'Ideal' shape for low drag
 

The 'ideal' shape for 2-D flow of struts/symmetrical wing sections was obtained by S. Goldstein,in 1938, and is presented in Fig. 2.28, page 244, of 'AERO-HYDRODYNAMICS OF SAILING, INTERNATIONAL MARINE PUBLISHING,Camden,Maine, copyright 1979, 1988, by C.A. Marchaj.
The drag is governed by surface friction and pressure drag. There exists a single iteration of the struts which embodies the least drag.If it is lengthened,the surface drag increases,along with an overall drag increase.
If the strut is shortened,pressure drag increases,increasing the overall drag.
This one geometry of struts is in the bottom of the drag 'bucket.'
--------------------------------------------------------------------------------------
The same holds true for 3-D flow streamline bodies of revolution. Both surface friction and pressure drag bottom out with a body of Length-divided by Diameter = 2.5. Yielding Cd 0.04. As a half-body,in ground reflection,this body produces Cd 0.08. Adding wheels increases drag to Cd 0.13.
According to Hucho,this drag can be reduced by integrating the wheels into the body. According to Goro Tamai of M.I.T.,a proper set of wheel fairings can reduce the wheel drag approximately 70%, allowing in the neighborhood of Cd 0.095. Honda achieved Cd 0.10 in 1994 with their 'Dream' solar racer.
The 2013 Cambridge University Eco Racer CUER solar car was measured at Cd 0.11.
-----------------------------------------------------------------------------------
The 'template' is intentionally fashioned after the Cd 0.04,2.5 L/D, streamline body of revolution.
Hucho says aerodynamics is about pressure drag.
Hucho says pressure drag is about separation.
The 'template' is incapable of producing separation,by default.
Without separation it achieves the drag minimum recognizable,as Hucho puts it.With tighter wheel fairings it's quite likely that she would have mimic'd CUER's Cd 0.11.
================================================
If you want to believe that aerodynamics is a 'black art',I can't stop any of you. The evidence suggests that it's quite easy to construct really low drag automobiles with out any further research or expenditure. Off - the -shelf technology if you will.
And this argument would date to 1922.

Not such a good post. Starting to:

- use rules of thumb (eg wheel fairings can reduce drag by 70 per cent and then applying that figure to another body)

- make conceptual jumps from theory to real road cars (eg solar race cars are not practical road cars)

- develop strawman arguments (who said aerodynamics was a black art?).

Further to this, I would add: this continued insistence on reporting "Hucho says..." (this or that gross oversimplification) really rubs me the wrong way.

Guess what? Hucho is still alive and working. If you want to know what he says, you can ask him! I decided to do that a couple months ago, and he was gracious enough to reply. When I asked him specifically about using his textbook as a guide to reduce the drag of our cars--and remember, as Ecomodders our focus is necessarily limited to modification of existing cars since we aren't OEMs--he wrote back:

(emphasis his)

He did not mention boat-tailing; he cautioned that "a posteriory attached details" were not the focus of his work. He advised that I would find the papers published on his website, aerowolf.de, more useful. He also said that "cD = 0.20 seems to be possible for cars manufactured in large volume. And that’s it."

That's what Hucho says. If you want to argue that we should all follow the template, fine. Go right ahead, but don't couch it in an appeal to authority. Otherwise you are putting words in someone else's mouth who isn't here to defend him or herself. Own your own words unless you proffer a direct quote with a proper citation.

Rant over.

Thanks for the explanations. Would you share page citations from Hucho?

Not such a good...........
 

