Fineness ratio vs drag coefficient
 

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
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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 ).

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.

take your pick
 

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.

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

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.

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

someone
 

Perspicacity deficit disorder.

Reality
 

' [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:
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.

how much lower
 

Cd 0.09.

it's cooling system limitations as well ICE requires a bigger cooling system then a EV

Will never happen in a production car. Hucho doesn't think so, either--I asked him, and he wrote, "Now, more than 40 years later, cD = 0.20 seems to be possible for cars manufactured in large volume. And that’s it."

You can go on wishing for unicorns to become real. Or...you can be happy we have horses and look for the best horse possible.

the average car is 185inches long(i.e 2020 civic, corolla ) you can fit a Chevy suburban 225" in the average garage

I've skipped through a lot of this thread but I 100% agree with the above. "The juice has to be worth the squeeze."

At some point it is not worth the small increase in efficiency to lose comfort, capability, style, safety, etc.

The law of diminishing returns is obvious.

Someone asked me the other day what aero drag improvements I would recommend on a Tesla Model S.

I said: "None that I'd be confident with."

That was in the context of Rab Palin (Tesla aerodynamicist on the Model S) telling me he'd been having discussions with an aftermarket company developing Tesla Model S bits to - purportedly - reduce drag. Ones he said, that would not do so.

And hell, who would better know than he?

But still leaves us many billions of vehicles where it's easy to reduce drag (and/or lift)...

Your'e obsessed with posting about lift. Is it because you've got the instrumentation to measure it?
In the realm of economic driving, the destabilizing effects lift are not an issue. Neither is adding downforce.
What we care about here is drag. We do indeed measure it with adding mods, then doing tuft testing, A-B-A testing, coast down tests. Long term gathering of data over the same ground, day after day.
We don't have many million dollar wind tunnels.
In the speed ranges in question, measuring lift is a roundabout way of measuring a component of drag. It is by no means the source of all drag. Not even close.
I suggest you climb down off your well beaten dead high horse and try participating in a civil manner.
You have a great deal to add here. Try adding it and not contesting every other thing you see.

I can measure changes in drag and panel pressures too, and I often write about those as well.

Unless you never exceed 80 km/h, not true.

Well, any coast down testing that you can perform on a normal road is quite invalid - it's one of the things that Aerohead states here that is right. I am not sure what your point is with your other tests - they're great!

Neither do I - I am not sure of your point.

That does not appear to be the case. An example is the Porsche Taycan that has lower drag at higher lift (ie spoiler down).

I've never suggested that is the case

Well, the 'civil manner' of the group has allowed complete misconceptions and erroneous advice about car aerodynamics to flourish for a long time. Might be a bit better if BS is called out and not brushed under the carpet.

I contest only misconceptions, errors and misleading advice. Unfortunately, there is a lot of it.

We learned in the sixties and forgot this again: To fight is to feed it. Counter bad speech with good speech.

Propose a template framework for a first approximation across a range of use cases for airflow management.

Something like Morelli's Urban Car.

Why? For car modification, I think it is the completely wrong approach.

I'm building a clean slate travel trailer, and plan to use it.

Yes, I can see some use of low drag, generalised shapes in that context (as I have written here before.)

Cool. Is there a build thread or post?

I shall not repost my aerodynamic trailer design, but my current design is for a neighborhood trailer with a 25mph top speed. It would have an eyebolt for towing instead of a tongue and hitch.

It would be a pseudo-Airstream shape with Gothic arch bulkheads at the ends with an half-circular section through the middle.

it's all in my head and has been for a while. I hesitate to put it to paper due to many redesigns :)
The basic numbers are 26'-27' long by 7'6" wide, by 7'8" high with roof down.
It will have similar ground clearance as the tow vehicle at 12", with a very smooth finish on all sides.
When I start putting epoxy and glass to EPS foam on the prototype, I will get a thread going :)

I'm with you there. Thoughts move much faster than fingers.

