Fineness ratio vs drag coefficient

[aerohead]

According to Hucho, and with respect to vehicles that are developed for the lowest possible drag ( something which might be of interest at this forum ), fineness ratio is the single most important criteria.....
'Lower drag can only be achieved by extending the length of the vehicle's body.'
Hucho, 2nd-Ed., page 201.
After examining some commercially-available BEVs ( which lack conventional cooling systems, and enjoy smoother bellies), concepts, as well as converting some previously-tested, simplistic wind tunnel models, by adding Tesla-grade' side mirrors in order to simulate those bodies as BEVs, I compiled a list, which illustrates the correlation between fineness ratio and drag coefficient.
I used the Length/ Square-root of frontal area in all cases. This converts each vehicle to a square,rectangular bar section which can be assembled into a bar-graph as has been done by marine biologists to compare the hydrodynamic efficiency of, for instance, fast pelagic fish and marine mammals.
--------------------------------------------------------------------------------------
2006 i-MiEV.................................. 2.219827.................................. 0.35
2012 Chevy Spark......................... 2.510446.................................. 0.325
2017 Chevy Bolt............................ 2.705049.................................. 0.312
2012 Toyota RAV4.......................... 2.837229.................................. 0.30
2018 Jaguar I-Pace......................... 2.956080................................. 0.29
2018 Nissan LEAF........................... 2.908106.................................. 0.28
2019 Audi E-Tron (raked-roof).......... 2.98404................................... 0.28
2020 Porsche Taycan Turbo S........... 3.269224................................. 0.25
2020 Tesla Model S......................... 3.216863................................. 0.24
2014 VISIO.M................................ 2.745537................................. 0.24
2017 Tesla Model X......................... 2.76212................................... 0.24
2017 Tesla Model 3......................... 3.110366.................................. 0.23
2020 Tesla Model Y......................... 2.9182387................................. 0.23
2020 Lucid Air................................ No available data........................ 0.21
1941 FKFS K-5 (kurz-heck).............. 3.29665................................. 0.2166
1996 GM EV1................................ 3.1069501.................................0.197
1941 FKFS K-5 ( m-h).................... 3.69486.................................. 0.1813
2012 VW XL1................................ 3.011834................................ 0.1769
1992 GM Ultralite........................... 3.131837................................ 0.1733
1941 FKFS K-5 ( Lange-heck).......... 4.30126.................................. 0.1662
1993 GM 'Yellow-Ferret' LSR EV1...... 3.435861................................ 0.15
1981 VW 'Flow' body ( k-h )............. 3.346776................................ 0.16
1981 VW 'Flow' body ( lange-heck)... 4.432975................................. 0.15
--------------------------------------------------------------------------------------
In 1922 Paul Jaray demonstrated low drag via an elongated body form.
In 1933, Dr. Walter Lay established the relationship between drag and elongation.
In 1935, Dr. Kamm and Baron Fachsenfeld re-created Lay's research, re-establishing the link between drag and body elongation.
In 1978, Liebhold et al. demonstrated a drag reduction of the D-B,M-B C-111 III, from Cd 0.237, to Cd 0.178, with body elongation ( fineness ratio )
In 1981, Buchheim et al. demonstrated drag reduction of the VW 'Flow' body with elongation.
In 1991, the IT works Honda CRX-HF set a club record at USFRA World of Speed, and verified the body elongation as primary source of drag reduction, later backed up at the Chrysler Proving Grounds, East Chelsea,Michigan.
In 1993, General Motors Corporation established a land speed record, at the Bridgestone / Firestone Proving Grounds, Ft. Stockton, Texas; with their Impact/ EV1, in part through body elongation, achieving Cd 0.14, compared to Cd 0.197, for the standard production version.
In the last session at DARKO, the 'baby' template car demonstrated a drag reduction with elongation ( fineness ratio). ( Cd 0.151, to Cd 0.121 ).
In the same test session, Spirit of Ecomodder indicated from Cd 0.44, to as low as Cd 0.214, as a simulated BEV.
2015's Mercedes-Benz IAA demonstrated a drag reduction with elongation ( fineness ratio ).

