Want low drag? Don't follow a template.

[kach22i]

Originally Posted by aerohead

* I did investigate the HOT ROD Magazine Camaro land speed record car, which I believe Gary Eaker consulted on. The 'Bonneville' rear spoiler on that car happens to match the AST-II perfectly.

Would that be Roadkill?

2013
Roadkill Ep. 20 - Chasing a Landspeed Record at Bonneville with a 1100hp Camaro
https://www.hotrod.com/articles/road...1100hp-camaro/


2007
Car Aerodynamics - A2 Wind Tunnel - Wind Camp Tech Theory
https://www.hotrod.com/articles/car-aerodynamics/

Coefficient of drag, as simplified by Eaker, "can be thought of as the proportion of airflow energy not recovered by the car." In other words, the ideal situation would be to have the pressure at the front of the car be equally balanced by pressure at the rear of the car. That's impossible, but the closer you can get, the lower the Cd will be and the faster the car will go. The Cd measurement is independent of size; in other words, a 51/48-scale model of your car has the same Cd as the fullsize car.

[aerohead]

Originally Posted by JulianEdgar

I use the term 'wrapping around curved surfaces' (and similar) to explain the low pressures that are generated when attached flow follows a convex surface*.

In an email to me last week, former head of Porsche aerodynamics, Dr Thomas Wolf, puts it this way:

"According to the Coanda-Effect the air follows the curved (upper) surface. According to Newton’s first law, there must be a force which pulls the air downwards. This force is exerted to the air by the car body and pulls the air downwards. According to Newton’s third law (action = reaction) there must be a force of the same magnitude in the opposite direction acting on the body."

But hey, what would Dr Wolf know?

Unfortunately, a lot of what Aerohead so steadfastly believes is simply wrong, as some simple on-car measurements very quickly shows. It's just a crying shame that Aerohead has so effectively mislead so many people here for so long.

(*The opposite happens with concave surfaces - high pressures are developed.)

1) The Coanda Effect has nothing to do with mass- produced passenger cars.
2) Nor Newton's 'Impact Theory', if that's where Dr. Wolf is going. Hucho addresses this specifically in his 2nd-Edition.
3) On the forebody of an automobile, the air is held against the body as the vehicle attacks the air.
4) Once past the maximum cross-section of the body, there's no reason for the air to remain attached, as it's now in a hostile pressure gradient.
5) If the this pressure gradient exceeds a particular, 'critical' ( in the words of P. W. Bearman ),adverse pressure gradient threshold, it will trigger turbulent boundary layer separation, in accord with Dr. Hermann Schlicting's boundary layer theory. ( W.H. Hucho was assistant to Dr. Schlicting at the Technical University of Braunschweig from 1961-to-1968 ).
6) Only a 'streamlined' aft-body contour can produce the sub-critical pressure gradient which will guarantee attached flow for the total length of the aft-body.
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As the flow continues rearwards it is simultaneously gaining pressure as it loses velocity. If the flow remains attached all the way, when it does finally detach at the trailing edge, it will be at its lowest velocity, highest pressure over the rear upper surface and sides.
By extending the length of the body, along the streamlined contour, at 'full' length ( lang-heck ) the pressure will be the highest attainable, drag the lowest, and minimum lift.
If one wishes to induce direct downforce by means of a rear spoiler, for high-speed operation, then this is recommended practice. Or one may simply build downforce into the car's sheetmetal, as with the new, 2021, Maserati MC20 supercar.
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A reading of U.S. Patent # 4,533,168 will disabuse you of blanket statements about the virtues of 'concave' geometry.

[kach22i]

Originally Posted by aerohead

If one wishes to induce direct downforce by means of a rear spoiler, for high-speed operation, then this is recommended practice. Or one may simply build downforce into the car's sheetmetal, as with the new, 2021, Maserati MC20 supercar.

If there is a better reference article, someone please post it.

Maserati MC20 Refined Aerodynamics
https://www.invetr.com/dynamics-and-...d-aerodynamics

Maserati Innovation Lab engineers achieve these goals with the aid of state-of-the-art engineering techniques in both simulations and test methods:

Computational Fluid Dynamics (CFD)
Used in all development phases, it allows testing of multiple geometrical configurations in short times, monitoring a large number of physical parameters through complete analysis of the flow field.

Full size wind tunnel (FCA Orbassano)
Used in the detailed design phase, it reproduces the motion of the real air flow around the vehicle, enabling optimisation of even small-size components.

