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Old 11-13-2020, 05:24 PM   #1 (permalink)
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Want low drag? Don't follow a template.

I see Aerohead, completely undaunted, is back to spreading his misconceptions about his beloved template. Here's video I did on that subject a month or so ago.

(If you wish to comment, please watch the entire video, including the citing of five completely differently shaped low-drag templates.)



Using Aerohead's template as the guide in aerodynamic car modification and assessment is, unfortunately, quite absurd.

If only it were that simple!

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Old 11-13-2020, 05:32 PM   #2 (permalink)
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Not again!
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Old 11-13-2020, 05:57 PM   #3 (permalink)
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I enjoyed the video, thank you for posting it.

For reference a post of mine from 2017 questioning some of the same things.

You need to click the link to read the whole post including charts from a another forum reference.

https://ecomodder.com/forum/showthre...s-34840-4.html
Quote:
Originally Posted by kach22i View Post
I agree with what everyone else has been saying about reducing Cd and lift if you can. I'm offering this example for reference because it is so iconic and well tested.

Sourced base image below because I could not find the one out of a book Aerohead scanned into his forum gallery which shows a template overlay.

https://en.wheelsage.org/porsche/911...ctures/o3rqv5/

Then I overlaid the one of the forum's AeroTemplates from here (not sure which page):
http://ecomodder.com/forum/showthrea...ion-21952.html

The combination:
Aerodynamics by George Kachadoorian | Photobucket


This overlay was done to provide some context for the chart quoted below, which shows a lowering of Cd and lowering of lift with ducktail.

Spoilers: front and or rear - Page 2 - Pelican Parts Technical BBS


Going back to one of my first comments about getting the rear spoiler to fill out the template, it looks as the Porsche 911 RS kept it just below that line for some reason unknown to myself.

Maybe your testing can examine what happens when you fill that gap in your comparative Honda Civic project. At what point does the Cd stop being lowered and starts going back up.

If your 3D printer does not allow for curved or wavy top edge of rear spoiler, then try a square notched or comb filter pattern, something like a bread knife edge.

The point would be to see if there is less drag when the vortex coming off the Gurney flap is broken down to smaller elements and is less planar or a constant cylinder of rolling air mass.

836 Gurney flaps - Aerodynamics - Civil Engineering Handbook



Concept of wavy spoiler: Distribution of energy verses concentration of energy, which may help control oscillations.

If one "feathers" the edges as found in nature (bird feathers, whale fin tubercles) just to see what happens, there might be a nice surprise - or not.

Be brave, take a chance. Have no regrets because each failure is just another step in learning, and learning is a life long path - or at least it should be.

EDIT: One more chart.
Spoilers: front and or rear - Pelican Parts Forums


The 3.3 turbo tail is different than the spoiler type Ducktail, it extends out and is sometimes called a Teatray or Whaletail.

Similar to the below shown on a later model.
Spoilers: front and or rear - Pelican Parts Forums


I should note that there is less drag and less lift with the type shown on the red car (tray/wing) verses the earlier green car (duck).

If you go to Bonneville you are more likely to see rear mounted horizontal extensions than near vertical ones for the same reasons. Each one attempts to fill out the Aero-template in it's own way.
With the Jaguar and Tesla shown in the video, is air attachment of tufts absolute proof of the lowest drag possible?

Perhaps the very slight imperfection of a physical medium that has mass/weight and surface area (the yarn tuft) has not been overwhelmed by the ever so slight pressure differences.

It is their own weight that is holding them down (gravity) and not air pressure or air flow?

In a 1930's NCAC video posted earlier this month the demonstrator using a small wind tunnel used a wand with what looked like long stringy bird feathers much lighter and delicate than a yarn tuft, and this allowed him to actually show vortex formation around a cube.

Are yarn tufts infallible and indisputable?

Did these blimp derived shapes of the 1930's use yarn tufts at all to arrive at their shapes or to collect data?

Do present day automobile manufactures with wind tunnels use yarn tufts?

If yarn tufts are 90% accurate, that would leave 10% on the table, room for improvement.

How close to idea are yarn tufts?

EDIT-1

Looks like back in 2001 NASA was using polyester not wool, and glued not taped with very specific procedures and requirements such a syringes to keep the glue glob size down.

PDF Download:
https://www.google.com/url?sa=t&rct=...Ft9VnAkyPjVl77

.................................................. .....................................

EDIT-2



Getting an error on providing a link for the above image, found via Google image search with phrase "aerodynamic tuft accuracy".

I assume it's showing reaction sensitivity or response times of various tuft filaments.
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Chin Spoiler:
http://forums.pelicanparts.com/off-t...effective.html

Rear Spoiler Pick Up Truck
http://forums.pelicanparts.com/off-t...xperiment.html

Roof Wing
http://ecomodder.com/forum/showthrea...1-a-19525.html

Last edited by kach22i; 11-13-2020 at 06:23 PM..
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Old 11-13-2020, 07:43 PM   #4 (permalink)
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Does anyone here have access to a Mercedes A class sedan ?
The car has an excellent .22 Cd, yet not only deviates from the template, but also has a short trunk / boot that you would think would keep flow from reattaching.
I seemed to recall it having quite a bit of tumblehome to the rear, and thought this maybe what helps the flow reattach, but after looking at one that my neighbpr has, I see that is not really such a big factor.