[JulianEdgar]

A reminder: no current textbook on car aero even mentions 'fineness ratio', and even older textbooks give it only a very small space (1-2 pages in a whole book).

So take your pick:

Either those professional aerodynamicists, amongst the most respected and authoritative in the world, have forgotten to cover something of critical importance.....

...or Aerohead is embarking on one of his weird theories that has little or no substance.

You decide.

[aerohead]

Originally Posted by JulianEdgar

A reminder: no current textbook on car aero even mentions 'fineness ratio', and even older textbooks give it only a very small space (1-2 pages in a whole book).

So take your pick:

Either those professional aerodynamicists, amongst the most respected and authoritative in the world, have forgotten to cover something of critical importance.....

...or Aerohead is embarking on one of his weird theories that has little or no substance.

You decide.

Just for some insight.
In 1920s, General Motors dealerships were complaining to the corporation that shoppers were making noises about all cars beginning to look alike, with implications about the viability of new car sales. $$$$$$$$$$$$$$$$$$.
In 1926, General Motors embraced design-obsolescence as official corporate policy, whereas, all cars would have a different 'look' each year, every year.
It was like BAYER HEROIN to the buying public, and since GM was the largest automaker at the time, any company hoping to compete in the market would have to buy into the 'law of the Paris dressmaker', as Alfred P. Sloan expressed.
Ever since, and to this day, the exterior design of automobiles has been driven by 'styling', not technology.
In 1963, another real nutcase, by the name of Walter Korff, presented at the annual SAE Congress, advocating for real aerodynamics, as Alex Tremulis had advocated at the close of WW-II, and Carl Breer had going into WW-II.
H. Schmude ( sp?) of GM countered Korff with the counter-argument that, if all cars were designed 100% aerodynamic, that wouldn't they all look alike and the designer have nothing to do?
We're still at this impasse today.
Foreign competition and CAFE standards have forced incremental aerodynamic improvements over the ensuing decades, at least for the US market.
This is the context of Julian Edgar's experience that there's so little in the extant literature, which might direct attention towards 'template' automobiles that, if they did find public acceptance, would spell the end of styling and the end of the aerodynamicists who work under them.
You won't see Boeing or any other aerospace company roll out a new fuselage each year. Or a new nuclear submarine hull. Rectal suppositories. Or boutique, designer ammunition.

[JulianEdgar]

Originally Posted by aerohead

Just for some insight.
In 1920s, General Motors dealerships were complaining to the corporation that shoppers were making noises about all cars beginning to look alike, with implications about the viability of new car sales. $$$$$$$$$$$$$$$$$$.
In 1926, General Motors embraced design-obsolescence as official corporate policy, whereas, all cars would have a different 'look' each year, every year.
It was like BAYER HEROIN to the buying public, and since GM was the largest automaker at the time, any company hoping to compete in the market would have to buy into the 'law of the Paris dressmaker', as Alfred P. Sloan expressed.
Ever since, and to this day, the exterior design of automobiles has been driven by 'styling', not technology.
In 1963, another real nutcase, by the name of Walter Korff, presented at the annual SAE Congress, advocating for real aerodynamics, as Alex Tremulis had advocated at the close of WW-II, and Carl Breer had going into WW-II.
H. Schmude ( sp?) of GM countered Korff with the counter-argument that, if all cars were designed 100% aerodynamic, that wouldn't they all look alike and the designer have nothing to do?
We're still at this impasse today.
Foreign competition and CAFE standards have forced incremental aerodynamic improvements over the ensuing decades, at least for the US market.
This is the context of Julian Edgar's experience that there's so little in the extant literature, which might direct attention towards 'template' automobiles that, if they did find public acceptance, would spell the end of styling and the end of the aerodynamicists who work under them.
You won't see Boeing or any other aerospace company roll out a new fuselage each year. Or a new nuclear submarine hull. Rectal suppositories. Or boutique, designer ammunition.

