Scaling the Template

[ChazInMT]

Originally Posted by kach22i

You sure have a big head about yourself Charlie, living in an alternate universe must be hard on you.

Is that the best you got? Name calling? Trying to demean because you can't counter with something that makes sense? Could you just bring up one talking point to elaborate on? Or is it just "Template scales and still works good" "Square peg semi-round hole" "I have figured out this is stupid and everyone else is an Idiot" with you?

This is why it is frustrating to go back & forth with you cause you never acknowledge anything anyone puts out there.

[Xist]

Originally Posted by kach22i

Kach22i, for a moment, let's go back to the sideview of the template. Does Aerohead tell everyone to create boat tails that have the full taper? No, he tells us that it is impractical to have it go more than eighty or ninety percent, which is effectively as good as a boat tail.

Is a laterally hemispherical vehicle practical? Absolutely not. For anything that fits in a car lane, you would not be able to do anything with the outer quarter on each side, half of the width, so would it be worth increasing the frontal area? No, but it would completely eliminate vortices, or so I read.

Round is good, where possible.

[kach22i]

Originally Posted by Xist

Kach22i, for a moment, let's go back to the sideview of the template. Does Aerohead tell everyone to create boat tails that have the full taper? No, he tells us that it is impractical to have it go more than eighty or ninety percent, which is effectively as good as a boat tail.

Xist, that information gets lost when using the template as the simple tool that it is. Kind of says to me that there needs to be a truncated template used exclusively for boattails. The template as first introduced was as I understand it meant to be used as a template for creating boattails on existing automobiles.

However as shown in the cross section study I did, adaption from square to round is not even a consideration. This could be an extreme oversight, or not a big deal, the more I think about it, the more I see need for a remedy.

My contention is that square does not fit round, it should not be a "hard-sell", it should sell it's self.

Proposal: Two new templates need to be developed in 3D (eighty or ninety percent length?)
Industrial Design - Transportation Photos by kach22i | Photobucket

[kach22i]

I was doing a little research on Cd, scale, models and Reynolds numbers and found this paper.

This is curious, the "Estate Back" (think of a Chrysler 300 wagon) has a higher Cd than the "Fastback".

http://delphi.com/pdf/techpapers/2012-01-0168.pdf

Based on doing many many template overlays and just eyeballing it, I'd have to say the Estate Back fits the template, and the fastback falls short.

What's going on here?

[aerohead]

Originally Posted by kach22i

With all due respect Charlie I have stated the full sized template works as a stand alone entity, but is not the tool needed nor promoted in this forum by it's developers and their following.

I have stated many times in many ways that scaling down the template is scaling down success, and I suspect that there is a mathematical factor which can be applied to this.

I have suggested that a 3D boattail template/tool be develop to retrofit to existing cars/trucks and made suggestion as to what it may look like in cross section and so forth, therefore I have done much more that criticize - read up.

I admit that I need to learn more on the scaling of models (not doing any model testing at this time), however very few things posted in the forum lend a clue. The comment that a 1/6th scale model would need to be doing 120 mph means what exactly? An intriguing comment, one which I appreciate but no context or comparisons to add value or pattern. 120 mph verses what other speed of the full scale body, 20 or 250 mph? No clue, sorry if I missed it. Is the air going to detach on the scale model before the full scale model? Now that would be useful information and make some sense. Crickets.

I understand that one cannot fit a square peg into a half circle hole, which puts me one up on you Charlie - your emotions on this topic have blinded you to the painfully obvious.

The drag coefficient will not stabilize until the body reaches a critical Reynolds number which forces transition to the turbulent boundary layer.
Reynolds number is a function of body length and velocity with road vehicles.
*For a full-scale vehicle the critical Rn will occur at 20 mph.
*For a 1/2-scale 40 mph.
*1/4-scale 80 mph
*1/5-scale 100 mph
*1/6-scale 120 mph.
Aerodynamicists use the term verisimilitude to describe the necessary relationship among scale-models with which to produce an Rn which will produce meaningful data.
freebeard just posted a link to Suzuka's wind tunnel work which covers this phenomena.

[Sven7]

Originally Posted by kach22i

You sure have a big head about yourself Charlie, living in an alternate universe must be hard on you.

Now back to something more productive, pages 24-30-ish have several post by ERTW which is the missing link several people remembered but could not locate.

More than one person expressed misgivings about the template's application and usefulness other than in the abstract. I am not alone on that one.

You're like a kid who complains about what his mom cooks for supper, but when asked what he would like, replies, "I don't know- something good."

If the template doesn't satisfy your impeccable aerodynamic tastes, please do provide free samples of your new recipe. Buy a 1/18 scale model and some clay if you need to. But don't forget the little umbrellas.

[kach22i]

Originally Posted by aerohead

The drag coefficient will not stabilize until the body reaches a critical Reynolds number which forces transition to the turbulent boundary layer.
Reynolds number is a function of body length and velocity with road vehicles.
*For a full-scale vehicle the critical Rn will occur at 20 mph.
*For a 1/2-scale 40 mph.
*1/4-scale 80 mph
*1/5-scale 100 mph
*1/6-scale 120 mph.
Aerodynamicists use the term verisimilitude to describe the necessary relationship among scale-models with which to produce an Rn which will produce meaningful data.
freebeard just posted a link to Suzuka's wind tunnel work which covers this phenomena.

Great information aerohead, thank you for your post.

Here is the link and quote to freebeard's post in the Air-Tabs thread:
http://ecomodder.com/forum/showthrea...r-25687-4.html

Originally Posted by freebeard

Are you familiar with the work of Yoshi Suzuka?

I suspect the SAE paper I posted (#2012-01-0168) will cause me to abandon the Part-C template all together.

I've only skimmed it, but so far it looks like the smaller the aft area of the car (as long as you have flow/attachment) is more important than maintaining the template arc at full scale.

This makes sense, but like I said I have to read the paper in full and do some test overlays to confirm.

[ChazInMT]

Originally Posted by kach22i

I was doing a little research on Cd, scale, models and Reynolds numbers and found this paper.

This is curious, the "Estate Back" (think of a Chrysler 300 wagon) has a higher Cd than the "Fastback".

http://delphi.com/pdf/techpapers/2012-01-0168.pdf

Based on doing many many template overlays and just eyeballing it, I'd have to say the Estate Back fits the template, and the fastback falls short.

What's going on here?

Good paper. The thing that caught my eye was this chart of data.



Could someone please explain it? I don't understand what the Cp is so needless to say it is difficult to know what the lines stand for. At first I thought it was simply the pressure, but the data does not add up if this is the case cause I don't see how the pressure could be higher at the rear glass area of a notch back than an estate. So the Cp must be some other derived number, or something.

[Xist]

Originally Posted by ChazInMT

Yeah.....no. The third thing is in the paper itself, but still confusing. I do think thought that somehow, the pressure is greater at mid rear glass but it may be due to dynamic pressure as opposed to static. Hmmmm, puzzling fer sure.

At the end of the paper, under "NOMENCLATURE," it says:

cp
Pressure coefficient

.

I am going to delete my previous post, since it was useless.

[redneck]

ChazInMT

Could someone please explain it? I don't understand what the Cp is so needless to say it is difficult to know what the lines stand for. At first I thought it was simply the pressure, but the data does not add up if this is the case cause I don't see how the pressure could be higher at the rear glass area of a notch back than an estate. So the Cp must be some other derived number, or something.

This may be what your looking for.

http://www.aerostudents.com/files/in...efficients.pdf


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