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Old 08-16-2011, 06:24 PM   #181 (permalink)
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harm

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Originally Posted by Bicycle Bob View Post
A little knowledge is a dangerous thing, and this template is the least amount of information that can make people feel wise about airflow when they are not. I suspect that this thread does more harm than good.
Do you find fault with the supporting documentation for the development of the 'Template'?
This represents 35-years of work.If you have some better science to bring to the table we'd all appreciate it.This is no time to withhold knowledge.Thanks!

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Old 08-16-2011, 06:28 PM   #182 (permalink)
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homework

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Originally Posted by Grant-53 View Post
No substitute for during your homework starting with fluid mechanics basics then proceeding to aircraft design and automotive applications. Check local college libraries and book venders such as Amazon. The Society of Automotive Engineers list reference books and articles for a price. "Race Car Aerodynamics: Design for Speed" by Joseph Katz covers basics for DIY mechanics and has many references at the end of each chapter. Just remember modern race cars are more concerned about downforce than drag.
Is 35-years of homework enough?
And how about 35-years of road vehicle aerodynamics homework.Aircraft design is not germane to the issue of road vehicle aerodynamics.
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Old 08-16-2011, 06:42 PM   #183 (permalink)
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Originally Posted by kach22i View Post
Not to argue, but they look pretty flat to me, at least most of them. Does it have to be all of of the Tuffs laying flat?



EDIT: I do see that none of the Tuffs in the last 18" to the rear point are flat. Is this what you were pointing out?
"Flat" is a relative term, but they all have to be laying against the surface, not lifting up (moving left right is ok, lifting up is not). You can see in the picture that shortly after the extension, tufts are lifting off the surface, first a little, then more and more and more. Tufts lifting off the surface is detached flow, detached flow means no further aero improvement, thus the very little gain by adding the extension (in my humble aerodynamic opinion).
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Old 08-16-2011, 06:48 PM   #184 (permalink)
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Originally Posted by aerohead View Post
Vekke,the underside is covered in Hucho's section on diffusers.
The upsweep angle of the diffuser will depend on where the upsweep originates.
I believe that if before the rear axle,the angle can be as steep as 4-degrees.
If it starts after the rear axle,then 2.5-degrees is the limit.Please check Hucho's work to verify.
The longer diffuser likes a shallower angle, the shorter diffuser likes a steeper angle (thus the reason I had to look at which actually reduced the tail area more). Shorter likes maybe 3.66 degrees the most, longer about 2.66 degrees. But as I said earlier, anything up to 6 deg (long) and 11 deg (short) is an improvement over 0 deg. These numbers may go further, but this is how far Hucho covers in his book.
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Old 08-16-2011, 08:27 PM   #185 (permalink)
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Quote:
Originally Posted by wyatt View Post
Tufts lifting off the surface is detached flow, detached flow means no further aero improvement, thus the very little gain by adding the extension (in my humble aerodynamic opinion).
Even with detached airflow, wouldn't there still be a real benefit to filling in the wake? It wouldn't be optimal, but it looks a lot cleaner than simply leaving a large square hanging out back there to hold you back. I'd expect that "slightly detached" airflow would be a whole lot better than "completely abandoned" airflow. I'm thinking along the lines of "anything that brings you closer to the template helps" with the spoilers.

I'm actually asking a question here (not putting forth an argument)- I'm ignorant enough that I only have aerodynamic questions, not opinions.
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Old 08-16-2011, 09:07 PM   #186 (permalink)
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Quote:
Originally Posted by aerohead View Post
Is 35-years of homework enough?
And how about 35-years of road vehicle aerodynamics homework.Aircraft design is not germane to the issue of road vehicle aerodynamics.
Phil, I think I remember a thread where you derived the template from referenced material. Perhaps we are all getting a little confused at this point. I searched but could not find the thread that I recall. Can you give us a pointer to the thread.

BTW, I don't think anyone is being critical of the template idea, but it is natural among scientists to revisit the sources. After all we all keep picking up Hucho's book. I for one certainly appreciate your efforts to lead the way.
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Old 08-16-2011, 09:49 PM   #187 (permalink)
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I think complications come from having a 2D template with only a side view trying to define an optimal 3D form.

Just about every vehicle on that template pic is very aerodynamically efficient, yet just about every vehicle has a backlight that is steeper than the template. I don't think they'd have the low Cds if they suffered separation at the top of the backlight.

And then there are those like the Rumpler and the Boxfish that are aero but don't conform to the template very well at all.

Still, I like the template because if one uses it they won't go way wrong.
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Old 08-17-2011, 06:00 PM   #188 (permalink)
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Aerodynamics is tricky...

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Originally Posted by Fat Charlie View Post
Even with detached airflow, wouldn't there still be a real benefit to filling in the wake? It wouldn't be optimal, but it looks a lot cleaner than simply leaving a large square hanging out back there to hold you back. I'd expect that "slightly detached" airflow would be a whole lot better than "completely abandoned" airflow. I'm thinking along the lines of "anything that brings you closer to the template helps" with the spoilers.

I'm actually asking a question here (not putting forth an argument)- I'm ignorant enough that I only have aerodynamic questions, not opinions.
It would seem logical that filling in the wake would help, and I would normally agree, but if you make the angle wrong, it hurts. With between 10 and 15 degrees being the "optimal" for low drag, between maybe 20 and 30 degrees makes things worse (with 30 degrees adding a LOT of drag AND lift). After 30 degrees, you go back to having the wind not care, and it's like having a 0 degree back end (thought there is still added lift up to about 60 degrees). I guess what I am saying is, if done incorrectly, you can make things worse. From looking at the pictures of the van, they don't look to have done anything bad, just that it could have been done better. They didn't see much aero-improvement with the extension, and that should have been an indicator. I am guessing that if the corrected the extension to maintain attached flow, the benefit would be noticeable. (This is my aerodynamic opinion, since I can't actually test it.)
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Old 08-17-2011, 06:08 PM   #189 (permalink)
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Zero Lift

Quote:
Originally Posted by Frank Lee View Post
I think complications come from having a 2D template with only a side view trying to define an optimal 3D form.

Just about every vehicle on that template pic is very aerodynamically efficient, yet just about every vehicle has a backlight that is steeper than the template. I don't think they'd have the low Cds if they suffered separation at the top of the backlight.

And then there are those like the Rumpler and the Boxfish that are aero but don't conform to the template very well at all.

Still, I like the template because if one uses it they won't go way wrong.
If I remember correctly (and please do correct me if I am wrong), Aerohead has designed the template to provide a combination of low drag, and zero lift. The other vehicles may not have been as concerned with lift, or may have used other means of overcoming/compensating. For example, the Prius has a "steeper than the template" rear windshield, and to compensate for the lift being generated, there is a flat spoiler on the back. I suspect that the SedanKamm that I put on my car also generated a fair amount of lift, but I wasn't worried about losing control, because the duct tape holding it in place would have given way long before the rear tires came off the ground!
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Old 08-17-2011, 07:13 PM   #190 (permalink)
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Quote:
Originally Posted by Frank Lee View Post
I think complications come from having a 2D template with only a side view trying to define an optimal 3D form.
That's always tricky, as the sides and their interaction with the air going over the car, also contribute to the drag - or lack of it.

Winglet design shows just how much effect this interaction can have - and that's on a design where the basic 2D shape extends out for 30m / 60' or so.


The Schlörwagen shows how it's done though.

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