Want low drag? Don't follow a template.

[AeroMcAeroFace]

Originally Posted by aardvarcus

Since people like analogies so much… This conversation reminds me of something else…

Person A:
Did you know you can heat your house from the sun? South facing windows, size the overhangs to reject summer sun, extra insulation, etcetera.

Person B:
No. I do not believe it. Unless you pay extra for a special house with a special certificate from a governing body regulating low energy construction, no house can be made or modified to be heated by the sun. You can’t mimic details to get there. You cannot copy construction techniques to get there.

Person A:
But wait, it works. I applied some rules of thumb and some hard math and calculated…

Person B:
Rules of thumb don’t work. The sun does not work. You see, not every house is heated by the sun- it must be wrong! Other houses have south windows, overhangs, insulation but they require supplemental heat. Houses can not be heated by the sun. You must have central HVAC, you see all houses have central HVAC. All architects design to have central HVAC.

Person A:
But I actually built one, and it works. See, here are some plans.

Person B:
You might have accidentally found something that worked for you, but it will not work for anyone else. Nobody listen to this other guy, he is wrong. See this plan has 12% glazing and this one has 10% glazing. Inconsistent. They don’t agree. Must be wrong. Everything is wrong. Install central HVAC or I will taunt you some more.

However, Person B is cynical rather than sceptical. If Person A had showed how much heat the sun provides per square metre, showed the suns path through the sky in winter and then in summer, and also said, "it worked for me and I followed these rules, however that doesn't mean it will work for you" then that is fine.


In the UK we have houses like that, but because the sun is so low in the summer the overhangs need to be huge and are often not big enough and prolonged heat and sun has heated the houses to unbearable levels. Just because your rule of thumb works on your latitude, doesn't mean it works everywhere else.

No-one is saying following a template won't work, we are saying it is not always the best.

[aardvarcus]

Originally Posted by AeroMcAeroFace

However, Person B is cynical rather than sceptical. If Person A had showed how much heat the sun provides per square metre, showed the suns path through the sky in winter and then in summer, and also said, "it worked for me and I followed these rules, however that doesn't mean it will work for you" then that is fine.


In the UK we have houses like that, but because the sun is so low in the summer the overhangs need to be huge and are often not big enough and prolonged heat and sun has heated the houses to unbearable levels. Just because your rule of thumb works on your latitude, doesn't mean it works everywhere else.

No-one is saying following a template won't work, we are saying it is not always the best.

Maybe person A was expecting person B to read up on it a bit... not take the first sketch they find and build it.

I am aware of the occasional heat issues. Like when weather acts crazy and it gets summer weather in the winter. Have to block the windows for a few days. Annoying, but beats a $$$ utility bill. Similar to setting an angle for an extension in an aero pickup bed cover, add 1000lbs to the bed, watch that angle change 5 degrees or so. Something to be aware of, but if designed around not a deal breaker.

I would classify "using it to guide the shape is rubbish" and the dozens of other similar comments as synonymous with "won't work" not "isn't ideal."

[JulianEdgar]

Originally Posted by aardvarcus

Even funnier is that of the six cars being used to disprove the "template", five fit the AST-II diagram perfectly and the other one is streamlined from top view, begging the question why you thought it was appropriate to do a side view overlay on it?

Those professional aerodynamicists might not use the diagram, but boy their cars sure like to land on it...

I don't know why you blame the website for your use of your favorite AST-I diagram. Other people have successfully navigated to the prevalently used diagram. Most people click more than one link on websites to find their desired information.

What would be really really funny would be a remake of the video, same cars and same script, using the AST-II.

Well, why don't you make it? I don't have a monopoly on producing YouTube videos.

[JulianEdgar]

Originally Posted by Vekke

- Show where there is separated and attached flow on existing cars

Audi A6 A7 model aero mentioned here many times SAE paper

- Guide the shape of rear extensions

5 of 6 cars in the video match the AST-II template pretty accurate anyone can check themself

- Show how rear spoilers on sedans should be positioned and shaped

Many examples already mentioned lastly the hotrod magazine camaro (ok no a sedan car. Which rear spoiler just touches the AST-II template. First you need to tilt the picture little bit to get it first straigth.

Taycan turbo S also raises its rear spoiler in eco mode to go closer the template. I did not find a side picture to check how high it actually raises.

- Allow the assessment of the ‘aerodynamic purity’ of cars)

This is more tough one to answer but the the most aerodynamic models seem to follow the template even the aerodynamics havent used the template. Like I wrote before you usually cannot know what their limiting factors in the design have been, but the more aerodynamic the car is more likely it happens to follow the template.

Yesterday I checked ligthyear one with Cd under 0,20. Almost perfect match. Sligthly off to give space for rear passenger heads.

Why aerodynamics dont say the use the template--> Does olympic athletics tell what drugs or medizines they use or all the ways they train themself?

No they don`t. In many areas there are trade secrects and common ways how things are done. Maybe aerodynamics want to feel special by making lot of effort to sculpture better shapes in wind tunnel and try different things and then in the end end up exatly the same shape that could have been used in the first place and would have saved a lot of time, money, recources.

Rules of physics don`t change in the wind tunnel or in the road. There is a physical reason behind the template and thats why it works the best.

No remember, earlier you were (rightly) insistent that published scientific research evidence is required.

You have referenced none here - the SAE paper on the Audi does not in any way mention a template of any sort (so you've obviously not read it).

