Aerodynamic Streamlining Template: Part-C

[freebeard]

The Dymaxions weathervaned instead of veering downwind. Just no self-centering in the steering.

[aerohead]

Answering a question from long ago:
*the removable tail constituted 39.4% of the aft-body.
*it's length was 29.25-inches(742.95mm)
*The total aft-body is 76.1% of total length,or 74.198-inches (1884.629mm)
*Total body length 97.5-inches(2476.5mm)

[Cd]

Hello all, I didn't want to ask this question in a new thread, since the subject has been discussed to absolute death.
This appears to be the latest version on the Template, so I thought this might be the most up to date conversation.

Question : what is the maximum angle of curvature that you can still have attached flow on a roofline ?
What's that you say ? This has been answered a million times over ?
Well yes, I know this, but I plan to begin skinning my Kammback / boattail tomorrow afternoon, and have limited time to read the hundreds of replies for an answer.
Why I ask : We have discussed anomalies, such as the EV-1 and Pillbug design that fall out of the curve of the template, but here recently, I have begun to see more super slippery designs that ignore the template as well, such as the Mercedes EQ-S, the Lucid Air, the Model 3 and model X.
I must admit I favor the LOOK of a "faster" roofline that is closer to the template, but I am also looking to make my wake as small as possible.
( The smaller the wake, the lower the drag - correct ? )
I could just make a design and continue to lower it farther and farther off of the template curve till I get turbulent flow, but I have a very limited amount of Coroplast that I am using and would like to get it right the first time.
One thing I notice for certain, is that in each of these low drag designs that fall out of the template, there is a very gradual curve to the roofline, and then a quick wick downwards out of the curve.
It's just a guess, but I'm thinking the airflow follows the curve of the front portion of the roofline that IS in the template curve, and just continues onwards down the curve that is out of template, as if it had fallen off of a Kammback ( the roofline that IS in the curve )
On a hatchback car that has a roofline that has a very minor curve, I'm thinking that too steep a curve would result in detached flow, since the airflow is following the angle of the roofline more that the steep Kammback you might add to the rear.
What's your ideas on this ?
If I create a tail that is as steep as a Model X's curve, I can have a super small wake area.
( The height of my license plate ! ) rather than one that is 4-5 times that size if it follows the template curve.
- Thanks.

[Bicycle Bob]

Originally Posted by Cd

Hello all, I didn't want to ask this question in a new thread, since the subject has been discussed to absolute death.
This appears to be the latest version on the Template, so I thought this might be the most up to date conversation.
- Thanks.

The Template is just a lab curiosity. If you only want to learn one thing about aero, it is a good place to start, but from there on, everything is variations and exceptions to make a practical compromise.

[freebeard]

Question : what is the maximum angle of curvature that you can still have attached flow on a roofline ?

Grinds my gears. Twists my knickers.

en.wikipedia.org/wiki/Degree_of_curvature

An angle requires an arc length to define a curve.

As an example, a curve with an arc length of 600 units that has an overall sweep of 6 degrees is a 1-degree curve: For every 100 feet of arc, the bearing changes by 1 degree. The radius of such a curve is 5729.57795. If the chord definition is used, each 100-unit chord length will sweep 1 degree with a radius of 5729.651 units, and the chord of the whole curve will be slightly shorter than 600 units.

Your looking for the sweep, not the angle.

A progressive curve can be defined by two points with contol handles. -- Bexier curve.

https://en.wikipedia.org › wiki › Bézier_curve
Bézier curve - Wikipedia
A Bézier curve is a parametric curve used in computer graphics and related fields. A set of discrete "control points" defines a smooth, continuous curve by means of a formula. Usually the curve is intended to approximate a real-world shape that otherwise has no mathematical representation or whose representation is unknown or too complicated. Bézier curve is named after French engineer Pierre Bézier, who used it in the 1960s for designing curves for the bodywork of Renault cars.

Defined in 1960. Who knew?

