissan GU Patrol ST
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Quote:
Originally Posted by Sven7 View Post
Tesla, I think I get what you're saying. You're assuming both the side and top view teardrops have to end at the same point like a cone, right? I will continue assuming this... if I'm wrong let me know

You can't scale the template in x and not y- you need to keep the same ratio. When you set the template from the side you need the apex of the template (the 0% mark) at the apex of the roof. When you set the template on the top as in the MG above, you do the same, HOWEVER the car's corners should not stick out of the teardrop like they do above. You need to move the drop back far enough that is tangential to the rear quarter panels. (This actually applies to wagon profiles like your Patrol, too- the template should never "cut off" part of the car)

Yes, the apex from the side and the apex from the top will be positioned differently relative to each other- meaning, if the side apex is in the middle of the front door and the top apex is over the rear axle, that's okay. That's how the air needs to flow off the car.

Here is a Patrol with the ideal template. Since it tapers more on each side than on the top and bottom, the sides meet sooner. This is just fine! It will give you the ideal curvature on side as well as top, dissipating pressure evenly. Of course you'll need as much radius as you can get (the fore-body still has pressure differentials windward of the tail) but inevitably the rear will end in a vertical fin- this is natural if you use the template on all sides.

The boat tail will not automatically fix the pressure differentials- it will only bring the "split" air back together again when your vehicle passes. Looking at the macro I believe you will still have vortex-like airflow patterns, though not as strong as before. The radii aft of the apex are where you get rid of those differentials and therefore the vortexes.


Ideal Wagon Form by Tyler Linner, on Flickr

Does that help?

EDIT
Now you guys have me second-guessing the correct application of the plan view tapers, but the concept should still be the same.

Since the maximum taper is supposed to be 22*, shouldn't the tail's rear end meet at that angle? Therefore you would want the "wheels" of the template along the centerline of the car as in post #413. Otherwise your teardrop meets without hitting that golden angle, adding more skin drag than is necessary.

Bringing that tail in without tapering to 22* leaves geometry on the table! (But you still have to move the apexes back to be tangential with the body)

Discuss! (I love EcoModder)
That's it, and thanks for the Patrol images, still haven't worked out how to do the overlay's.

Regarding starting point 0, we tend to agree that there should be some flexability that it matches as close as possible to the max camber point and the natural taper which all vehicles have to some degree, so there are some compromises to be made here, because unless you drive a template, these two points wont be the same on any regular road vehicle.

As for the scaling, that's where my bigger question lies, I've read numerous times that the width dimension is used as the whole mirrored effect, so the template must be applied from the vehicles vertical centreline as the base for the template.

I've also read numerous times that the narrower of the two dimensions, length & 1/2 width is the major player, so this suggests we either apply that to both dimensions, resulting in a significantly reduced tail, or we have to apply different scaling to the sides and the top.

My thinking is that the x coordinate applies from the centre line and the y coordinate applies from ground level.

So like in my case with the Patrol, a point somwhere around the rear axle best conforms to a balance between max camber and start of natural body taper, I then took the measurements at multiple points up the sides and on top creating a 2d profile then tabled these coordinates and applied the same equation to all coordinates, some fade to 0 very quickly others take longer, but ultimately as your image shows a full tail would end up as a vertical fin.

It kind of came to me when I was thinking about the angled surfaces on a vehicle, are these vertical or horizontal or do both apply causing both narrowing and lowering, this seems to be the way to do it, because the air will want to go down as well as come in.

If we think of the ideal 5:1 tear drop body of rotation, it is tapered from all perspectives, not just vertically or horizontally, so therefore both coordinates must get the same treatment.

Still getting my head around this stuff, so well prepared to be wrong.
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