I suppose someone who believes in some pretty odd car aerodynamic theories was bound to also be a believer in a conspiracy theory about a lack of aerodynamic progress.

[IRONICK]

Aerohead, someone has to stop and it's not you.

[Vman455]

Originally Posted by aerohead

Just for some insight.
In 1920s, General Motors dealerships were complaining to the corporation that shoppers were making noises about all cars beginning to look alike, with implications about the viability of new car sales. $$$$$$$$$$$$$$$$$$.
In 1926, General Motors embraced design-obsolescence as official corporate policy, whereas, all cars would have a different 'look' each year, every year.
It was like BAYER HEROIN to the buying public, and since GM was the largest automaker at the time, any company hoping to compete in the market would have to buy into the 'law of the Paris dressmaker', as Alfred P. Sloan expressed.
Ever since, and to this day, the exterior design of automobiles has been driven by 'styling', not technology.
In 1963, another real nutcase, by the name of Walter Korff, presented at the annual SAE Congress, advocating for real aerodynamics, as Alex Tremulis had advocated at the close of WW-II, and Carl Breer had going into WW-II.
H. Schmude ( sp?) of GM countered Korff with the counter-argument that, if all cars were designed 100% aerodynamic, that wouldn't they all look alike and the designer have nothing to do?
We're still at this impasse today.
Foreign competition and CAFE standards have forced incremental aerodynamic improvements over the ensuing decades, at least for the US market.
This is the context of Julian Edgar's experience that there's so little in the extant literature, which might direct attention towards 'template' automobiles that, if they did find public acceptance, would spell the end of styling and the end of the aerodynamicists who work under them.
You won't see Boeing or any other aerospace company roll out a new fuselage each year. Or a new nuclear submarine hull. Rectal suppositories. Or boutique, designer ammunition.

Reality is, as reality is so often wont to be, much more complex than you suggest. As Hucho himself has written, "Low air drag is one target. Many others like styling, safety, cost, etc. have to be observed" (emphasis original).

Boeing does not use a direct-to-consumer business model, nor does the US Navy; they certainly work under similar cost and safety constraints, and when those parameters fail to hold similar influence in the product's design it can result in spectacular debacles, as the 737 MAX has demonstrated.

I'm not sure where you get the idea that cars are still redesigned every year; this has not been the case since the 1960s.

I assume you read the Mercedes S-class press release on aerodynamics that JulianEdgar posted (since you commented on the thread), but it sounds like you missed this part:

The three-dimensional airflow pattern around the vehicle was already calculated in high-performance simulation clusters using CFD (Computational Fluid Dynamics) in an early development phase. Shortly after this project started, during the dimensional drawing phase, several extensive DOE (Design of Experiments) studies were carried out on the basis of the preceding model, with up to 250 calculations per body area. In this process the aerodynamic engineers specify the parameters for certain components, e.g. the possible height of the boot lid.

Several hundred simulations were carried out over several days, fully covering the scope of the prescribed parameters. These simulations can be used to calculate a global or local optimum or, far more importantly in this phase, establish the influence of the individual parameters on the drag coefficient. Using the DOE method, specific aerodynamic requirements were reported to and discussed with personnel working on the dimensional concept and design in a very early phase.

(emphasis added)
That doesn't sound to me like an "impasse" between style and technology; rather, cooperation.

Originally Posted by IRONICK

Aerohead, someone has to stop and it's not you.

I very much disagree.

[Tahoe_Hybrid]

