Beware old and new aero data

[JulianEdgar]

Originally Posted by freebeard

[time for some popcorn. It's gonna be epic]

Honestly, there is a lot of misinformation discussed here (and of course some excellent information as well). I sometimes think that people don't even stop to think for a moment before accepting advice / statements that have been made. Then they apparently become gospel - weird.

- Streamlined bodies don't develop lift? What, then, is an aircraft wing?

- Airflow over the curved upper surfaces of a car doesn't develop lift? Really, when you can see in any pressure measurement diagram of a car all that top surface lift?

- Percentage drag reductions can be precisely quantified as fuel economy gains? And that's apparently irrespective of the car?

- Kamm tails (as opposed to the use of simple separation edges) are somehow great for drag? Really? What's an example, and don't give me those K cars from the 1930s that now measure at 0.37!

- There is a perfect streamline template shape? And this is irrespective of the thickness of the boundary layer (which will be car-dependent)?

- Box cavities are a panacea? And that despite almost no mention in the technical literature?

- Coast down tests are accurate? And that's when even the experts hate them (unless they're using very sophisticated equipment).

- Full wheel covers are always best for low drag? There are specific, recent tech papers than contradict that.

It's a bit frustrating when you see so many people putting in work and enthusiasm to modify the aero of their cars when there is so much poor advice being given out.

Note that I am not throwing the baby out with the bath water - there is some excellent information here, and some very good testing (especially mileage over long periods). And that's great. But it's so frustrating reading older threads where suggestions are made, I think to myself 'that won't work' (or - often - 'the difference will be so small, forget it'), and then after a lot of time and effort, the OP says 'that didn't make any difference'.

[freebeard]

Honestly, there is a lot of misinformation discussed here (and of course some excellent information as well). I sometimes think that people don't even stop to think for a moment before accepting advice / statements that have been made. Then they apparently become gospel - weird.

You may want to avoid The Lounge.

There've be a few times that other posters confirm my suspicions. One was when HHOTDI moved the folding tent from the top of his New Beetle to a hitch mount and the drag was lower than no tent at all. https://ecomodder.com/forum/showthre...tml#post390588

FreeBeard was right...what would we do if the MPG goes up!?!?
Thanks FreeBeard! I just returned home from a 770 mile round trip to Portland, lake Oswego and home! I returned home late last night and checked the hard mileage and found 770 miles, 15.6 gallons netted 49.3MPG! So as Freebeard suggested, what will we do if the mileage goes up?!?
Beetle w/o any attachments/slick top= ave 48mpg :{)
Beetle with factory VW Roof racks and CVT tent= 38mpg :/
Beetle with new areo-hitch mounted rack/CVT tent= 49.3MPG WhoooRah!
That is a gain of 11.3 mpg boys and girls!

tinypic.com ate the pictures. Another one was when someone CFD'd this and got some improvement over a flat transom.


It uses longitudinal rolling vortexes just like the Cybertruck.

[aerohead]

Originally Posted by JulianEdgar

Well, of course. But 'contextual' is the whole issue here: the context is that in some cases, full wheel covers increase drag. Isn't that the point?



I am sorry, but I have seen lots of advice here - including from you - that is stated without any caveats whatsoever. Here's an example that was being used in a signature:

"At 55 mph, a 10% drag reduction translates to a 5% increase in fuel economy. At 70mph,a 10% drag reduction translates into a 6% increase -Phil Knox (Aerohead), Aerodynamics Seminar #2"

No caveats there.

(Nothing on what proportion of total drag is made up by CD vs rolling resistance - which is car-dependent? Nothing on the change in engine efficiency - BSFC if you like - as the rpm changes? For a discussion of this sort of thing, see Barnard, Road Vehicle Aerodynamics Pages 54-55, and Stone, Motor Vehicle Fuel Economy, Page 136.)

