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Old 01-27-2013, 03:47 PM   #451 (permalink)
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Quote:
Originally Posted by sgtlethargic View Post
This thread is pretty spread out. I might reread it and try to decipher it. I'm craving a concise summary.

Regarding the front end- I'm looking at the template as if I was going to build a car from scratch. I may try to model it using CAD software.

It seems there would've been text instructions to go along with the diagram in the original source.
The original source is Aerohead, he has compiled a vast knowledge through his studies and practical experience in the field.
It is a composite fulfilling a number of aero guidelines and as he has said it is a quick and safe guide to build a boattail that works, it's not the only shape and maybe not even the best, but it is tested and it works.

If you look back a few pages, 4, 5, 6? I was looking for the original source and an equation so I could easily produce dimensions, the best I came up with was the NACA 0039 airfoil, very close to template, but not exactly the same.

There really is no such thing as concise when you want to know how and why something works, because concise is not complete, so you still end up asking all the questions to get to a complete understanding.

If you want to do some serious aero work, you will need to gain a complete understanding of the template form, even if you do not use it in the end, you will still need to understand it fully so that you can make informed decisions in your design process.

Short cuts always take longer in the end.

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Old 01-27-2013, 04:02 PM   #452 (permalink)
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Quote:
Originally Posted by aerohead View Post
Your concern for crosswind stability is something which has attracted attention since the 1920s.
Depending upon the position of a cars center of gravity,and center of pressure,gusts and crosswinds could destabilize a car at speed.
Professor Kamm was obsessed with stability,and his last project,around 1966,was with a DKW sedan which he modified to a K-form roofline.Kamm added his signature dual,split-fin stabilizers (Egyptian round-hulled sailing ships used these 'steering-rudders' 4,500 years ago) to guarantee crosswind stability.
Ten years after Kamm's DKW,Professor Morelli determined that Jaray's downward sloping nose,rounded fenders,and Von Mises' reflexed-camber tail could easily mitigate any crosswind funny business.
General Motors Research Labs ascertained that in the United States,we experience an annual mean-averaged 7-mph crosswind condition during driving.
On My trip to Bonneville last September,I drove through a storm front in the Texas panhandle which killed a family of five.I maintained 70 mph through 50 mph crosswind unabated until I got into Santa Rosa ,New Mexico that night.Sure,at times it was 'busy',but at no time did I feel endangered.No one else kept up with me.
My opinion is that a boat tail is zero liability.
I have noticed in my lupo 3L that it more unstable at highspeeds 80-100mph in crosswinds with 800mm long kammback spoiler. On slower speeds no problems whatsoever. My car is light about 900 kg with driver and small. Without the spoiler the car goes like in tracks but with the spoiler I have to make small corrections every now and then specially in open sections and bridges etc. Its nothing that you cannot correct at least in 100mph speeds if you have done some rallying but you need to be more alert on windy days.

Just to give more info I will chop my lupo roof according to the template .
https://www.facebook.com/photo.php?f...type=3&theater

So just to be sure is that template assembled correctly?

Now that means that I will lose about 72mm total height and I need to cut about 122mm from the back to get correct angle for the roof. So I will cut the a pillar corners and tilt the whole roof from there downward...
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Old 01-27-2013, 05:11 PM   #453 (permalink)
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sgtlethargic -- I think re-reading all that would just be to dig the hole deeper. Reposting the referenced templates with red arrows attached would make the questions easier to follow.

How about this: Bottom line The Template is a developed prolate hemisphereoid. All the good and bad things devolve from that: Less subsonic noise, but poor payload packaging and nothing said about the interface with the road*. I think the template is like the geodesic dome; a mathematical purity that when placed in the the real, physical, world dirt starts to cling to. And you know what that does to laminar flow.

Vekke -- Before you go cutting it, look at the GTI W12-650:


*Edit: I want to distinguish aerohead's contributions from the teardrop mirrored into the road surface.

Last edited by freebeard; 01-27-2013 at 06:02 PM..
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Old 01-27-2013, 05:14 PM   #454 (permalink)
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Originally Posted by Sven7 View Post
.....Yes, the telescoping would need some soft portion but I was thinking the shells would be hard- ABS or fiberglass. You could actuate them any number of ways, one of which could be to pull it out by hand before you drive on the highway or push it in after you park. Some latches would keep it in place and rubber flanges/gap fillers would keep the wind out.