Originally Posted by aerohead

According to Hucho, and with respect to vehicles that are developed for the lowest possible drag ( something which might be of interest at this forum ), fineness ratio is the single most important criteria.....
'Lower drag can only be achieved by extending the length of the vehicle's body.'
Hucho, 2nd-Ed., page 201.
After examining some commercially-available BEVs ( which lack conventional cooling systems, and enjoy smoother bellies), concepts, as well as converting some previously-tested, simplistic wind tunnel models, by adding Tesla-grade' side mirrors in order to simulate those bodies as BEVs, I compiled a list, which illustrates the correlation between fineness ratio and drag coefficient.
I used the Length/ Square-root of frontal area in all cases. This converts each vehicle to a square,rectangular bar section which can be assembled into a bar-graph as has been done by marine biologists to compare the hydrodynamic efficiency of, for instance, fast pelagic fish and marine mammals.
--------------------------------------------------------------------------------------
2006 i-MiEV.................................. 2.219827.................................. 0.35
2012 Chevy Spark......................... 2.510446.................................. 0.325
2017 Chevy Bolt............................ 2.705049.................................. 0.312
2012 Toyota RAV4.......................... 2.837229.................................. 0.30
2018 Jaguar I-Pace......................... 2.956080................................. 0.29
2018 Nissan LEAF........................... 2.908106.................................. 0.28
2019 Audi E-Tron (raked-roof).......... 2.98404................................... 0.28
2020 Porsche Taycan Turbo S........... 3.269224................................. 0.25
2020 Tesla Model S......................... 3.216863................................. 0.24
2014 VISIO.M................................ 2.745537................................. 0.24
2017 Tesla Model X......................... 2.76212................................... 0.24
2017 Tesla Model 3......................... 3.110366.................................. 0.23
2020 Tesla Model Y......................... 2.9182387................................. 0.23
2020 Lucid Air................................ No available data........................ 0.21
1941 FKFS K-5 (kurz-heck).............. 3.29665................................. 0.2166
1996 GM EV1................................ 3.1069501.................................0.197
1941 FKFS K-5 ( m-h).................... 3.69486.................................. 0.1813
2012 VW XL1................................ 3.011834................................ 0.1769
1992 GM Ultralite........................... 3.131837................................ 0.1733
1941 FKFS K-5 ( Lange-heck).......... 4.30126.................................. 0.1662
1993 GM 'Yellow-Ferret' LSR EV1...... 3.435861................................ 0.15
1981 VW 'Flow' body ( k-h )............. 3.346776................................ 0.16
1981 VW 'Flow' body ( lange-heck)... 4.432975................................. 0.15
--------------------------------------------------------------------------------------
In 1922 Paul Jaray demonstrated low drag via an elongated body form.
In 1933, Dr. Walter Lay established the relationship between drag and elongation.
In 1935, Dr. Kamm and Baron Fachsenfeld re-created Lay's research, re-establishing the link between drag and body elongation.
In 1978, Liebhold et al. demonstrated a drag reduction of the D-B,M-B C-111 III, from Cd 0.237, to Cd 0.178, with body elongation ( fineness ratio )
In 1981, Buchheim et al. demonstrated drag reduction of the VW 'Flow' body with elongation.
In 1991, the IT works Honda CRX-HF set a club record at USFRA World of Speed, and verified the body elongation as primary source of drag reduction, later backed up at the Chrysler Proving Grounds, East Chelsea,Michigan.
In 1993, General Motors Corporation established a land speed record, at the Bridgestone / Firestone Proving Grounds, Ft. Stockton, Texas; with their Impact/ EV1, in part through body elongation, achieving Cd 0.14, compared to Cd 0.197, for the standard production version.
In the last session at DARKO, the 'baby' template car demonstrated a drag reduction with elongation ( fineness ratio). ( Cd 0.151, to Cd 0.121 ).
In the same test session, Spirit of Ecomodder indicated from Cd 0.44, to as low as Cd 0.214, as a simulated BEV.
2015's Mercedes-Benz IAA demonstrated a drag reduction with elongation ( fineness ratio ).

my big ole SUV has the same drag coefficient as a Geo metro

[aerohead]

Originally Posted by JulianEdgar

I suppose someone who believes in some pretty odd car aerodynamic theories was bound to also be a believer in a conspiracy theory about a lack of aerodynamic progress.

Perspicacity deficit disorder.

[aerohead]

Originally Posted by Vman455

Reality is, as reality is so often wont to be, much more complex than you suggest. As Hucho himself has written, "Low air drag is one target. Many others like styling, safety, cost, etc. have to be observed" (emphasis original).