So where is the published evidence showing that a template can be used to do these things:

- 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)

You'd expect the peer reviewed literature to be full of it.

[JulianEdgar]

Originally Posted by aardvarcus

When you are pouring hundreds to thousands of hours into a build, you take these design details seriously. You read literature. You buy books. You do the best you can with the resources you have.

So where in the literature and the books is all this information about the template and applying it in this way?

- 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)

Surely you must be able to quote lots of publications that show all this?

[JulianEdgar]

Originally Posted by kach22i

A wing in free flow and rear spoiler acting with attached flow preceding it will be more effective than wings/rear spoilers in disturbed flow (I think).

Yes, and that's how all modern car rear spoilers work - in attached flow. It's been that way for decades. (This 'reaching up to the template line' is just baloney.)

[JulianEdgar]

Originally Posted by aardvarcus

Since people like analogies so much… This conversation reminds me of something else…

Person A:
Did you know you can heat your house from the sun? South facing windows, size the overhangs to reject summer sun, extra insulation, etcetera.

Person B:
No. I do not believe it. Unless you pay extra for a special house with a special certificate from a governing body regulating low energy construction, no house can be made or modified to be heated by the sun. You can’t mimic details to get there. You cannot copy construction techniques to get there.

Person A:
But wait, it works. I applied some rules of thumb and some hard math and calculated…

Person B:
Rules of thumb don’t work. The sun does not work. You see, not every house is heated by the sun- it must be wrong! Other houses have south windows, overhangs, insulation but they require supplemental heat. Houses can not be heated by the sun. You must have central HVAC, you see all houses have central HVAC. All architects design to have central HVAC.

Person A:
But I actually built one, and it works. See, here are some plans.

Person B:
You might have accidentally found something that worked for you, but it will not work for anyone else. Nobody listen to this other guy, he is wrong. See this plan has 12% glazing and this one has 10% glazing. Inconsistent. They don’t agree. Must be wrong. Everything is wrong. Install central HVAC or I will taunt you some more.

You can use all the analogies you want, but it's best to pick parallels from car modification. And there are plenty - rules of thumb / inappropriate historic parallels / incorrect logic are often used in car modification by people who want simplistic answers to complex questions. Here are some parallels to the template:

- Car air/fuel ratios - A lean cruise AFR of 16:1 worked on Fred's car and so that's what I am going to use on my car. Also I've seen that many standard cars use 16:1 so that must be the right value.

- Car ignition timing - That ignition timing map worked really well on the car I used to have and so I am going to cut and paste it for my new engine - it will be great, after all, it was before and that's all the evidence I need.

- Suspension natural frequency - Frederick Lanchester found about 100 years ago that a suspension natural frequency of 1Hz is best for ride comfort so I am going to set my car up with 1Hz natural frequencies. After all, the research was all done 100 years ago!

Subwoofer design - I was listening the other day to John's sub and it sounded great. So I've bought the same driver and I am making a new box for it. The box will be about one-quarter the size of his but I know it will sound good because it's a great driver and I heard it sound good.

Gearbox ratios - I've been looking at what car makers do with their top gear ratios. I found that a ratio of 0.72:1 is really commonly used - it's like it's a 'secret ratio' that all designers know! So I am going to use that ratio because all those designers can't be wrong. What's that? No, I haven't decided on my final drive ratio - does that matter?

...and so on and so on.

Rules of thumb / inappropriate historic parallels / incorrect logic all make for poor car modification approaches.

[aerohead]

Originally Posted by JulianEdgar

I've given you two references.

But what's the point? If they don't agree with you, they're sure to be wrong, aren't they?

If you are referring to the book, and want to use it as prima facie evidence, you'll either have to submit it, as discovery material in your brief, for your kangaroo-court, or write, in your own words, the essence of the material ( a few lines should suffice ), otherwise all I can perceive is, the actions of a coward, hiding under the skirt of a 3rd-party, pretending knowledge.
I give you chapter, verse, page numbers, and publication. I have expectations of reciprocity in a discussion among gentlemen. A term I use loosely sir.

[aerohead]

Originally Posted by JulianEdgar

I don't think that intuition is a good approach to learning about anything much.

In car aero, the Coanda Effect is primarily used to describe the tendency of a stream of fluid to follow the contours of a convex surface rather than continue moving in a straight line.

It is therefore present on all normal cars.

I the very brief fluid mechanics literature, concerning Newtonian physics, there is no mention of Henri Coanda. A satisfactory explanation for the phenomena of a fluid following a convex profile has a fully-vetted logic and history which has no need of a 'Coanda Effect' to affect an understanding.
It's bizarre that Dr. Wolf would waste a comment on it. Although, some PhDs I know, say that it's no big thing to be a PhD. One can be completely wrong in the doctoral dissertation and still receive the degree.

[aerohead]

Originally Posted by JulianEdgar

Mmmm. Yep. Mistaken again - but I am quite certain you won't take the correction. It's just that the textbooks and experts will be wrong again...

An aircraft application of a Coanda-esque device would be the 'blown' slots on the Japanese, Shin Meiwa PS-1 and US-1, STOL aircraft, by Kawanishi. It required only 3,060- horsepower to provide the necessary airflow volume to the wings. Stupid aeronautical engineers! All they needed was to know the illustrious Dr. Wolf and his magic technology.
Let us know how that works out on your car.