Want low drag? Don't follow a template.

[aardvarcus]

Originally Posted by JulianEdgar

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.

I see you are ignoring the part of my post about not using the AST-II template, which fits several of your example cars in your video per my overlay.

Apologies I did not precisely define all terms in my question.

I do not understand your correlation of a photographed wake size to drag, as a photograph can not lend insight into how the airflows that become the wake are exerting pressure on the vehicle.

I realize that your post acknowledges the effects of the steep extension not aligning with this expected result of smaller wake equals lower drag.

I am aware of what is in your aero book, seeing as how I own a copy. Care to comment on why there are so many examples of modifications made to match "the template" in the DIY section?

[JulianEdgar]

Originally Posted by aardvarcus

I see you are ignoring the part of my post about not using the AST-II template, which fits several of your example cars in your video per my overlay.

Why pick that one? Why not pick one of the five differently shaped ones shown in the video? No doubt we could find lots of cars that match those five too. So what does all that mean? Absolutely nothing. Using a template - any template - is just absurd. If only it were as simple as following a template....

I do not understand your correlation of a photographed wake size to drag, as a photograph can not lend insight into how the airflows that become the wake are exerting pressure on the vehicle.

The angled extension reduces the wake size, as evidence by the dust. Typically, a reduced wake size = reduced drag. As you say that is not definitive, but it's a guide that is right most of the time. But by all means do some pressure testing x area and let us know the results - I don't have a monopoly on testing.

I am aware of what is in your aero book, seeing as how I own a copy. Care to comment on why there are so many examples of modifications made to match "the template" in the DIY section?

I am not sure what you mean? None of my modifications in the book were done following a template (any template) and no template is mentioned (let alone lauded) in my description of any modifications in the book. That reflects the professional literature, where 'pure shapes' are given typically only a few pages in a full book - they're simply not that important in the real world. Reading only professional literature and doing my own testing, I'd never even heard of a template when I wrote the book. Had I done so, I would have spent some pages debunking the approach.

Basically, people here have been sucked-in over a long period of time by a completely fallacious approach, vehemently argued by someone who hasn't even read a professional aero textbook more recent than 1987.

As I said in the video, you need do only the most rudimentary testing on a variety of cars to see that much of what is said here about the template is just rubbish. It's no coincidence that those people here who have actually done that testing are also those that most question the validity of the template approach.

[freebeard]

Fair enough. I'm down to one Beetle anyway. It's still a pile of parts, I'll have to come up with my own d*mn plan.

If I had your resources I could test a Coanda nozzle and vent the exhaust into the wake of the rear tires. Just to see what happens.

I wondered about the yellow arrow as well. It appears to be a normal, but I suspect you intended a resultant vector.

Are you assuming Ecomodder is always and only about templates? Maybe you should get out more.

[JulianEdgar]

Originally Posted by freebeard

Fair enough. I'm down to one Beetle anyway. It's still a pile of parts, I'll have to come up with my own d*mn plan.

If I had your resources I could test a Coanda nozzle and vent the exhaust into the wake of the rear tires. Just to see what happens.

My aerodynamic test resources cost very little.

But to be fair, my aerodynamics reference materials cost more (spread over 25 years) and my home workshop cost a lot (spread over 35 years).

But what you are describing shouldn't cost much.

All car modification has costs, so I figure if you're broke it's not a good hobby to want to pursue! These days, I spend $1000 a year on modifying my Insight. Bought the new lower ratios gearbox about a year ago, and just last week bought the clutch and gearbox rebuild kits. So that's the thousand for this year.

I wondered about the yellow arrow as well. It appears to be a normal, but I suspect you intended a resultant vector.

Not sure what you are describing.

Are you assuming Ecomodder is always and only about templates? Maybe you should get out more.

My comments relate only to the aero sub forum. I don't read anything else on the site.

[aardvarcus]

Originally Posted by JulianEdgar

Why pick that one? Why not pick one of the five differently shaped ones shown in the video? No doubt we could find lots of cars that match those five too. So what does all that mean? Absolutely nothing. Using a template - any template - is just absurd. If only it were as simple as following a template....



The angled extension reduces the wake size, as evidence by the dust. Typically, a reduced wake size = reduced drag. As you say that is not definitive, but it's a guide that is right most of the time. But by all means do some pressure testing x area and let us know the results - I don't have a monopoly on testing.



I am not sure what you mean? None of my modifications in the book were done following a template (any template) and no template is mentioned (let alone lauded) in my description of any modifications in the book. That reflects the professional literature, where 'pure shapes' are given typically only a few pages in a full book - they're simply not that important in the real world. Reading only professional literature and doing my own testing, I'd never even heard of a template when I wrote the book. Had I done so, I would have spent some pages debunking the approach.

