How To Line Up Template - Sports Car Design

[winkosmosis]

Originally Posted by euromodder

What makes you think 0.28 is good ?
Shorter cars like the Audi A2 have done better despite their poorer L/D ratio.

The 2012 Mercedes B-class is 0.24 (with eco package, 0.26 without) despite it being rather short as well.

The Schlörwagen - which is more or less the template in 3 dimensions - only has about half that Cd.

But the 911 is disadvantaged by huge wheel wells and big spokey wheels

[Patrick]

Originally Posted by kach22i

3. Targa roof on - see below......................is this any better than the first design on page one? What does that roll bar step do to the aerodynamics?

Industrial Design pictures by kach22i - Photobucket

No. You will have separation at the rollbar and therefore increased drag.

[autogyro]

The angle at which air flow starts to leave is important, and that is as claimed; 27 degrees. However, the mindset of a designer says this is a 15 degree fastback, not a 27 degree fastback.

Here's another flow video, with the same "average" inclination,but a flat slope of 18.4 deg. note that the flow just barely starts to become detached. The lesson here seems to be that the air likes convex, or positive curvature, shapes. and is less happy with flat ones.

http://www.youtube.com/watch?v=lKHURePuSZo&NR=1

Of course this is only in 2D But it does reveal how dynamic the whole thing is.

[Patrick]

It doesn't look flat. Is that the right video?

[autogyro]

Oops! My Bad. Heres the right one

http://www.youtube.com/watch?v=rt3bD4_VWdQ

[ChazInMT]

@ Winkosmosis, think all the intuitive ideas you want, stay passionate about this, keep thinking deeply and trying to understand things. This is a good thing. Don't think for a second you have it all figured out exactly. Eventually, it will dawn on you the concepts which Aerohead is trying to convey, and your line of thinking will converge, and you'll be all like. "Oh." Least thats how it was for me.

One key point to bear in mind, the template modeled cars are the optimum designs for the lowest Cd. Don't confuse attached flow with lowest Cd. I, like you, am not convinced that the template is the only path to attached flow, I do however now firmly believe the template is the path to lowest Cd.

[kach22i]

Autogryo, I trust you don't mind my marking up some of your images so that I can explain the whole tangent line thing as I see it.

Your first one:
Automobile pictures by kach22i - Photobucket


Your second one:


Here is my car at 30 degrees - approx.


Some other P-Cars:


An old racing Porsche modified with roof line per race rule allowances.


I stand by my earlier statement that 10-15 degrees is best, you can get away with 20 degrees on a good day, and that 30 degrees has air lifting off of it quite easily at speed. I also think the template works and is an "Idea" we should strive for but maybe not marry.

[autogyro]

For me, the take away is, whatever the tangent, positive curvature is a good thing on the trailing end. The air likes to stay attached.
BTW: I used to own a 912: two of them in fact.

[kach22i]

Originally Posted by autogyro

BTW: I used to own a 912: two of them in fact.

The old long hoods?

The pre-1969 short wheelbase cars?

I bet they were a hoot to drive back in the day before giant SUV's roamed the roads.

[winkosmosis]

Originally Posted by ChazInMT

@ Winkosmosis, think all the intuitive ideas you want, stay passionate about this, keep thinking deeply and trying to understand things. This is a good thing. Don't think for a second you have it all figured out exactly. Eventually, it will dawn on you the concepts which Aerohead is trying to convey, and your line of thinking will converge, and you'll be all like. "Oh." Least thats how it was for me.

One key point to bear in mind, the template modeled cars are the optimum designs for the lowest Cd. Don't confuse attached flow with lowest Cd. I, like you, am not convinced that the template is the only path to attached flow, I do however now firmly believe the template is the path to lowest Cd.

But my point is that the template is meaningless in this context. People are overlaying it arbitrarily on their cars and then thinking that where the lines fall happen to describe some kind of ideal shape. The shape of the template is only ideal for the whole shape described by it.

It's completely backward reasoning.

You're right, it's not just attached flow that matters. It's attached flow that allows the car to leave as small and clean a hole as possible in the atmosphere once air is done flowing around it.

You can do better with common sense. Start with a horizontal surface (most roofs), increase the curve gradually, and don't exceed 20°, or 15°, or 10°, or whatever (depending on airspeed). Trying to align a template arbitrarily with a whole front end that has zero relevance to a normal car is a waste of time and energy.