Scale wind tunnel (Dallara)
Used in the preliminary design phase, it supplies initial experimental feedback and guides development along the correct lines through in-depth analysis of aerodynamics in various vehicle configurations.

[aerohead]

Originally Posted by AeroMcAeroFace

All I can reasonably take from that is, the template is low drag, especially without wheels, and even more so nowhere near the ground.

But actually, reading through the bits of that book I can access, it says "Klemperer (1922) recognized that flow over a body of revolution... changed drastically and lost symmetry when the body came close to the ground"

Seems pointless to follow something like that to me when designing cars, which live on the ground.

"Despite their extreme length, flow separates from the rear of streamlined cars. By truncating the rear shortly upstream of the location where separation would take place, shapes of acceptable length were generated with no drag penalty"

So that says to me that using a template may actually increase drag, a bit like extending a kamm-back can be pointless.

1) We're not discussing a body of revolution near the ground with the 'template.'
2) We're discussing a 'half-body' derived from a streamlined body of revolution, which completes the streamlined body in mirror-image of its ground reflection.
3) The streamlined body from which the 'template' is derived has the lowest drag measured, at supercritical Reynolds number, and capability for forward outward vision from the driver's position.
4) If you watched Julian Edgar's video, and have access to Hucho's 2nd-Edition, you'll know that :
* The VW-Blunt Body ( solid border schematic ) is the AST-II contour.
* The VW-Blunt Body ( dashed line schematic ) is Wolfgang Klemperer's 1922 'minivan', of which it's contour is co-opted by Hucho in 1978 for his half-body, and you'll see it used also on the 1937, Jean Andreau' Peugeot 402, and 1971 Renault ALPINE A 110 1600S. Klemperer's body can be thought of as a 'curved' version of the Ahmed body, used in vortex-drag research.
* The VW-Drop Shape ( dotted line border ) has the AST-I roofline.
* The VW-Flow Body ( short-tail ) is very close to the AST-I contour.
* The VW-Flow Body ( long-tail ) Cd 0.14, shown opposite Hucho's nested 5-car image, is also AST-I-esque.
* The AVA ( Gottingen ) Streamline Shape is from the 1938 Schl'o'rwagen. developed under Ludwig Prandtl's perview, developer of the 'Lange' car, of which Porsche borrowed from. Cd 0.186, as with the VW XL1, and Renault Vesta-II of 1987.
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1) Of course, when the body runs out, the flow no longer has a surface of attachment.
2) When the flow does leave a streamline half-body, it's at maximum deceleration, highest pressure, with a drag penalty strictly due to surface friction. Lift should be lowest as well.
3) I've yet to see a body truncation which did not result in a measurable drag increase. I'll ignore the knife-edged Morelli shape of 1976. What they cut off would even allow enough room for a European license plate.
4) The long-tail Kamm car was Cd 0.12- 0.13.
5) The 'length' issue of the long-tail was addressed by Fachsenfeld with his 'extensible' tail technology of the 1930s. Hucho shows it in the chapter on commercial vehicles.
6) On the T-100, along with a half-tonneau cover, and rooftop cargo carrier in the bed of the truck, this inflatable tail helped pushed the Toyota to over 30-mpg on the open road.
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To get drag lower than the 'template' requires very small vehicles, going so slow, ( 15-mph average ) that they remain at subcritical Reynolds number.
The lowest drag car reported so far, the 2015, ECORUNNER- V, by Delft Technical University, Cd 0.0512, even uses the AST-II for most of it's roofline.

[aerohead]

Originally Posted by JulianEdgar

It's amazing how to a hammer, everything looks like a nail...

The Leaf has far better undercar aero than the Prius - something that perhaps might explain its lower lift? You seem to be forgetting half of the body shape!

As for the Taycan having upper body separation - you are joking, aren't you? Where's your evidence for that - comparison with your make-believe ultimate template? If so many people hadn't been sucked into believing such rubbish, it would be just laughable.

It's really obvious that no matter what anyone shows through testing, or whatever anyone shows through technical references, or what anyone says in citing current car aerodynamicists, you will not change your views. To be honest, that is just gobsmackingly arrogant.

