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Old 05-20-2020, 01:16 PM   #1 (permalink)
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'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.

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Last edited by aerohead; 05-20-2020 at 01:19 PM.. Reason: add data
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Old 05-20-2020, 05:49 PM   #2 (permalink)
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Quote:
Originally Posted by aerohead View Post
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?).
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Last edited by JulianEdgar; 05-20-2020 at 08:08 PM.. Reason: grammatical error
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Old 05-20-2020, 08:29 PM   #3 (permalink)
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Quote:
Originally Posted by JulianEdgar View Post
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:

Quote:
The book is written by engineers working in the development of cars produced in large volume. Low air drag is one target. Many others like styling, safety, cost, etc. have to be observed.
(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 thatís 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.
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Old 05-21-2020, 10:31 AM   #4 (permalink)
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Quote:
Originally Posted by aerohead View Post
... 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. ....
Thanks for the explanations. Would you share page citations from Hucho?
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Old 05-22-2020, 11:17 AM   #5 (permalink)
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Not such a good...........

Quote:
Originally Posted by JulianEdgar View Post
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?).
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.
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Old 05-22-2020, 11:20 AM   #6 (permalink)
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citations

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Originally Posted by California98Civic View Post
Thanks for the explanations. Would you share page citations from Hucho?
Each talking point was given it's own thread,and those contain the specific citations.
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Old 05-22-2020, 11:41 AM   #7 (permalink)
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Quote:
Originally Posted by aerohead View Post
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. ...
Thanks. Here is an image of that CUER car. That sure is the template!



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.

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Old 05-22-2020, 12:05 PM   #8 (permalink)
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sure is

Quote:
Originally Posted by California98Civic View Post
Thanks. Here is an image of that CUER car. That sure is the template!



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.
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.
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Last edited by aerohead; 05-22-2020 at 12:08 PM.. Reason: add PS
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Old 05-26-2020, 06:29 PM   #9 (permalink)
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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.



Free streem optimization of NACA 4412 versus wig optimization of the NACA 4412
Quote:
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.
This studdy agrees with your sediment as I understand it.
Dream big chisel down to reality
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Old 05-26-2020, 07:00 PM   #10 (permalink)
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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.)

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