My target is a Shepherd's* trailer (benches over the wheels) with a 6'6"x16' floor and taper fore and aft of the two axles. I estimate $300 for a mortise and tenoned rectangle with a plywood diaphragm ready for 5" of insulation and decking. I'm agnostic about what happens above that. The big problem is I don't have space to build it. :(

So where does one start then? Scott Adams emphasizes the importance of process above goals.

* Why isn't that pronounce 'sheferd'? :confused:

Just exactly like with any other car modifications. You measure what you have, then develop changes from those starting points and then assess the result.

If you're modifying the engine management, you don't apply some rule of thumb to altering ignition timing and air/fuel ratios - you measure what you have, make changes from those starting points and then assess the result.

If you're modifying car suspension, you don't apply some rule of thumb - you measure what you have, make changes from those starting points and then assess the result.

With car aero it's in fact easier than most car modifications, because making trial changes can be made cheaply and easily and then assessed.

The 5th edition does not list fineness in the index but does discuss d/l ratio. Since an automobile is not a body of rotation tapered to a point this is more applicable to aircraft. Compound curved panels are more costly to stamp than flat panels generally speaking. My interest is chiefly in body shells for commuter bicycles. Drag and stability are issues to be addressed.

This is one approach to making very low drag, compound curved shapes. It was covered on a website I edited (Autospeed) but I am not sure the articles are still up. It used foam rubber heated and pulled into shape.

https://i.postimg.cc/P57VmqgL/foam-1.jpg

https://i.postimg.cc/gJFMTTkm/foam-10.jpg

https://i.postimg.cc/MT7s2mvR/foam-12.jpg

https://i.postimg.cc/X77smF3G/foam-17.jpg

What makes it rigid? I've had 4 mm coroplast deflect from wind pressure at the front.

There's a lot of work in that form. It reminds me of Neile Blanchard's CarBEN prototype.

I developed a plan for formless construction. Using math[s].

https://ecomodder.com/forum/member-f...07-7-35-02.png

Pushing a simple ocathedron would be high drag. But at 4 to 6v it closely approximates a compound curve. I'm curious how a 2 or 3v would fair.

d/L
 

*The inverse is the fineness ratio for the mirror image, what the air always
'sees'
* The convention is to take half, as a half-body,and loft up to whatever ground clearance is chosen.
* For bicycle stability, aeronautical engineer, Al Voight chose a trike for his Vector, which HUFFY purchased and sold for $10,000/ pop. Cd 0.11 was claimed.

Cd 0.20
 

Hucho published that Cd 0.09 was technically feasible as of 1986. What a carmaker builds depends upon the technical specification of the committee which has oversight and authority over such decisions.
You may be entirely correct in what is decided, however it would not be driven by aerodynamic considerations. Those are for a different thread.

225"
 

There are vehicles on the roads today in the USA, of 260" length. 'Extensible' tails as depicted by Fachsenfeld in the 1930s would enable 'parking'.

thanks for nothing
 

After more name dropping you succeeded in providing us absolutely zero actionable information. Really appreciate it.

Taycan
 

Julian, do you know if the only difference between the Taycan Turbo, and Turbo S is the flip-up spoiler? Is there any other feature, like narrower tires in the equation? Thanks in advance!

It had foam stiffeners inside, and of course the trike frame helped support it.

Most of the forces, except for a tiny area of the nose, would be outwards.

The graph in the paper Dr Wolf sent me is for the same car (Taycan Turbo), with the spoiler in up and down positions. Lowest drag occurs with the rear spoiler down, giving highest rear lift.

Well I for one really appreciate hearing from the professionals actually working on cars - especially ones like the Tesla. Since Rob has now left Tesla, and is very frank in his communications, I thought it quite interesting that he could see no easy aftermarket way of lowering the Model S drag.

As I said, the law of diminishing returns is starting to apply with these cars.

spoiler down
 

That makes sense. While perhaps the best contour ( excepting racing cars ) the Taycan's is still a little too aggressive. Like Panamera, Cayman, etc..

easy
 

Easy would be a slip-in, receiver-hitch, truncated boat-tail. Cd 0.18 would be a breeze.
A gap-filled, full-boat-tail trailer could take it to Cd 0.12, with extra batteries to boot! You'd be getting better mpg with the trailer , than without.