Scaling the Template

[kach22i]

I did some template overlays to study the shapes, the estate/wagon shape is close to but slightly higher than the Part-C template. Meaning slightly more adhesion at the roof than we need. It also results in a higher rear wake area resulting in the marked higher Cd number.

Industrial Design - Transportation Photos by kach22i | Photobucket


Scaling the template as silly and as improbable to succeed as it at first may appear, does a good job at predicting attachment (see Audi TT wind tunnel images posted earlier).

Industrial Design - Transportation Photos by kach22i | Photobucket


From the SAE paper we see the Fastback has the best numbers. My take on this is that it uses the best combination of least amount of rear area (wake), and largest template shape.



The template path to a Kammback works (history supports this), but if you can maintain attachment plus get the smallest rear area possible to reduce your Cd, then you are on the right track.

This is all conjecture as I've done no scientific testing, but it's conjecture based on old school observation. Learning to observe (without bias) is the basis for science and art alike.

This same sort of methodology started the Renaissance Era and gave us guys like Leonardo da Vinci. If any of us can be a fraction of as a good observer as he, then we are having a pretty good day.

[freebeard]

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."

"The food here is terrible."

"I know, and such small portions!"


Frontal area trumps Cd, right?

[kach22i]

Originally Posted by freebeard

Frontal area trumps Cd, right?

That's the thing we need to remind ourselves of. For instance if some car has an average Cd number but is lower and narrower than most it's CdA or actual drag may still be well below average.

Going to the rear area of the car we may have a similar situation, that being the path there may be great, but still leaves a big hole compared to a pseudo-Kammback of lesser rear area.

There must be a chart involving rear area and Cd as well as one with frontal area and Cd. Getting the best balance of the two should net the best results over just focusing on one or the other end of the car. Just an idea, think about it.

EDIT: I'm trying to put in context the meaning of this passage..........any takers?

Aerodynamics

It is, in particular, important to know whether the results stated were obtained with a full-scale car, rather than with a 1/5 scale model (whose aerodynamic results are generally about 20% better), but also whether the car was a real one, and loaded.

On this one, I think they mean "longer body", not larger.

In 1980, with the introduction of the GSA, Citroën were building the most aerodynamic 5 door saloon on the market. Audi matched the 0,31 Cx of this car with the much larger 4 door Audi 100 - it is much easier to achieve good aerodynamic results with a large vehicle than with a small one.

If CxS = CdA (think this might be a translated article) then this the first time read of a rear wing increasing mpg.

The spoiler improves the CxS by 2.7 %, the aerofoil by 7.5 %. Both together improve the CxS by 10 % and reduce petrol consumption by 7.5 % at 120 km/h (74.6 mph).

This article originally appeared in Double Chevron #59 © Automobiles Citroën 1980

[freebeard]

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

Dodge Magnum?

rather than with a 1/5 scale model (whose aerodynamic results are generally about 20% better), but also whether the car was a real one, and loaded...it is much easier to achieve good aerodynamic results with a large vehicle than with a small one.

..........any takers?

It's just a poorly written article? But it is possible to use more relaxed curves on a larger vehicle and elements like doors and headlights don't scale.

The spoiler improves the CxS by 2.7 %, the aerofoil by 7.5 %. Both together improve the CxS by 10 % and reduce petrol consumption by 7.5 % at 120 km/h (74.6 mph).

If they re using the spoiler and aerofoil together, what the odds the aerofoil is at the front, e.g., air dam/partial bellypan? "improve the CxS by 10 % and reduce petrol consumption by 7.5 %" might mean high rolling resistance. Or the Cd%/2=mpg% rule of thumb may assume a lower speed. 60mph vs 74mph might account for it.

[kach22i]

It's funny when things come to you, I just started cleaning my cast iron pan for lunch and I guess found this relaxing enough to have an "Aha!" moment.

I'd like to thank everyone for their participation in this thread, without some of the push and shove I may not have been challenged enough to have this little epiphany.

Industrial Design - Transportation Photos by kach22i | Photobucket


A closer look at the sketches.


Feel free to ask questions, I'm now the one in the position of defending a theory (please be gentle).

Before this all I really had was the right questions, and the mistakes I made only helped me learn more and lead to the final outcome.

I'm just about positive there is nothing new here, just new to me. In fact I'd bet my lunch money that some guy 100 years ago did all the calculations and tests on this.

The mathematical equations and so forth may all be out there in some old text book. However in terms I can relate to, and the twist of perspective I've lent to this issue may help some of us want-a-be aerodynamicist, or at least I can only hope.

Click on the images to make them larger and more readable.

[freebeard]

Two different threads that are the same is making my head hurt.

[Xist]

Good point, but at least Blowncopcar does not have no-longer-welcome guests arguing in his living room.

What can we do about it?

[freebeard]

Do what I do, reply here instead of there?

Edit: Example—Blows. My. Mind.

The Secret Life of Vortices: Evolution and Destruction of Vortical Flows

2nd Edit: Hold the phone, stop the presses! This one only had 2 views when I got to it. I can't view videos in the forum, but I understand they don't Autoplay, so... break out your 3D glasses!

The Secret Life Of Vortices - Mono And Stereo

[kach22i]

Originally Posted by freebeard

Two different threads that are the same is making my head hurt.

Sorry about that, I have not figured out a solution.

No time to talk today, did some quick sketches and a composite image sheet this morning, enjoy.

Industrial Design - Transportation Photos by kach22i | Photobucket

[some_other_dave]

In the last picture, the vortexes (vortices?) are due primarily to the sharp corner between the vertical sides and the flat top. Round the corners, and the vortexes get smaller. Round them more, they get even smaller. Make the front profile a half-circle, and the only ones left are down at the bottom of the car--and they are much smaller.

There are lots of interactions when it comes to aero. My aero engineering friends tell me that fluid dynamics is the most empirical (e.g., "fiddle with equations until the numbers seem to come close to what we measure") and least intuitive of all of the engineering disciplines.

For example: In the aircraft world, significant reductions in drag have been found simply by adding a fillet between the wing and fuselage of the aircraft. (That's a transitional radius between the two shapes, not part of a fish. ) The interactions between air flowing over the wing and air flowing along the fuselage created significant drag, and adding a piece to make a small curve instead of a (nearly) right angle reduced that to very little.

-soD