*The 5%,10% etc. rule of thumb is from PhD, Gino Sovran,General Motors Research Laboratory, contributor to SAE publications,and personal friend and mentor of Wolf Heinrich Hucho.These relationships have been shared by GM,Ford,Chrysler,and Hucho,extending into recent times.
*The relationship is predicated solely upon drag reduction,and I've addressed the implications of BSFC degradation for not gear-matching in the seminars,as per Gino Sovrans admonition in the SAE literature.Hucho repeats this cautionary tale in his textbooks. I'm quite familiar with the relationship. Once upon a time, I had a personal relationship with Glen Scharf,of GMs Aerodynamic Laboratory.I could simply call on the phone,share data,and he was gracious enough to help me navigate the drag implications of delta-drag,based upon delta-mpg.Glen is the individual who stated that my modified CRX had Cd 0.235,based upon my Bonneville speed record, coastdown data from the Chrysler Proving Grounds,and tank mileage.

[aerohead]

Originally Posted by JulianEdgar

Honestly, there is a lot of misinformation discussed here (and of course some excellent information as well). I sometimes think that people don't even stop to think for a moment before accepting advice / statements that have been made. Then they apparently become gospel - weird.

- Streamlined bodies don't develop lift? What, then, is an aircraft wing?

- Airflow over the curved upper surfaces of a car doesn't develop lift? Really, when you can see in any pressure measurement diagram of a car all that top surface lift?

- Percentage drag reductions can be precisely quantified as fuel economy gains? And that's apparently irrespective of the car?

- Kamm tails (as opposed to the use of simple separation edges) are somehow great for drag? Really? What's an example, and don't give me those K cars from the 1930s that now measure at 0.37!

- There is a perfect streamline template shape? And this is irrespective of the thickness of the boundary layer (which will be car-dependent)?

- Box cavities are a panacea? And that despite almost no mention in the technical literature?

- Coast down tests are accurate? And that's when even the experts hate them (unless they're using very sophisticated equipment).

- Full wheel covers are always best for low drag? There are specific, recent tech papers than contradict that.

It's a bit frustrating when you see so many people putting in work and enthusiasm to modify the aero of their cars when there is so much poor advice being given out.

Note that I am not throwing the baby out with the bath water - there is some excellent information here, and some very good testing (especially mileage over long periods). And that's great. But it's so frustrating reading older threads where suggestions are made, I think to myself 'that won't work' (or - often - 'the difference will be so small, forget it'), and then after a lot of time and effort, the OP says 'that didn't make any difference'.

