I want to build a 100-mpg car (and I want it to look vintage) - Page 13 - Fuel Economy, Hypermiling, EcoModding News and Forum

Grant-53 · 10-06-2020, 11:30 PM

Like aerodynamics, crash safety depends on the interaction of a number of factors. Material selection and shell geometry is key. A template is a starting point for experimentation much like a review of previous designs. That is why people do testing. Modern computer simulations can help but at some point there is a need to road test thoroughly. When i was a 12 year old boy my father took me to an auto show in the city. There was the car and designer of a land speed record jet car. I asked Art Arfons what was the most difficult part of building such a vehicle. He said raising the money.

aerohead · 10-09-2020, 04:09 PM

Quote:

Originally Posted by JulianEdgar

Happy to talk about the Insight, even though it's a typical red herring to the argument.

1. The Insight has a sharper downwards rear angle than The Template, and so according to Aerohead, it should have separated flow across the hatch.

2. Of course it doesn't - The Template cannot be used to predict separated / attached flow.

3. According to Aerohead, rear spoilers 'reach up' to separated flow. Of course, that applies only in very old cars with separated flow on the back, not a modern shaped car like the Insight.

4. So, according to Aerohead's theories, the spoiler on the back of the Insight cannot work - its rear edge is no higher than the standard rear lip.

5. But of course, it does work - it's operating in attached flow. (Just as with the recent, properly measured, data on the Prius and its trialled rear lip*.) It works by changing attached flow direction, not creating flow attachment.

6. If I'd followed the theories of Aerohead and The Template, the spoiler would look nothing like it does. A spoiler made to his theory of reaching up would also have increased drag with its larger wake.

You're quite wrong. I am - in your words - 'trashing' incorrect theories and misleading advice.

Would you rather people were continually led astray?

* testing that of course Aerohead immediately denigrated.

1) according to Hucho the flow over the aft-body is technically compromised.
2) the 'template' can be used as a Go NoGo for attached flow.
3) in virtually every case for the non-high-performance car spoilers depicted in Hucho's 2nd-Edition, they were located in a region of separated flow. The 1st-gen Insight is compromised, for the reasons explained by Hucho.
4) the rear spoiler you've put on the Insight functions for the exact reasons as spelled out by Hucho.
5) I believe that your 'attached flow' nomenclature is amiss. There's an extremely high probability that you've mistaken 'downwash' for 'attached flow.' And for the exact reasons Hucho spells out. The sub-template Prius measured in Don Sherman's DRAG QUEENS had higher rear lift than the on-template LEAF. The sub-template, Mercedes-Benz notchback CLA 250 had the highest rear lift of the five cars measured.
6) a larger wake is of no greater drag if its base pressure is higher. For exactly the reasons as explained by Hucho. And until you master boundary layer theory, there's no way you'll ever understand the underlying premise for the template, and how it can be the canary in the coal mine of separation. Again, for the very reasons Hucho gave it to us. You may never get it. As mentioned by others, we throw buckets full of dots your way, and you remain incapable of connecting any of them. It's like terminal perspicacity - deficit disorder.
Please get a hold of Hucho and ask him about Hermann Schlicting's body of work.

Vman455 · 10-09-2020, 05:23 PM

Quote:

Originally Posted by aerohead

1) according to Hucho the flow over the aft-body is technically compromised.
2) the 'template' can be used as a Go NoGo for attached flow.
3) in virtually every case for the non-high-performance car spoilers depicted in Hucho's 2nd-Edition, they were located in a region of separated flow. The 1st-gen Insight is compromised, for the reasons explained by Hucho.
4) the rear spoiler you've put on the Insight functions for the exact reasons as spelled out by Hucho.
5) I believe that your 'attached flow' nomenclature is amiss. There's an extremely high probability that you've mistaken 'downwash' for 'attached flow.' And for the exact reasons Hucho spells out. The sub-template Prius measured in Don Sherman's DRAG QUEENS had higher rear lift than the on-template LEAF. The sub-template, Mercedes-Benz notchback CLA 250 had the highest rear lift of the five cars measured.
6) a larger wake is of no greater drag if its base pressure is higher. For exactly the reasons as explained by Hucho. And until you master boundary layer theory, there's no way you'll ever understand the underlying premise for the template, and how it can be the canary in the coal mine of separation. Again, for the very reasons Hucho gave it to us. You may never get it. As mentioned by others, we throw buckets full of dots your way, and you remain incapable of connecting any of them. It's like terminal perspicacity - deficit disorder.
Please get a hold of Hucho and ask him about Hermann Schlicting's body of work.

