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Old 02-25-2011, 07:38 PM   #141 (permalink)
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spring effect

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Originally Posted by NeilBlanchard View Post
Hmmm, I missed that part of the discussion, I guess. It is odd to think that the air is "bumped" out of the way and has a spring affect? One would think that a given volume moves out of the way a given distance, and then moves back.

There must be a repelling force between the molecules in the air that is acting as a spring. So, it is not just the static volume of air that gets displaced, but rather the speed and shape of the front that has a lot to do with it.

So, then the shape of the front does have an indirect affect on drag -- blunt front ends "punch" the air harder, and may *improve* the air flow on the trailing side, by creating higher pressure? But at high(er) speeds, this leading pressure wave becomes critical, and the front must be pointy to split the air, rather than "punch" it?
I think 'spring' is a pretty good analogy in this perverted sense.The atmosphere is a valve-lifter and spring pushing against a camshaft,or breaker block in an old distributor.
There is a rpm at which the spring is unable to close the valve or maintain contact between the breaker-points rubbing block and distributor cam,and you'll have 'float.'
If the body of a car's aft-body drops off too suddenly,the air 'spring' of the atmosphere will be unable to prevent 'float' resulting in separation.

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Old 02-25-2011, 08:19 PM   #142 (permalink)
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Nice rule

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Originally Posted by aerohead View Post
Neil,once the boundary layer goes over to turbulent the Cd is fixed for a body up until around 250-mph for cars,when compressibility issues come into play. Angles, good at any sub-transonic velocity will not alter drag anywhere within this velocity domain.
In fluids,the barometric pressure will affect air density,which is part of the drag equation.
The pressure drag,which is what we're going after,is ruled by separation.If you can eliminate separation you've eliminated pressure drag.
If you get an angle which will work at 25 mph,she'll still be good at 225.
That is a nice rule of thumb. The 25-250 mph regime is good to know. I thought that was about the lower bound but had no idea about the upper bound. Thanks, Phil!
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Old 02-26-2011, 05:48 PM   #143 (permalink)
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While doing some overlay work, I've noticed that the template is actually tilted slightly. The base isn't quite level. The base of the template is tilting up at the rear by about .6 degrees, according to GIMP's measure tool.

Here's the rotated image:



I found the tilt when overlaying the template to a 3rd gen Prius, since I'm using it as inspiration for my Aspire project. I'm thinking if the Prius goes a bit steeper than the template and still manages good flow, maybe I can get away with something similar.

The image of the Prius wasn't level either, but I tweaked the images to work. I didn't make it level, since it was making it look distorted.

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Old 02-26-2011, 06:08 PM   #144 (permalink)
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AeroModder -

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Originally Posted by AeroModder View Post
While doing some overlay work, I've noticed that the template is actually tilted slightly. The base isn't quite level. The base of the template is tilting up at the rear by about .6 degrees, according to GIMP's measure tool.

Here's the rotated image:

I already took that into account in earlier posts. Here's post #119 with a super-clean image for transparency overlays :

http://ecomodder.com/forum/220442-post119.html


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Old 02-26-2011, 06:34 PM   #145 (permalink)
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Sorry, I missed that. I thought that it was just the numbers that were cleared up.
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Old 02-26-2011, 11:29 PM   #146 (permalink)
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Here's an interesting comparison:

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Old 02-27-2011, 09:53 AM   #147 (permalink)
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OK, so I've been scanning through this thread and need some answers to help my brain wrap around this streamline template thing so I can do some figuring with my truck......

Is this template from post 119(as shown by cfg83) the correct/best to use for a side shot?



Is this template from post 95(the double one) the correct/best to use to look at from the top?



What software do I need to use to be able to make this happen? and can it be downloaded from cnet.com(I've read good things about that website)

Thanks for any help...
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Old 02-27-2011, 01:03 PM   #148 (permalink)
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The double one is pretty much taking a full teardrop and adding the ground in straight down the middle. It's used in the same way. The newer template is edited so it's easier to use and understand.

As for software, I use GIMP, a free art program. It takes a bit of getting used to, but it's quite good.
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Old 02-27-2011, 02:40 PM   #149 (permalink)
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LUVMY02CREW -

Quote:
OK, so I've been scanning through this thread and need some answers to help my brain wrap around this streamline template thing so I can do some figuring with my truck......
That is a good question. I think that aerohead should pick from among *all* of our templates, and a moderator can modify the first post to show *the* best image for the job.

In terms of the image I created, the main benefit is that it has the 0.6 degree rotation/correction that aeromodder pointed out and I reduced it down to a 2-color image. That means it works well in a transparent overlay.

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Old 02-27-2011, 09:27 PM   #150 (permalink)
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AeroModder
- Just to be sure I'm not confused, would you please clarify what you meant by "The newer template is edited"...was it the double template that I reposted or another one that I missed????
- AND thanks about the "GIMP" software, I looked into that a bit earlier and that might be the way I go...free stuff is great

cfg83
- I agree that it would be helpful if aerohead and the mods could update and clarify which to use. Everyone "reading off the same page" can only be a good thing in the long run.

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