I think the side wind issue is that you get more "frontal" area on the sides. Since that area is behind the rear axle it tends to use your entire car as a lever... and the wind has the mechanical advantage! I noticed this with the Anal Probe (40"/1m boattail). It tended to want to turn itself slightly when hit by a strong crosswind.

If you want to run a trailer you may have a hard time making up for the extra rolling resistance, weight, etc. but it has been done. Aerohead did it on his T-100 and there's surely a very detailed thread about it somewhere
Still floating ideas around for the expandable type short boattail, does seem to need a solid skin, or reinforcing underneath to get a semi solid skin, which may be even better, like fish and birds, to allow some dampened pressure movement, but not oscillations.

As for the trailer idea, it is primarily as a trailer for utility that is aerodynamic, not the boattail. I did read Aeroheads entire trailer thread, an epic story like War and Peace, or as I said elsewhere "Gone with the Wind"

I am looking at a variety of options, so my current thoughts are permanent framework on rear of vehicle with short Kammback/start of boat tail. Then depending on requirements I could attach to this either hitch mounted or inflatable medium boattail or go even further when I need to use the trailer.
Sort of a multipurpose aero module plug in system.
So utility is the primary factor and trying to design some aero for a side benefit.

Quote:
Originally Posted by Vekke View Post
I have noticed in my lupo 3L that it more unstable at highspeeds 80-100mph in crosswinds with 800mm long kammback spoiler. On slower speeds no problems whatsoever. My car is light about 900 kg with driver and small. Without the spoiler the car goes like in tracks but with the spoiler I have to make small corrections every now and then specially in open sections and bridges etc. Its nothing that you cannot correct at least in 100mph speeds if you have done some rallying but you need to be more alert on windy days.
There have been a number of reports of crosswind issues, but as many or more of increased stability, this area is worth more investigation to try to nut out the differences and identify potential pitfalls.
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Old 01-27-2013, 06:07 PM   #455 (permalink)
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Quote:
I am looking at a variety of options, so my current thoughts are permanent framework on rear of vehicle with short Kammback/start of boat tail. Then depending on requirements I could attach to this either hitch mounted or inflatable medium boattail or go even further when I need to use the trailer.
Sort of a multipurpose aero module plug in system.
Forget the hitch, go for the bumper brackets.


Quote:
As for the trailer idea, it is primarily as a trailer for utility that is aerodynamic, not the boattail.
So start the thread already. Did I post 'the Rosewell' yet?

Last edited by freebeard; 01-27-2013 at 06:17 PM..
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Old 01-27-2013, 06:44 PM   #456 (permalink)
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Originally Posted by freebeard View Post
Forget the hitch, go for the bumper brackets.


Have had a look at your VW pic's and did take into account the bumper mount, but will use that for the short Kamm idea, the hitch thing is to go beyond that, and the advantages it has is fitted lights, legality etc.


Quote:
Originally Posted by freebeard View Post
So start the thread already. Did I post 'the Rosewell' yet?
Oh, but I am just mere "Grasshopper" and I have much to learn yet.
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Old 01-27-2013, 07:08 PM   #457 (permalink)
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Originally Posted by freebeard View Post
sgtlethargic -- I think re-reading all that would just be to dig the hole deeper. Reposting the referenced templates with red arrows attached would make the questions easier to follow.
...
I agree with the deeper sentiment. I was thinking about posting pictures when I first asked-- I will now.

#1: The "meat" of the template; profile (side) view; for adding a boattail to a vehicle. To use: Line up the "max. roof camber" point with the tallest part of the vehicle body, adjust (scale) size of template #1 to line up the ground planes of the template and the vehicle.

#2: The top or plan view template; to shape the sides of the vehicle. To use: On a top view of the vehicle, line up with roof as in #1, adjust (scale) dimension "d" on template #2 to width of vehicle (make sure to scale the length proportionally).

#3: How is "L" found, what is it related to?