*Look to THEORY OF FLIGHT,by Richard Von Mises, DOVER Publications, for an introduction to the concept of the streamline body, conceived in 1907,by Frederick W. Lanchester, who was one of only two people on Earth with a true command of aerodynamics, Ludvig Prandtl being the other.
*Airfoils have nothing to do with streamline bodies.
*And again, if you will re-visit 'THEORY OF WING SECTIONS Including a Summary of Airfoil Data, Appendix-IV, by Abbott and Von Doenhoff, you'll be re-acquainted with 118-families of airfoils, all of which are zero-lift, depending on angle of attack, also addressed in Von Mises work.
*Wait until you see the pressure profile over a streamline body and it's zero-lift before you condemn them.
*Yes, if properly gear-matched, a 10% drag reduction will approximately relate to a 5% improvement in constant velocity,55-mph, highway driving (not to be confused with any EPA Highway testing protocol);for any vehicle which has undergone only a shape change.
*As to 'Kamm' tails, technically, it would be 'Koenig' tail, as Reinhard Koenig-von Fachsenfeld holds the patent for the 'K' truncated tail. Kamm is given the credit, but he's really known for his low-drag cooling system. And yes, in the context of the model studies conducted at FKFS, the K-tail might as well be considered ideal.FKFS starts with the entire streamline body, then begins lopping off sections, documenting the drag change as a function of the degree of truncation.'Verjungunsverahaltnisses'. Hucho states that this is the only path to low drag, and the premise of aerodynamic streamlining.
* The Langenburg Castle K-car is not Kamm's K-5,Cd 0.23 car, it is the K-3.It was Jerry Sloniger of ROAD & TRACK,August,1982,page 66, who tracked down the proper nomenclature of these cars. Kaselbach is incorrect in his data presentation. Kamm's car was Cd 0.23,diesel-powered,overdriven,85-hp,and top speed of 113-mph.HOT ROD Magazine featured photos of the K-5 in 1963,depicting tuft-testing.The flow is virtually ideal up until it reaches the rear radii of the tail, something not depicted in in the FKFS drag tables.
*There is a perfect streamline template shape. Hucho gave us all the critical elements for its construction in his 2nd Edition of 1987.At 1/3rd-scale,it returned Cd 0.1201 at DARKO,with compromised wheel fairings. It's a sure thing. You don't have to worry about boundary layer or anything else.It's pre-tested, at any scale you choose, up to 250-mph.
*Would you like to show me where I've ever implied that box-cavities are a panacea?
* I have only recommended that people approach CAR and DRIVER, and see if they'll conduct the coastdowns at Chrysler Proving Grounds, E.Chelsea, Michigan.They know the SAE protocols,have scientific-grade weather system,and optical fifth-wheel, as well as access to Chrysler's all-wheel scales, a dedicated test-track, where they conduct the rapid, back-to-back coastdowns, plus custom software for the data reduction.Top speed is conducted on the 8-mile oval, with only stop-watch,and no aerodynamic excrescences of any kind. They tested my CRX in 1991,afer the speed record at Bonneville. Call for current pricing and availability.
*As to the wheelcovers, your messengers have yet to re-test previously-measured static structures on a rolling-road wind tunnel, and publish the discrepancy between the two readings. I've yet t see any bona fides,prima facie evidence for the superiority of aspirated covers,except in specific cases.And curiously, when GM,Ford,or Volkswagen set out to establish land speed records, their choice of wheelcover is the MOON disc, even though they have access to virtually any 'secret-weapon' design. And to the rotating wheel supremacy issue, Dennis Semanaitis,of ROAD & TRACK,August,1982.p.35,reported:'[Rotating wheel] Tests have shown that wheels and tires of an ordinary passenger car generate added drag of 0.005 or less....The idea of a moving belt has been explored,though most believe its complication outweigh any potential improvement in realism.'
Hucho, in 1987, states,'The rotation of the wheels isusually not taken into consideration in wind tunnel testing; testing is carried out with the wheels stationary.When the wheels are integrated into the body the roration of the wheels appears to have negligible influence upon the forces and moments acting upon the vehicle (Second Edition, page 419).

[freebeard]

Kamm is given the credit, but he's really known for his low-drag cooling system.

As at Permalink #4, I solicit documentation. My favored search engine only points to the Meredith Effect.

The major effect of a rolling wheel appears at the leading edge of the contact patch[es]. Shouldn't have much effect on the body itself.

[aerohead]

Originally Posted by freebeard

As at Permalink #4, I solicit documentation. My favored search engine only points to the Meredith Effect.

The major effect of a rolling wheel appears at the leading edge of the contact patch[es]. Shouldn't have much effect on the body itself.

Automobile Magazine (I think) published two, hardbound books which included brief histories of both Jaray and Kamm. I believe that this is the source of the information. Both books will be easy to locate,I'll look. Since Germany lost WW-I because it ran out of oil and rubber, Hitler was very concerned that his up-coming war might be jeopardized the same way (which turned out to be case).In exchange for all Fachsenfeld' aerodynamic patents,he was to be given free rein at the FKFS laboratory/ wind tunnel in Stuttgart (it's right next door to Daimler-Benz (Mercedes-Benz). Kamm's low drag cooling system would also help the war effort.Kamm would commute back and forth to Berlin in his K-5,while Porsche commuted there and back in his 60K10 Berlin-Rome racer.

[freebeard]

Both books will be easy to locate,I'll look.

Thnx.