It was Hucho who spoke to this issue on page 201. Goro Tamai simply provides some quanta which tends to support Hucho's thesis.
Since the 2013,Cambridge University CUER Eco Racer is a 'template' car,and incorporates a full suite of swept wheel fairings,it's Cd 0.11 represents the fruition of this line of investigation/technology transfer.
The CUER would make a fine production vehicle.
Much of popular automotive journalism espouses that aerodynamics falls within the realm of the 'black arts.' Fed by public relations men from U.S. automakers.
The 'conceptual leap' to Spirit allowed for up to 39.9-mpg,up from 26-mpg. This is why aerodynamicists publish. This is what the SAE exists for.

citations
 

Each talking point was given it's own thread,and those contain the specific citations.:)

Thanks. Here is an image of that CUER car. That sure is the template! :)

https://ecomodder.com/forum/attachme...1&d=1590161983

As for Hucho pages, I think your citation here answers the question, which was only about this post/thread. I will dig out my partial copy (photocopier edition) and see what I see.

Cheers.

sure is
 

Yes, it's very close. At the very back,just past the Perspex/Lucite/Lexan,whatever it is, fenestration for the PV array,there's just a touch of von Mises,reflex-camber,upsweep in the tail.I did this also on 'Baby', with a smidgeon of truncation,allowing for tail/stop lights and license plate. We know from Morelli's research of the 1976 CNR 'banana' Morreli body,that losing this portion is compensated for with the 'phantom' tail',and zero drag penalty.:)
PS Notice also,that nearly the entire belly of CUER is 'diffuser.' This is something Cybertruck will be able to incorporate,with it's all-wheel air-suspension.

In order to understand automotive air dynamics one must throw out most of what know of airplane aerodynamics and only consider WIG(Wing In Ground effect) aerodynamics

This team took the NACA 4412 "best in Ground effect" and optimized it for just above road going vehicle altitudes of 20% cord ground clearance ratio. EG 100" bumper to bumper = belly pan 20" off the deck

Shape Optimization of an Airfoil in Ground Effect for
Application to WIG Craft


Yilei He,1 Qiulin Qu,2,3 and Ramesh K. Agarwal1
1 Department of Mechanical Engineering and Materials Science, Washington University in St. Louis, St. Louis, MO 63130, USA
2 Institute of Fluid Mechanics, Beijing Institute of Aeronautics and Astronautics, Beijing 100191, China
3Washington University in St. Louis, St. Louis, MO 63130, USA
Correspondence should be addressed to Ramesh K. Agarwal; rka@wustl.edu
Received 31 May 2014; Accepted 19 October 2014; Published 8 December 2014
Academic Editor: Jian-han Liang


Even at 80% cord ground clearance ground effect must be considered in design
Eg 100" bumper to bumper @80" belly pan clearance is still in full ground effect and free stream aerodynamics cannot apply.

https://ecomodder.com/forum/attachme...1&d=1590530532

Free streem optimization of NACA 4412 versus wig optimization of the NACA 4412https://ecomodder.com/forum/attachme...1&d=1590531923
This studdy agrees with your sediment as I understand it.
Dream big chisel down to reality

To be honest, I am not really sure how this is relevant to people modifying their road cars. If we were designing solar race cars, maybe. (And even then, I'd look at successful solar race cars, rather than wing in ground effect studies.)

Thanks for the find. That's how I read the lit, too. Including Hucho: "dream big, chisel down to reality."

Hucho's 4th edition from 1999 has a few things to say about ideal shapes and research into road vehicle aerodynamics. On pages 224-229, and a few other places, Hucho describes aerodynamics as a field in which not all relevant research has a specific car or specific part in focus. There are ideal shapes derived from basic research. In fact, Hucho's book can be read as a history of the rise of “detail optimization” and “shape optimization” as ways of compromising “ideal" shapes with market and other design requirements.

As gumby79 said: “dream big, chisel down to reality.” Julian’s throttle stop testing method is one of the good testing methods for chiseling, and a knowledge of some of this theory is also useful.

Hucho (4th Ed.), p.51:
https://ecomodder.com/forum/attachme...1&d=1590557199

I honestly cannot see what optimising a wing to provide maximum lift/drag ratio for an aircraft flying in ground effect has to do with the shapes of cars.

But I can see several reasons why it wouldn't be useful, and in fact could be quite deceptive.