#4: This is what I refer to in post #448 when asking, "What does the other (smaller than the outermost airfoil, larger than the vehicle; not dimensioned) airfoil represent?". This was pretty well answered in post #447, "The template is half of a symetrical airfoil, top half represents a vehicle on the ground, because of ground effect the air acts as if the same image is mirrored below ground."

#5: This is what I refer to in post #448 when asking, "What does the outermost airfoil shape represent?" I still don't understand this. I do see that the overall length equals 2.5 d.

#6: This is #1 through 5 lined up in order from top to bottom to try to give an overview of my mess. The scales should not have changed. I tried to align them vertically (blue and red lines) but they're off some.

Please pardon the drawings. Hopefully this helps clarify things.
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Old 01-27-2013, 08:13 PM   #458 (permalink)
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Quote:
Originally Posted by sgtlethargic View Post
I agree with the deeper sentiment. I was thinking about posting pictures when I first asked-- I will now.

#1: The "meat" of the template; profile (side) view; for adding a boattail to a vehicle. To use: Line up the "max. roof camber" point with the tallest part of the vehicle body, adjust (scale) size of template #1 to line up the ground planes of the template and the vehicle.

#2: The top or plan view template; to shape the sides of the vehicle. To use: On a top view of the vehicle, line up with roof as in #1, adjust (scale) dimension "d" on template #2 to width of vehicle (make sure to scale the length proportionally).

#3: How is "L" found, what is it related to?

#4: This is what I refer to in post #448 when asking, "What does the other (smaller than the outermost airfoil, larger than the vehicle; not dimensioned) airfoil represent?". This was pretty well answered in post #447, "The template is half of a symetrical airfoil, top half represents a vehicle on the ground, because of ground effect the air acts as if the same image is mirrored below ground."

#5: This is what I refer to in post #448 when asking, "What does the outermost airfoil shape represent?" I still don't understand this. I do see that the overall length equals 2.5 d.

#6: This is #1 through 5 lined up in order from top to bottom to try to give an overview of my mess. The scales should not have changed. I tried to align them vertically (blue and red lines) but they're off some.

Please pardon the drawings. Hopefully this helps clarify things.
Hope I haven't missed something, but you only have a couple of questions,

#3, this is about L, L is determined by the vehicle dimension, think I mentioned it earlier, it is 1.78d. so for width it is 1.78 x Width and for height from ground it is 1.78 x 2 x height, because of ground mirror, so the two may well end up as different lengths, then you do your best to merge side taper with top taper in construction.

#5 The outermost shape is the template profile, the inner sketchings of vehicle front and wheels is what Aerohead has drawn in to represent what an ideal vehicle may look like, this has taken into account other knowledge of aerodynamics and performance in crosswinds etc.

So the template size is governed by the dimension you are applying it to, the 2.5d is for the full length of airfoil/symetrical body in rotation, but for boattail purposes you only need to work with the 1.78d as this is the length from max camber (0) to tip of tail on the ground (100%).

Hope this gets you another step of the way.
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Old 01-27-2013, 09:30 PM   #459 (permalink)
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Quote:
#3, this is about L, L is determined by the vehicle dimension, think I mentioned it earlier, it is 1.78d.
The only thing I would add is; vehicle dimension before any truncations.



Maybe this will help. Fatter or skinnier still works, 2.5 is the sweet spot where two curves cross.

Here's the one I use. It's hard to make out the date but I think it's 2011.



Includes instructions on how to handle the top vs the bottom. Doesn't address combination forms.
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Old 01-27-2013, 10:39 PM   #460 (permalink)
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Quote:
Originally Posted by aerohead View Post
MN,if you were above the Template looking down,the front of the vehicle and its sides would have the same 'roundness' and curvature as the side elevation view of the Template,just like an NFL regulation football viewed from any 'side.' The windshield would have compound 3-dimensional curvature as on Sunraycer and the sides would taper to the rear just like the Hindenburg.
Based on this statement, I now believe that the template application from the top view would fall somewhere in between my full width application and the base line in the center & mirrored application.


Bigger Versionto Oogle
Or


Bigger Version to Ponder

The baseline center thing just felt way too fast, but my using the full width of the car didn't seem to make sense either in that it seemed long. I think using the car height and applying it to the sides makes sense because the air will be closing in at the same relative speed thus reducing the chance of a pressure differential which would cause the air to spin.

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