[JulianEdgar]

Originally Posted by aerohead

*Look to THEORY OF FLIGHT,by Richard Von Mises, DOVER Publications, for an introduction to the concept of the streamline body, conceived in 1907,by Frederick W. Lanchester, who was one of only two people on Earth with a true command of aerodynamics, Ludvig Prandtl being the other.
*Airfoils have nothing to do with streamline bodies.
*And again, if you will re-visit 'THEORY OF WING SECTIONS Including a Summary of Airfoil Data, Appendix-IV, by Abbott and Von Doenhoff, you'll be re-acquainted with 118-families of airfoils, all of which are zero-lift, depending on angle of attack, also addressed in Von Mises work.
*Wait until you see the pressure profile over a streamline body and it's zero-lift before you condemn them.
*Yes, if properly gear-matched, a 10% drag reduction will approximately relate to a 5% improvement in constant velocity,55-mph, highway driving (not to be confused with any EPA Highway testing protocol);for any vehicle which has undergone only a shape change.
*As to 'Kamm' tails, technically, it would be 'Koenig' tail, as Reinhard Koenig-von Fachsenfeld holds the patent for the 'K' truncated tail. Kamm is given the credit, but he's really known for his low-drag cooling system. And yes, in the context of the model studies conducted at FKFS, the K-tail might as well be considered ideal.FKFS starts with the entire streamline body, then begins lopping off sections, documenting the drag change as a function of the degree of truncation.'Verjungunsverahaltnisses'. Hucho states that this is the only path to low drag, and the premise of aerodynamic streamlining.
* The Langenburg Castle K-car is not Kamm's K-5,Cd 0.23 car, it is the K-3.It was Jerry Sloniger of ROAD & TRACK,August,1982,page 66, who tracked down the proper nomenclature of these cars. Kaselbach is incorrect in his data presentation. Kamm's car was Cd 0.23,diesel-powered,overdriven,85-hp,and top speed of 113-mph.HOT ROD Magazine featured photos of the K-5 in 1963,depicting tuft-testing.The flow is virtually ideal up until it reaches the rear radii of the tail, something not depicted in in the FKFS drag tables.
*There is a perfect streamline template shape. Hucho gave us all the critical elements for its construction in his 2nd Edition of 1987.At 1/3rd-scale,it returned Cd 0.1201 at DARKO,with compromised wheel fairings. It's a sure thing. You don't have to worry about boundary layer or anything else.It's pre-tested, at any scale you choose, up to 250-mph.
*Would you like to show me where I've ever implied that box-cavities are a panacea?
* I have only recommended that people approach CAR and DRIVER, and see if they'll conduct the coastdowns at Chrysler Proving Grounds, E.Chelsea, Michigan.They know the SAE protocols,have scientific-grade weather system,and optical fifth-wheel, as well as access to Chrysler's all-wheel scales, a dedicated test-track, where they conduct the rapid, back-to-back coastdowns, plus custom software for the data reduction.Top speed is conducted on the 8-mile oval, with only stop-watch,and no aerodynamic excrescences of any kind. They tested my CRX in 1991,afer the speed record at Bonneville. Call for current pricing and availability.
*As to the wheelcovers, your messengers have yet to re-test previously-measured static structures on a rolling-road wind tunnel, and publish the discrepancy between the two readings. I've yet t see any bona fides,prima facie evidence for the superiority of aspirated covers,except in specific cases.And curiously, when GM,Ford,or Volkswagen set out to establish land speed records, their choice of wheelcover is the MOON disc, even though they have access to virtually any 'secret-weapon' design. And to the rotating wheel supremacy issue, Dennis Semanaitis,of ROAD & TRACK,August,1982.p.35,reported:'[Rotating wheel] Tests have shown that wheels and tires of an ordinary passenger car generate added drag of 0.005 or less....The idea of a moving belt has been explored,though most believe its complication outweigh any potential improvement in realism.'
Hucho, in 1987, states,'The rotation of the wheels isusually not taken into consideration in wind tunnel testing; testing is carried out with the wheels stationary.When the wheels are integrated into the body the roration of the wheels appears to have negligible influence upon the forces and moments acting upon the vehicle (Second Edition, page 419).

This is just a dump largely of misunderstandings, outdated references, and irrelevant citations.

To save time, I'll pick out just five completely wrong statements.

Originally Posted by aerohead

1. Airfoils have nothing to do with streamline bodies.

Well then, you must have a unique definition of what comprises a streamline body. Earlier you defined a streamlined body as one with attached flow. (Which is correct.) And aerofoils have attached flow. Hmmm.

Originally Posted by aerohead

2. Wait until you see the pressure profile over a streamline body and it's zero-lift before you condemn them.

I've seen lots. One example is a wing, and they allow aircraft to fly.