This is ridiculous. Hucho didn't "give" us a template! Here, direct from a 2010 paper Hucho posted on his own website:

Quote:

For moderate angles of inclination of the tail slope, a minimum is established for the resistance; the tip vortices are weak. The minimum is very flat and thus gives the designer a certain amount of leeway when choosing the angle of inclination. The modern hatchbacks are all in this flat area.

His example of cars with attached flow due to vortex-induced downwash but high drag? Extreme angles such as this, yet with clearly attached tufts:

Which looks nothing like modern hatchbacks such as the Prius and Insight I.

As far as Hucho's depictions of spoilers, I'll say it again: You have done yourself a tremendous disservice by using such an old edition of "his" book (I use quotation marks because he was adamant that it is not "his"). The last edition he edited, published in 1998*, has several pages on spoilers fitted to production cars in attached flow:

This despite your often-repeated claim that Hucho wasn't concerned with lift. Note that the car referenced here, the 1988 VW Corrado, was still in development when the 2nd edition was published.

He also devotes several pages, with vector diagrams of the flow behind various car shapes, to vortex shedding from the rear.

If this was all as simple as using a template to predict flow, there would have been no need for a 900+ page book.

* I'm thinking about dropping the money to buy the newest 2012 edition since this one is now so far out of date.

JulianEdgar · 10-09-2020, 06:03 PM

Quote:

Originally Posted by Vman455

This is ridiculous. Hucho didn't "give" us a template! Here, direct from a 2010 paper Hucho posted on his own website:

His example of cars with attached flow due to vortex-induced downwash but high drag? Extreme angles such as this, yet with clearly attached tufts:

Which looks nothing like modern hatchbacks such as the Prius and Insight I.

As far as Hucho's depictions of spoilers, I'll say it again: You have done yourself a tremendous disservice by using such an old edition of "his" book (I use quotation marks because he was adamant that it is not "his"). The last edition he edited, published in 1998*, has several pages on spoilers fitted to production cars in attached flow:

This despite your often-repeated claim that Hucho wasn't concerned with lift. Note that the car referenced here, the 1988 VW Corrado, was still in development when the 2nd edition was published.

He also devotes several pages, with vector diagrams of the flow behind various car shapes, to vortex shedding from the rear.

If this was all as simple as using a template to predict flow, there would have been no need for a 900+ page book.

* I'm thinking about dropping the money to buy the newest 2012 edition since this one is now so far out of date.

Glad someone else is challenging Aerohead's endless BS in this area. Aerohead is completely wrong in his beliefs about airflow over the rear half of any typical car shape (squareback, fastback or sedan) of the last few decades, and seems incapable of realising this. As I have said, and as you and I have easily shown with tuft and pressure testing, it takes only a few short tests to show how incorrect his theories are. (Or just read any aero text of the last few decades!)

Re the Insight: rather than just theorising out of a book, I actually did a major amount of testing and development and so I can say with certainty (not just guesswork) that:

1. The Insight has attached flow over the rear hatch in standard form (tuft testing).

2. Tested rear spoilers that increased wake size increased drag (throttle-stop testing).

3. The Insight's current spoiler, that has a trailing edge no higher than the standard Insight lip, is working in attached flow (tuft testing).

4. The Insight's current spoiler reduces drag (throttle-stop testing) and increases pressures on the rear hatch (direct pressure measurement).

To reiterate what I said earlier: had I followed Aerohead's advice and theories on rear spoilers, my actual on-road testing (not guesswork) shows that I would have gone backwards with the Insight.

Aerohead is spreading misinformation and people putting their faith in his statements (not all, but most) to guide their car modifications are going to get poorer results than they should. We're not just talking about an abstract theoretical argument - we're talking about flat-out wrong theories and guidance!

Re the 2012 edition - save instead for the fifth edition (2016) edited by Thomas Schuetz. It's huge (over 1200 pages) and unfortunately expensive, but keep looking and it does turn up at less than the SAE new-book price.