The reason I am asking is to see if someone would be willing to do a tuft test at the rear of the car to show what happens with the airflow. *

The .25 Cd LS-430 had an even steeper back window, but it has a longer trunk / boot that could help the air to reattach.

* or some CFD images of the rear of the car.
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Old 11-13-2020, 08:23 PM   #5 (permalink)
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Mercedes-Benz A-Class sedan has lowest drag-coefficient of any production car
https://www.firstpost.com/tech/auto-...r-4822801.html
Quote:
Mercedes-Benz has announced that its upcoming A-Class sedan will be the most aerodynamic car on sale. The company claims that the new A-Class sedan has a coefficient of drag (Cd) 0.22 and a front area of 2.19 square meters. Mercedes-Benz says the new A-Class sedan's class-leading aerodynamic properties can be attributed to the effort gone into refining the shape of the car. The designers spent countless hours in the wind tunnel while the use of virtual modelling techniques gave further insight into developing an optimal aerodynamic shape.
From the above they did a lot of work where you just don't see it.

https://www.mercedes-benz.com/en/veh...-class-saloon/


Quote:
The new A-Class Saloon takes advantage of the favourable conditions provided by its long rear end to undercut even the already exemplary A-Class with hatchback (Cd value from 0.25). With a benchmark of Cd = 0.22 the new saloon model even equals the original world record of the CLA Coupé. Thanks to the frontal area of 2,19 m², smaller than that of the CLA, the new A-Class Saloon has the lowest aerodynamic drag of all production vehicles worldwide.
Great numbers, but far from theoretical idea.
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1977 Porsche 911s Targa
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Chin Spoiler:
http://forums.pelicanparts.com/off-t...effective.html

Rear Spoiler Pick Up Truck
http://forums.pelicanparts.com/off-t...xperiment.html

Roof Wing
http://ecomodder.com/forum/showthrea...1-a-19525.html
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Old 11-13-2020, 10:02 PM   #6 (permalink)
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Quote:
Originally Posted by IRONICK View Post
Not again!
That is exactly what I thought when I saw Aerohead posting all the same old rubbish. Surely, I thought, you're still not trying to mislead people in this way?!
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Old 11-13-2020, 10:04 PM   #7 (permalink)
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People don't change.

It only took 10:29 to burn my dinner. I think I punched in 4000 on the microwave.

Anyway, I assert you have fallen into the same pitfall as others. Nobody looks at the front view with it's hemicircular profile. It's as hard and fast a rule as the profile. It makes it a proper windshield header for side-by-side seating difficult. VW Beetle has an optimal profile with a flat windshield for optical clarity (ignore the blind spots behind the A-pillars).

Anyways, I made an argument for a superelliptic profile here. I noticed that Blender's geodesic generator has superilleptic options. Maybe I should revisit that.

edit:
I found Ducted radiators for low drag more helpful.

So what do you think of the difference between a velocity driven radiator system vs a power-driven fan and air cooling? IIRC VWs boxer four had 1500cu ft/min displacement to work with.
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Old 11-13-2020, 10:42 PM   #8 (permalink)
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Am I the only one to notice that you aren't even using the current version of "the template" and that some of the pictures of cars in your video fit the AST-II?


Do you really believe what you stated in the video that aerodynamics works off the ratio of photographed dust in a wake balanced against a perpendicular vector of the rear angle?
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Old 11-14-2020, 12:29 AM   #9 (permalink)
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Quote:
Originally Posted by aardvarcus View Post
Do you really believe what you stated in the video that aerodynamics works off the ratio of photographed dust in a wake balanced against a perpendicular vector of the rear angle?
I don't even know what "aerodynamics works off" means....

If it means "aerodynamic drag" then yes, a reduction in wake size (usually giving less drag) is one of the things that needs to be balanced in the rear extension design against the created lift-induced drag component (that gives more drag).

In the case of the photographed Roomster, too steep an extension angle (note: still with attached flow) gave higher measured drag, despite the smaller wake.

Incidentally, that Roomster example is in my aero book, and was specifically cited by three of the professional aerodynamicists who reviewed the book as a good example. In fact Dr Hucho liked the Roomster wake pics so much that he asked me for high res versions.
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Old 11-14-2020, 12:37 AM   #10 (permalink)
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Quote:
Originally Posted by freebeard View Post
People don't change.


edit:
I found Ducted radiators for low drag more helpful.

So what do you think of the difference between a velocity driven radiator system vs a power-driven fan and air cooling? IIRC VWs boxer four had 1500cu ft/min displacement to work with.
[Groan]

Freebeard, I never just guess stuff. I measure things, and I read formal references.

I know it's become de rigueur on this sub-group to knowledgeably pontificate in a context completely absent of any actual evidence, but I am not part of that.

Answer to your question: I have no idea.

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