From the paper:

...two optimization objectives are considered—maximization of the lift coefficient and the lift to drag ratio. - Neither is the aim with car aerodynamics.


The airfoil shape that results in lowest value of 100 x Cd/Cl... gives the shape of the optimized airfoil. - So, as you'd expect, they are chasing maximum lift and minimum drag. We don't want maximum lift in a car!

If the paper was about the best shape, in proximity to the ground, for lowest drag and lowest lift, I'd be all ears. (But even then, of course, they'd need to replicate the relatively rough surface of a car versus the dead smooth surface of an airfoil.)

I guess if you want your car to fly with the least power being consumed, this looks like an ideal paper to read!

There really is more for you to learn from foundational research about lift and drag, but you routinely just dismiss all of it. "Replicate the relatively rough surface of a car" is not not the path to lowest drag and lift. It is the path left available by an automotive industry, regulatory regime, and consumer market that makes more "ideal shapes" not practical on the road.

EDIT: But I have to admit no interest in driving a "template" shape as my DD, even though that whole discourse has been a pathway for teaching me valuable aspects of the aerodynamics of my car. I love the shape of the 90s Hondas. My goals are also a little like yours, Julian: optimization through testing mods, tweaking, retesting.

If you increase lift WITHOUT increasing drag, you have generally increased overall efficiency, possibly going to all laminar flow which AFAIK, THE HOLY GRAIL. At that point you get to start attacking/approaching the zero lift angle which should reduce drag further and mutes the lift argument.

I notice that both of these ordinates are significantly undercambered. I am not sure why that is relevant. Could be a traction enhancement.

lift/drag
 

* ' [T]here is not a simple relationship between overall lift and drag.' Hucho,page 123, 2nd-Edition,1987.
* ' [B]ecause the aspect ratio... is very small for vehicles a breakdown of the flow field into two-and three-dimensional components is not possible.The effect of the vortex field can therefore not be separated from that of the other flow field,as is possible for aircraft wings with high aspect ratio.' Hucho,page 122, 2nd-Edition, 1987.

Edgarwit's
Are a prime example of a WOG( wing in ground effect.) Your front and rear fenders appears to lack the roughness you refer to being present preventing such research from applying to your specific or any use case with wheels. one of us has the wrong context
You challenge the community to find you a better airfoil profile the community responded, Your response to your challenge being answered,is the subject is not relevant do to roughness of automobiles?
I'll push you a very specific question. Witch one has the best chance of maintaining attached flow mounted 20-80% of cord off the sheet metal the GEO 222, NACA 4412, or the optimised NACA 4412



testing you proved your 222 was not the right design . at any angle of attack it cannot achieve attached flow Beyond 1/3-1/4 cord . I guess only functioning 25% is considered a great success to a "published professional". Imagine how much better your guide plains would function if you use the correct airfoil, preventing the added additional Dynamic frontal area caused by the seperation. ( I do acknowledge that they have already showen a reduction the factory Dynamic frontal area, your testing shows 66 to 75% has been left on the table. thank you for presenting us with the evidence, now follow through and make your changes necessary to get your best result unless you are satisfied with 25% )

Use case 2
Rear spoiler
This profile may give you much better results on your rear wing then a profile that is designed to be a multi element 360 degree angle of attack as used.
Unless you don't think the back of your Prius is smooth enough or there's no ground effect off your back glass before you wing.

Use case 3
Flat floor
Take me into account the pressure wave rebound and making the floor not quite so flat in a way to take advantage of this rebound energy that is demonstrated in the pressure and velocity graphs of this document that you say can never provide useful energy formation to wheeled vehicles.

Use case 4
Building a West coast-style mirror (old school tow mirror)that acts like your edger wits but at the A-piller.

Of course I can provide useful use cases for the presented paper cuz I presented it I saw useful I thought others obviously not you but others could benefit from the things I see

dump out a bucket of dots I'll draw you a picture, all one has to do is connect them

I am comparing the relatively rough surface of a car with the surgically smooth surface of an aerofoil.

