Kammback Angle Okay?

[kach22i]

Originally Posted by ChazInMT

If you dive down to 22° when you should be at only 8°. it will not be a good thing.

To clarify even further, of course the transition should be gradual.

Taking the images posted and estimating with an adjustable triangle on my computer screen, we can see that the angles of inclination do increase.

Automobile 2 - Odds And Ends Photos by kach22i | Photobucket


I think there is a matter of scaling to consider which we should not leave out.

Say for instance we mount a 1/6 scale model of the car to the roof of the full sized car. The Cd of the model should be close to that of the full sized car without the roof attachment, right?

The frontal areas of the full sized car and model will be dramatically different, but the Cd's should be similar.

Now for argument's sake, graph on a "known to work" roof transition without regard to scale.

Automobile 2 - Odds And Ends Photos by kach22i | Photobucket


My hypothesis here is that when thinking about "scale", be it for a more aerodynamic mirror, roof luggage, trailer or an entire car body, we may have far for liberties at hand than normally accepted following the template at the standard scale.

What the Geo Metro and Pontiac Firefly roof garnishments prove to my eyes, is that re-scaling principals are already being applied, although nobody is calling them that just yet.

[ChazInMT]

Originally Posted by kach22i

Say for instance we mount a 1/6 scale model of the car to the roof of the full sized car. The Cd of the model should be close to that of the full sized car without the roof attachment, right?

My hypothesis here is that when thinking about "scale", be it for a more aerodynamic mirror, roof luggage, trailer or an entire car body, we may have far for liberties at hand than normally accepted following the template at the standard scale.

Yeah...No.

I thought the same thing once.

At the end of the day, we're not running scale models through the air, we're running full size cars. The Template is designed to be under the wheels and at the top of the highest point of the car, because that is what the gross volume of air is being affected by. Garnishments will obviously affect the overall shape and thus the Cd, but we cannot shrink the template onto small features and do elemental analysis on them to see if they'll optimize the entire shape.

I had a hard time with this myself a few years back and really thought as you do, and when I came to the realization that I was wrong, it sort of set a good fundamental understanding of aerodynamics for me which has not changed since. This whole conversation took place HERE.

Of course it is entirely possible that I just misconstrued what you were trying to say, and if so I apologize. These are good talking points worthy of hashing out for sure if we are to come to a good understanding of how to optimize the shapes of our vehicles to reduce drag.

Oh, and the Reynolds Number, which I'll admit is a bit of a mystery to me in detail, but the basics of it state that: A 1/6th scale model may indeed have a very different Cd than the full sized version. That's my take away, maybe someday I'll edumacate muhself on the vagaries of it, but for now, I'll just understand it to know that scale definitely changes things.

[kach22i]

Originally Posted by ChazInMT

This whole conversation took place HERE.

Interesting, thank you for providing the link.

Originally Posted by ChazInMT

Attached flow is NOT the goal, it is a characteristic that must be present in order to optimize the Cd. The goal is to eliminate drastic pressure changes, allowing the air to build in behind a vehicle as gently as possible creating as little turbulence as possible.

An important goal indeed, but in this situation he (the original poster) wanted the flow to stay attached. Maybe his goals should be re-examined.

Originally Posted by ChazInMT

A 1/6th scale model may indeed have a very different Cd than the full sized version.

I will look into this.

Below is an example of what I had in mind, not saying it is the best thing to do to obtain an aero-design. Just saying if adding on, maybe scaling down the template is better than nothing at all.

18 Gurney Bubble $35
Cushman Competition


Panoramio - Photo of Gurney bubble on a GT40


Reproduction GT40 for sale in California - SB100 *SOLD* - GT40s.com

[ChazInMT]

Originally Posted by kach22i

An important goal indeed, but in this situation he (the original poster) wanted the flow to stay attached. Maybe his goals should be re-examined.

You misunderstood me I think. I am saying that attached flow will always be required for the ideal aerodynamic car, but, It is a consequence of good design, it is not the main goal. Everyone gets all hairbrained about attached flow like that is the only thing we need to accomplish in order to achieve aerodynamic nirvana, but it is not. Of course he wants attached flow, he probably has it now, but just because the flow is attached does not mean it is optimized. You need to carefully consider how fast the air is being asked to close in behind the shape, if it is too fast, it creates low pressure. If this low pressure is in close proximity to an area of high pressure, the air will move up from the high to the low and if strong enough a difference, will set a vortex in motion which will be a real drag.

And Yes, If you have something sticking up on a car, a mini template is the best way to mitigate the damage done. I always look at AC units on RV roofs and think "That should be mini-templated". But, when you look at the major structures of cars, like the trailing edge of the roof, you can't just start with a mini-template there and expect it to be optimal, it probably wouldn't hurt, but it won't minimize the drag for the space that it takes up.

