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Old 05-24-2013, 05:20 PM   #533 (permalink)
aerohead
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intention

Quote:
Originally Posted by ChazInMT View Post
I have analyzed Phils Gen II template and there are some niggling issues I think are worth mentioning in case they need to be corrected.

I'm afraid what may have happened is that in the course of scanning the new images in, they got their aspect ratios messed up, and what we're left with is a fatter/shorter version of the previous template.

With the new Car Template Dwg, It seems to have an aft body ratio of 1.7 d when before Phil was indicating 1.78d for this same ratio. If you look at this drawing I made using essentially the same shape as Phils teardrop, but shifting forward the breakover point to make the aft body 1.78, there is a bit of a difference.









Big Version Here to look at.



What the new template does in essence is shift the alignment point on top of the template back 5% from where it was before with respect to the 1.78 aft body ratio.

Here are the 2 different teardrops, one with 1.71, the other 1.78. The difference in this picture is not discernible.



In the Bigger version Here you can sort of see a difference.

And here You can see it when zoomed in.



If this is the intention to go with a 1.71 aft body ratio then who the heck am I, as a plebian compared to Phil, but to say anything other than "Let's Roll".

If this is not the intention, than I think I can easily generate a new very precise drawing to fill in where Phil is using a drawing board and pencil.

Here's the New (My longer version) template on the old "Car" to see the change.



Bigger Version of Same

Here is my analysis of the teardrop GenII drawing.



I hope this makes sense what I'm trying to point out, the difference seems to be right on the edge of significant since the new template as drawn deviates from the old by 5%. From all that Phil has said to this point, it seemed he wanted to simply tweak the shape making faster in the first 30% and flatter in the aft 70%.
*Wind tunnel flow photos and tuft testing suggested that the 'Template' architecture was too conservative.This troubled me as much as any of the members.
*Some vehicles 'fit' while others seemed too 'fast' as Hucho warns us about.
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*This 'other' teardrop was still within the realm of the 2.5:1 free-air fineness ratio,would have Cd 0.04,but had a bit more 'aggressive' contour,more like Prius and 1st-gen Insight,and respects W.A.Mair's 22-degree limit to protect the boundary layer.
*It's 'slower' downstream.It doesn't hit 22-degrees until down in the area chopped away for ground clearance.
*This newer contour would allow a bit better rear visibility through the backlight.
*It really needs to be thought of based on its own original architecture,independent of the original drop.
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*I've done comparisons with both and it appears that one will work better than the other depending on vehicle.
*So I wanted to present both.
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*There are some other contours that we could consider for 'Templates' which I'll present in the future,but none of them can provide the drag reduction of the two I've posted and still allow for outward visibility or ground clearance.
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Thanks for all the imaging magic! No way do I have the facility to enhance the visuals as you have done.We're miles and kilometers ahead of my chicken scratching.
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