Basically, people here have been sucked-in over a long period of time by a completely fallacious approach, vehemently argued by someone who hasn't even read a professional aero textbook more recent than 1987.

As I said in the video, you need do only the most rudimentary testing on a variety of cars to see that much of what is said here about the template is just rubbish. It's no coincidence that those people here who have actually done that testing are also those that most question the validity of the template approach.

I would pick "that one" because it is the one most people on here use. Also based on your comments on what you believe it is supposed to represent it would be the more accurate one to use. (Not that I believe the claims asserted of what "the template" means.) I realize the main basis of your video is that "the template" doesn't fit the cars you posted, thus it would be counterproductive should the template start fitting several of them closely.

I have built and tested several "angled extensions" on several vehicles, the results are already documented on this site. Results logged over many thousands of miles. Does this qualify as the most rudimentary testing?

I have several "basic shapes" saved that I use in my planning. Tools in a toolbox for design. I do not have the time or computing/testing resources to develop original aero designs from scratch. I am not hung up on proving any of them right or wrong. I have repeatedly used the AST-II and/or the AST top view and the resulting projects hit my projections for drag change reasonably closely and made the handling changes I desired. Thus I will continue using them.

Since you apparently don't know, FYI there are photographs of "template based" projects in your book. You may want to edit them out in the second edition.

[freebeard]

My aerodynamic test resources cost very little.

My $1K for this year is to own and operate a 1990 XFi. The most recent model year I've owned to date. The Arcimoto FUV will bust my budget, [insert deity] willing.

But what you are describing shouldn't cost much.

The closest I've gotten. It was measured and destructed, I never took it on the road, obviously.

My comments relate only to the aero sub forum. I don't read anything else on the site.

ecomodder.com/forum/instrumentation.html

[JulianEdgar]

Originally Posted by aardvarcus

I would pick "that one" because it is the one most people on here use. Also based on your comments on what you believe it is supposed to represent it would be the more accurate one to use. (Not that I believe the claims asserted of what "the template" means.) I realize the main basis of your video is that "the template" doesn't fit the cars you posted, thus it would be counterproductive should the template start fitting several of them closely.

I am not sure how you reached that conclusion. The 'main basis' of the video is that the template is used here in the following ways (the text is straight from the slide in the video):

It is claimed to allow you to:

- Show where there is separated and attached flow on existing cars
- Guide the shape of rear extensions
- Show how rear spoilers on sedans should be positioned and shaped
- Allow the assessment of the ‘aerodynamic purity’ of cars

All these purported uses are simply rubbish!

I have built and tested several "angled extensions" on several vehicles, the results are already documented on this site. Results logged over many thousands of miles. Does this qualify as the most rudimentary testing?

Yes, sure.

I have several "basic shapes" saved that I use in my planning. Tools in a toolbox for design. I do not have the time or computing/testing resources to develop original aero designs from scratch. I am not hung up on proving any of them right or wrong. I have repeatedly used the AST-II and/or the AST top view and the resulting projects hit my projections for drag change reasonably closely and made the handling changes I desired. Thus I will continue using them.

Sure - but that is completely different from pretending that the template shape can be used for any of the purposes listed above. What you have said is more along the lines of "I fitted 150lb springs front and back and I am happy with the results". Great - but that doesn't mean they gave you the best results, or that other people fitting the same spring rates to their cars will get good results.

[freebeard]

Agreed about your four claims. I think what value the default template has is for canopies, blisters, aerocaps, external hinges & etc. With a higher fineness ratio.

This is as close as I could find a fit for the 'default' template:




A sectioned Type II could be center steer, between the front wheelwells. It fits the hemicircular front profile.

My "Template":



Three-quarters the wheelwell drag, but it needs to be truncated to a box cavity.

[ME_Andy]

Julian, please take a deep breath and DO NOT reply to my post.

We are lucky to have a template from (as I understand it) a retired professor. It's been useful in many cases. I'm sure it's not perfect for all cases but it's the best tool contributed so far.

I would welcome some input from somebody else about evidence or scientific basis behind the template.

Edit: here's the background for the template. https://ecomodder.com/forum/showthre...rt-c-9287.html

Now that disagreement has been expressed I hope we can move on.

[ME_Andy]

Quote:
I placed Sunraycer under the template,it was a perfect match.Bill Watson's airship "White-Dwarf" of 1984 is about 2.57:1 ratio. Watson worked with Burt Hibbs,aerodynamacist for Sunraycer. Both of AeroVironment. This shape seems to end up on some of the most efficient vehicles known in the world,I believe it to be a shoe -in for aero-modding.Those with more advanced aerodynamic toolbags will will no doubt venture out into some of the more exotic shapes,but for amateurs,I believe this form can serve us well.