Originally Posted by aerohead

3. Yes, if properly gear-matched, a 10% drag reduction will approximately relate to a 5% improvement in constant velocity,55-mph, highway driving (not to be confused with any EPA Highway testing protocol);for any vehicle which has undergone only a shape change.

It doesn't even need any reference citations to prove this is absurd. (But as a bonus I previously gave two references that show this not to be the case.) Just think about it for a minute. Given that rolling resistance (the other determiner of fuel consumption) changes little with cars of different CD, how can this rule of thumb validly apply to cars that might vary in CD from 0.5 to 0.25? To put it simply, the proportional change might be to something making up half the total vehicle resistance, or one-quarter! And yet the outcome is the same?

Originally Posted by aerohead

4.The Langenburg Castle K-car is not Kamm's K-5,Cd 0.23 car, it is the K-3.It was Jerry Sloniger of ROAD & TRACK,August,1982,page 66, who tracked down the proper nomenclature of these cars. Kaselbach is incorrect in his data presentation. Kamm's car was Cd 0.23,diesel-powered,overdriven,85-hp,and top speed of 113-mph.HOT ROD Magazine featured photos of the K-5 in 1963,depicting tuft-testing.The flow is virtually ideal up until it reaches the rear radii of the tail, something not depicted in in the FKFS drag tables.

No test of any of these K cars in a modern wind tunnel has given what we would now call low drag values. Hardly surprising. And yet you still quote these drag figures as if they are valid. More spreading of misconceptions.

Originally Posted by aerohead

(5. As to the wheelcovers, your messengers have yet to re-test previously-measured static structures on a rolling-road wind tunnel, and publish the discrepancy between the two readings. I've yet t see any bona fides,prima facie evidence for the superiority of aspirated covers,except in specific cases.And curiously, when GM,Ford,or Volkswagen set out to establish land speed records, their choice of wheelcover is the MOON disc, even though they have access to virtually any 'secret-weapon' design. And to the rotating wheel supremacy issue, Dennis Semanaitis,of ROAD & TRACK,August,1982.p.35,reported:'[Rotating wheel] Tests have shown that wheels and tires of an ordinary passenger car generate added drag of 0.005 or less....The idea of a moving belt has been explored,though most believe its complication outweigh any potential improvement in realism.'
Hucho, in 1987, states,'The rotation of the wheels isusually not taken into consideration in wind tunnel testing; testing is carried out with the wheels stationary.When the wheels are integrated into the body the roration of the wheels appears to have negligible influence upon the forces and moments acting upon the vehicle (Second Edition, page 419)

Seriously, this is flat earth stuff.

Pick up any current textbook on car aero.

Talk to any current professional aerodynamicist.

Think for a moment why every major car manufacturer in the world has spent millions (billions?) upgrading their wind tunnels, or building new ones, that incorporate the facility to have turning wheels on their test cars.

And they're all wrong - because of something you read in Road and Track in 1982?

[freebeard]

This is just a dump largely of misunderstandings, outdated references, and irrelevant citations.

You're beginning to understand the aerohead we all know and love. Asterisks for paragraphs but never a space between sentences.

Early American, English, and other European typesetters' style guides (also known as printers' rules) specified spacing standards that were all essentially identical from the 18th century onwards.

https://en.wikipedia.org/wiki/Senten...al_typesetting

[2000mc]

Originally Posted by JulianEdgar

It doesn't even need any reference citations to prove this is absurd. (But as a bonus I previously gave two references that show this not to be the case.) Just think about it for a minute. Given that rolling resistance (the other determiner of fuel consumption) changes little with cars of different CD, how can this rule of thumb validly apply to cars that might vary in CD from 0.5 to 0.25? To put it simply, the proportional change might be to something making up half the total vehicle resistance, or one-quarter! And yet the outcome is the same?

Sure you could have vehicles with the same rolling resistance and wildly different CD, but when I think of specific examples they seem to have a positive correlation. You say .5 CD I’m thinking hummer H2 has some serious rolling resistance, You say .25, I’m thinking insight. I always thought the 5/10 was more just a rule of thumb