Boeing does not use a direct-to-consumer business model, nor does the US Navy; they certainly work under similar cost and safety constraints, and when those parameters fail to hold similar influence in the product's design it can result in spectacular debacles, as the 737 MAX has demonstrated.

I'm not sure where you get the idea that cars are still redesigned every year; this has not been the case since the 1960s.

I assume you read the Mercedes S-class press release on aerodynamics that JulianEdgar posted (since you commented on the thread), but it sounds like you missed this part:

(emphasis added)
That doesn't sound to me like an "impasse" between style and technology; rather, cooperation.



I very much disagree.

' [T]he drag of the basic body ( Cd 0.09 ) is achievable. To what extent this can be approached in the development of a production vehicle is therefore more a question of the balance of the requirements of the specification than of technical feasibility.' Hucho, 2nd-Edition, page 209, circa 1986
The 'law of the Paris dressmaker' is what stands between us and low drag.

[Vman455]

Originally Posted by aerohead

' [T]he drag of the basic body ( Cd 0.09 ) is achievable. To what extent this can be approached in the development of a production vehicle is therefore more a question of the balance of the requirements of the specification than of technical feasibility.' Hucho, 2nd-Edition, page 209, circa 1986
The 'law of the Paris dressmaker' is what stands between us and low drag.

I really think you've put yourself at a disadvantage working from such an old copy of the book. I have the 4th edition (1998), which isn't even the newest but is the last edition edited by Hucho.

On p. 53, under the heading 1.4 Aerodynamics and Design, Hucho writes:

This discord between physics and art was resolved only when the automobile manufacturers started to do their own aerodynamics. As members of a vehicle engineering activity aerodynamicists became converted to vehicle engineers. The daily contact with design gave them an understanding of this kind of art. The aerodynamicists learned to respect the designers' creativity, and they realized how much the designer's freedom is already restrained by requirements, be they technical or legal. The designers, on the other hand, realized that aerodynamics is not a black art, but a rational discipline. The strategy of detail optimization, the step-by-step procedure, provided a sound basis for cooperation. Both the need to improve fuel economy and the desire to make the economics of a car visible by its style made designers open to aerodynamics. This openness increased as they realized that this technology is far more reliable than the temper of fashion. The plot of drag coefficients versus model year in Fig. 1.58 shows how fast the change of attitude progressed.

However, the strong position now held by vehicle aerodynamics is not welcomed by everyone. Two arguments are repeatedly made against aerodynamics: one technical and the other emotional. Aerodynamicists are well advised to carefully consider both of them if they do not want to lose the status they have now achieved.

You can go ahead and think that car design is all some sort of conspiracy, and that aerodynamicists have no say or are blinded somehow by corporate culture into producing shapes that have much higher drag than the lowest-drag experimental models or concept cars. But the reality is they work under constraints: people, who are motivated by cultural norms, buy cars; people buy cars for practical reasons and emotional ones; exterior design is dictated by both stylistic considerations (4 wheels, a standard-ish dimensional footprint, proportional wheels, aggressive faces, etc.) and technical ones (crash test performance, NVH, fuel economy, handling and performance, etc.).

The reason we don't have cars that look like tadpoles is simple: hardly anyone would buy one. Why? They look "abnormal." With a long tail, they won't fit in a standard garage, and if you make them small enough to do that they end up being tiny inside. They compromise packaging, interior volume, sightlines, and practicality.

Probably the best example of this so far is the original Honda Insight. That car is as close to a no-compromise car as have ever been brought to market. It had low drag and low weight. Consequently, it was a sales flop. People didn't want a two-seat car that didn't have an insane amount of power. People didn't want to pay extra for an all-aluminum body. People didn't want a tiny car with wheel skirts that looked different than every other car at their local Honda dealership.

It's stupid, yes, and buyers consistently buy impractical cars with feeble justifications--but this is not because of some conspiracy preventing their rational behavior. It's because we're irrational already.

We're seeing this start to change, as low-drag design becomes a premium feature, with cars like the Model S and 3, Taycan, and now S-class returning lower drag coefficients without significant changes to exterior design features. It remains to be seen how much lower they can go.