View Single Post
Old 01-24-2013, 07:22 PM   #416 (permalink)
Master EcoModder
Sven7's Avatar
Join Date: Aug 2011
Location: Warren, MI
Posts: 2,456

Boo Radley - '65 Ford F100
90 day: 13.28 mpg (US)
Thanks: 782
Thanked 665 Times in 408 Posts
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.

*Image replaced with what should be correct*

Ideal Wagon Form by Tyler Linner, on Flickr

Does that help?

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)
He gave me a dollar. A blood-soaked dollar.
I cannot get the spot out but it's okay; It still works in the store

Last edited by Sven7; 01-24-2013 at 08:29 PM..
  Reply With Quote