No production car will ever have such a smooth surface, as the production costs would be too high for a mass-produced item.

As surface roughness increases, so does the thickness of the boundary layer. The thicker the boundary layer, the more likely flow separation will occur. When flow separation occurs, the airflow is no longer following the shape.

Therefore, the surface roughness of a car needs to be considered when assessing 'ideal' shapes.

An 'ideal' shape that implicitly assumes a very thin boundary layer (as aerofoils are modelled to) has grave shortcomings when applied to a car.

Yes I did consider this. So in that case we'd:

1. Start with an aerofoil that has been optimised for maximum lift/drag ratio in ground effect for an aircraft

2. Change its angle of attack until we'd achieved zero lift (ie stop the aerofoil doing the very thing we've just optimised its shape for)

3. Claim we've developed a low drag shape for cars.

I don't see why such a design would have lower drag than a shape developed to have low lift and drag near the ground - as the primary criteria for its development.

It seems rather like developing a ladder for maximum height, laying it on the ground - and then saying we'd developed the best possible chair.

Because it would be optimized for the conditions experienced not just a calculated set of non infinite parameters. Or as aerohead calls it rolling floor effects. If you take your calculated profile and then modify it for local conditions, either the ordinates would be copies or very similar depending on how the aspect ratio is dealt with. Otoh, I have seen 1/1 aspect ratio low drag profiles and they are........ odd.

By the way, look out the window of a commercial airline in flight and notice how lumpy the upper surface is. Definitely not surgical smooth. Ditto for one piece composite or military although those are smoother.

From the EM archive (the wind tunnel, smoke thread)
 

Approximations of Hucho's CD 0.16 “ideal" shape in the wind tunnel (from our thread for wind tunnel images) are below. Also attached are 4 pages from Hucho, 4th edition (pp.45-46, 212-213), which include wind tunnel images (p.212 might be best). They describe such ideal shapes in the R&D process. I would love to hear from aerohead about the benefits he sees to the template shape in comparison to the one Hucho often seems to emphasize, calling it a "basic body." Respectfully, james.

http://korecologic.files.wordpress.c...1200&h=&crop=1

http://i1271.photobucket.com/albums/...titled9_25.jpg

HUCHO, 45-6, 212-3:

https://ecomodder.com/forum/attachme...1&d=1590677333

https://ecomodder.com/forum/attachme...1&d=1590677359

https://ecomodder.com/forum/attachme...1&d=1590677379

https://ecomodder.com/forum/attachme...1&d=1590677398

I don't have that edition (5th edition is on its way - I've been waiting over 2 months) but the 1987 edition I do have covers similar ground.

This diagram, for example, (P201):

https://i.postimg.cc/k4VTZ8Nd/IMG-0450.jpg

Obviously, there isn't just one 'template' shape that is best.

(In this diagram note also how the plan views of the shapes differ considerably.)

I don't understand your point. Taking an aerofoil designed for aircraft flying in ground effect as the starting point for a low drag shape for a car doesn't seem to me to make much sense.

That's right, and I'd be very surprised if the design of the practical wing shape doesn't take that surface roughness into account. (As opposed to a simulation of a perfectly smooth aerofoil.)

Note also how solid line shape (VW blunt body) likely has a large wake and AWA shape likely has very small wake - and yet the two shapes have the same drag.

If you look at the direction and strengths (ie vectors) of the likely lift forces occurring on the two shapes, you can start getting a feel for why this is so.

Lift is very important!

Cd 0.16
 

Sure could use an overhead, plan-view of this model. It began with Klemperer in 1922. When Buchheim et al. re-investigated it in 1981,as the Volkswagen 'Drop' shape, in plan-view,the car was 100% boat-tailed.Which is lost on the viewer when limited to the side elevation . The car's performance is completely predicated upon the full-boat tail.