Again, I'm not saying here that the existing Kamm is a disaster, just that it is not optimal. It is giving back 60-70% of the potential gain for a structure of it's size and type when it could be doing 100%

[kach22i]

Originally Posted by ChazInMT

You need to carefully consider how fast the air is being asked to close in behind the shape, if it is too fast, it creates low pressure. If this low pressure is in close proximity to an area of high pressure, the air will move up from the high to the low and if strong enough a difference, will set a vortex in motion which will be a real drag.

This is how my roof wing (see link in signature) works.

I had not really considered that if drawing too much air downward via pressure differences that a large drag causing vortex could be formed.

To tell the truth, it is difficult to see large trailing vortexes in wind tunnel smoke streams.

And CFD models mostly look like irrelevant swirls of psychedelic colors to me.

The CFD models based on pressure (verses flow) are even more ambiguous to my eyes.

Essentially everything aft of the car just looks like a mess to me, but I am slowly learning the differences.

[Sven7]

Just, whatever you do, please refrain from scaling the template like this. It will just confuse others and help to concrete bad habits in others. If you're unsure how to use it, re-read this thread.

Not trying to get all down on you or anything. I just want to make sure that the info we're posting is going to be useful to others who may happen upon it.

[Xist]

Originally Posted by kach22i

18 Gurney Bubble $35

How much for a Hammond bubble?

Sven7, who made that image? Did you properly discipline them?

[kach22i]

Originally Posted by Sven7

Not trying to get all down on you or anything. I just want to make sure that the info we're posting is going to be useful to others who may happen upon it.

When you say useful, you mean following conventional wisdom, right?

As pointed out in this thread there are several Japanese hybrids which truncate the arc over the roof at a greater angle than "our template". The reason for this is still being debated, and that debate is healthy in my opinion.

Back when we talked about altering the roof of a +2005 Mustang to fit the template the idea of re-scaling the template for the canopy somehow gained more acceptance than in this thread.

My goal in examining the re-scaling of the template is to discover the exceptions to the rules, not to redefine the rules or aero template.

There is something to be learned here, I just haven't quite figured out what yet.

The Gurney Bubble, wheel blisters and so forth are part of a micro aerodynamic package inside/outside of the overall general aero template.

How large is "micro", and may it include an entire passenger cabin canopy?

Still learning the answer to that one.

[Sven7]

Those interested in Geo Metro Kammback advice can skip this post.

Originally Posted by kach22i

When you say useful, you mean following conventional wisdom, right?

As pointed out in this thread there are several Japanese hybrids which truncate the arc over the roof at a greater angle than "our template". The reason for this is still being debated, and that debate is healthy in my opinion.

Back when we talked about altering the roof of a +2005 Mustang to fit the template the idea of re-scaling the template for the canopy somehow gained more acceptance than in this thread.

I take it Conventional wisdom would be the culmination of decades of aerodynamic research by seasoned professionals? Then yes, I would suggest sticking to conventional wisdom.

Yes, we do know each car is designed and modified on a case-by-case basis, HOWEVER, major changes such as the ideal angle of the rear glass are taken on by seasoned professionals in full-size wind tunnels in huge design and engineering facilities. They are tested and re-tested. They are built from the ground up to be a cohesive form.

Adding a kammback to an existing car without wind tunnel testing is an entirely different beast and we need to play it safe, using tried-and-true methods and forms in order to make things work as well as possible. We need to build off of known values and "conventional wisdom" to be reasonably confident the things we build will work as intended.

Throwing your hat in the ring because you think maybe unconventional changes might work in the right circumstances is not productive in this thread. Don't post an unfounded opinion as some sort of "alternate" fact.

My goal in examining the re-scaling of the template is to discover the exceptions to the rules, not to redefine the rules or aero template.

There is something to be learned here, I just haven't quite figured out what yet.

Until you test these theories and exhibit some form of real-world expertise on them, please refrain from posting them in otherwise serious threads. Moving drawings around in Photoshop does nothing to advance aerodynamic theory and only serves to confuse those who don't know better.

If you haven't learned what you're trying to learn it might be good to learn it before trying to teach others.

We have the Unicorn Corral for spitballing about this kind of stuff, and I personally think that's where this "debate" should stay. Until it's proven.

Until then, we have no reason to believe that the template is not serving its intended purpose. We have no reason to discard it or modify it. Next time, if you're thinking about using the template improperly, just STOP and think about what you're doing.

Just follow WWJD (What Would Jaray Do). Imagine having to explain to Paul Jaray why you're doing what you do. Saying "it looks like it'll work" or "why not?" isn't good enough.

[Sven7]

Originally Posted by Xist

Sven7, who made that image? Did you properly discipline them?

Kach22i.

Seriously, we do airflow testing here at work and some stuff comes out the exact opposite of what you'd think. I obviously can't go into detail. Just be aware that in many aero circumstances, how a thing looks has little bearing on how it actually works. If you're trying something that deviates from conventional wisdom, you need to test it.