* It 'might? 'perhaps'?
* Is the LEAF underside also superior to the Tesla Model S?
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* As to the Taycan, there's a probability that it experiences the same phenomena as the 2010 Audi A7. And for exactly the same reason.
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* As to the make-believe, ultimate template. It's simply a tool for non-millionaires, used to create cars like the Insight and Prius, both lineal descendants of the FKFS 'templates', which themselves were lineal descendants of Walter E. Lay's 'templates' of 1933, which took their inspiration from Jaray and Klemperer's research ' templates' of 1922.............. Otto Lilianthal's , 1899, Alexandre Gustav Eiffel's, 1898, .............................
* It's very curious that one lauds the progeny of the FKFS, while simultaneously attacking the methods employed giving birth.

[aerohead]

Originally Posted by aardvarcus

Try fitting the AST-II.

I've got the 2009, VW L1 in front of me. It's dead nuts on.
The 2011, XL1, ditto.
This was never intended to be difficult or confusing.
Even though all the templates were derived from 2.5:1 fineness ratio streamline bodies of revolution, some are more conservative / liberal. Just as production cars. Providing choice has obviously muddied the waters. For that I apologize.

[kach22i]

Aerodynamics: The best value of all current Porsche models
https://newsroom.porsche.com/en/prod...ics-18554.html

The Taycan also has active aerodynamics at the rear: depending on the driving situation, the spoiler extends to three different positions and thus influences the car's drag and rear lift. This means it is possible for the new Taycan to be conditioned for best efficiency and range, with the lowest possible drag that is useful on long journeys. It can also be set up with the lowest possible lift on the rear axle to deliver characteristic Porsche sports car performance, for achieving the fastest lap times on a race track.

I wonder if falling short of the aero-template provides additional opportunity to make the rear adjustable spoiler more effective for additional down-force.

VIDEO (different model)

https://www.youtube.com/watch?v=Xv9O3-rvj58

[aerohead]

Originally Posted by AeroMcAeroFace

So there are different templates, and the AST-II is the one to use? Why have people been encouraged to use a different one for the past 10 years?

Does this solve the PRIUS debate? Does that fit the different template? If it does then maybe the template is usable, but on the centreline where flow is front to back, not necessarily elsewhere.

When I published the second contour in 2013, I explained the methodology for the second option.
Something like the 1981 UNI-CAR has a very 'conservative' roofline.
Next year's 2021 Maserati MC20 is especially challenging. It's a more aggressive AST-II profile until it collides with the body at the beltline, then reflexes ( von Mises ) to create direct downforce by extending half-way to the AST-I contour as a sheetmetal spoiler. The 2022 model will include a 195-mph BEV variant, and nobody would dispute the value of added downforce at these velocities, no matter what the static weight of the vehicle is.
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* As to the Prius, I've not re-visited that yet.
* I have done the 1st-gen Insight blueprint. At the trailing edge of the hatch, the glass is within 24.5mm of the AST-II contour. I don't know the roof length off hand. The Mitsubishi Lancer EVO had 19-mm boundary layer thickness at the roof's trailing edge according to one report. If the Honda was similar, it would place the local streamline a smidgeon above the boundary layer, making the TBL a bit 'nervous.'
* The 2nd-gen Insight is dead-on the AST-II.
* 2010 Honda FCX Clarity, ditto.
* 2011 Honda Crosstour, ditto.
* I believe that the 2nd-gen ACURA NSX is also AST-II.

[aerohead]

Originally Posted by kach22i

Would that be Roadkill?

2013
Roadkill Ep. 20 - Chasing a Landspeed Record at Bonneville with a 1100hp Camaro
https://www.hotrod.com/articles/road...1100hp-camaro/


2007
Car Aerodynamics - A2 Wind Tunnel - Wind Camp Tech Theory
https://www.hotrod.com/articles/car-aerodynamics/

That's it! Thanks!

[JulianEdgar]

In an email to me last week, former head of Porsche aerodynamics, Dr Thomas Wolf, puts it this way:

"According to the Coanda-Effect the air follows the curved (upper) surface. According to Newton’s first law, there must be a force which pulls the air downwards. This force is exerted to the air by the car body and pulls the air downwards. According to Newton’s third law (action = reaction) there must be a force of the same magnitude in the opposite direction acting on the body."

Originally Posted by aerohead

1) The Coanda Effect has nothing to do with mass- produced passenger cars.

There he goes again - now Dr Thomas Wolf of Porsche is wrong... but of course Aerohead is right!

On my count that's the sixth professional, highly esteemed car aerodynamicist that Aerohead has said is wrong. And people here pay attention to him? That's just amazing.