[Cd]

Ok Freebeard. I learned something new.
I understand having to correct someone when your gears get ground.
For me, it's when people call their truck a 'car'.
Or when people write "That was so funny that I can't breath ! "
This last one I see far too often.

I think that my answer to the most efficient angle for the tail is

42

Faw - Tay - Teeew ??

[aerohead]

Originally Posted by Cd

Hello all, I didn't want to ask this question in a new thread, since the subject has been discussed to absolute death.
This appears to be the latest version on the Template, so I thought this might be the most up to date conversation.

Question : what is the maximum angle of curvature that you can still have attached flow on a roofline ?
What's that you say ? This has been answered a million times over ?
Well yes, I know this, but I plan to begin skinning my Kammback / boattail tomorrow afternoon, and have limited time to read the hundreds of replies for an answer.
Why I ask : We have discussed anomalies, such as the EV-1 and Pillbug design that fall out of the curve of the template, but here recently, I have begun to see more super slippery designs that ignore the template as well, such as the Mercedes EQ-S, the Lucid Air, the Model 3 and model X.
I must admit I favor the LOOK of a "faster" roofline that is closer to the template, but I am also looking to make my wake as small as possible.
( The smaller the wake, the lower the drag - correct ? )
I could just make a design and continue to lower it farther and farther off of the template curve till I get turbulent flow, but I have a very limited amount of Coroplast that I am using and would like to get it right the first time.
One thing I notice for certain, is that in each of these low drag designs that fall out of the template, there is a very gradual curve to the roofline, and then a quick wick downwards out of the curve.
It's just a guess, but I'm thinking the airflow follows the curve of the front portion of the roofline that IS in the template curve, and just continues onwards down the curve that is out of template, as if it had fallen off of a Kammback ( the roofline that IS in the curve )
On a hatchback car that has a roofline that has a very minor curve, I'm thinking that too steep a curve would result in detached flow, since the airflow is following the angle of the roofline more that the steep Kammback you might add to the rear.
What's your ideas on this ?
If I create a tail that is as steep as a Model X's curve, I can have a super small wake area.
( The height of my license plate ! ) rather than one that is 4-5 times that size if it follows the template curve.
- Thanks.

1) The EV1, @ Cd 0,197, achieved Cd 0.137 by elongating the tail out to the AST.
2) The Schlorwagen is a bit better than the Renault Vesta-II, and VW XL1 of similar drag.
3) The M-B EQS is less efficient than the Tesla Model S Plaid, Lucid Air, CNR-PF, and modified Audi A2, by Coventry University.
4) The Lucid Air is less efficient than the CNR-PF, EV1, VW L1, and Coventry Audi A2.
5) The Tesla Model 3 is less efficient than the Alfa Romeo Giulia Advanced Efficiency, and Tesla Model Y.
6) The Tesla Model X, with 3-position rear deck spoiler, is a AST form, and has no peer for it's fineness ratio.

[aerohead]

I'm just moving this thread to page-one so Vman455 can more easily access it.

[wax87]

I am new to all of this, Is the optimum curvature a function of the speed that you want to travel at or is it pretty universal for automobiles?

[aerohead]

Originally Posted by wax87

I am new to all of this, Is the optimum curvature a function of the speed that you want to travel at or is it pretty universal for automobiles?

For road vehicles, and between 20-mph, and 250-mph, and because of the relationship between their length and speed, the coefficient of aerodynamic drag, Cd [ based on the vehicles frontal projected area ( the size of the imaginary hole it burrows though the air, as measured by the shadow it casts from a coherent light source from ahead, projected onto a screen behind it )] remains constant.
A long way of saying that, no, it doesn't change, regardless of shape.
Technically, it has satisfied it's 'critical Reynolds number' criteria for constant Cd ( because of a turbulent boundary-layer, and because the air will remain incompressible throughout this velocity range ).
The 'optimum' curvature ( contour / silhouette ) IS the major defining factor of the Cd. It has to do with boundary-layer theory and effective fineness ratio.