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Old 08-16-2012, 05:56 PM   #31 (permalink)
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Sorry, I was out for a while. Thanks for spell checking, all the typos are hopefully fixed now. I made some minor changes, wrote short instructions and I also (finally) gave credit to Aerohead. And I also probably made some more typos.

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Old 08-16-2012, 07:09 PM   #32 (permalink)
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basis

Quote:
Originally Posted by ausias View Post
maybe you will like to play with equation if it is a NACA airfoil.
NACA airfoil - Wikipedia, the free encyclopedia


Is The Air template basis the upper half of a NACA 0025 airfol?
Does this template has almost the same lift (cL, coefficient of lift) than half symetric NACA?
I did some basic calculations on Solidworks CFD Flow Simulation of a wing shapped large van with flat door sides (naca extrusion) went around cd=0.37 (i don't remember lift). FAr better than a a brick shape 3D van but I got very dissapoinnted. I found airfol is very useful, on 2D shape slices to construct a 3D shape. But maybe the 3D rear wake it's wider than taller, so the more important naca shape should be zenital view of the car other than side view if enough air come from under the car to reduce lift or add aerodynamic load.
We are almost at 40șC here, sorry if overheat some brains as well as mine because Aerodynamics is a hobby for me even in summer. so I'm sorry if there are some mistakes in my words.
The 'Template' is based on a 3-D,streamline body of revolution of length/diameter=2.5,Cd 0.04 (free-flight),and Cd 0.08 in ground proximity as a 'half-body,' with ground clearance cut away,and length/height=5.
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Old 08-28-2012, 12:58 AM   #33 (permalink)
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Thanks for this tool, I love it.

I want to make a comment about using the template - hopefully some of the aero experts here will chime in.

While fitting my cars I kept wondering if I couldn't slide the template a little left of the roof's apex, to bias somewhat in favor of lower tail surface area. Essentially the question is, if my design is limited by length, is it better to err on the side of slightly greater turbulence (or should I say mildly detached flow) or slightly greater tail surface area?

(Asside from Cd, doing so would bias towards lift, but as long as the "fudging" is small then the lift may be small too. I doubt a Kammback would have more lift than a sedan. For a full boattail, they usually end up steeper than 10 deg on the bottom for clearance anyways, which should counteract lift.)

I thought to check this against some low Cd production cars - where I'd expect to see Cd-optimized length-limited designs.


The Prius is quite biased.

The 2011 Honda Insight is the closest to the template of these three. The fit is close enough it may be hard to see the deviations right away, but the template is a little left of the apex, and the tail still slopes just a little steeper than the template.

The Volt, even when I interpret it's little spoiler as the tail, is also biased in the same way, not as much as the Prius.

Cd's, if anything can be believed, might be 0.28 for the Volt, 0.32 for the Insight, and 0.30 for the Prius. (I get the impression from the pic this was test with mirrors on - Cd seems like an awfully fuzzy number anyway.) gm-volt.com/2009/12/04/chevy-volts-coefficient-of-drag-is-0-28-beats-prius-and-insight/

I'm tempted to conclude that if the full-time professionals are pinching the curves when they're short on length, then it's probably the best solution, even from a Cd point-of-view. By my fitting, they only pinch a little. But it doesn't take too much pinching on the curves to net several inches reduction in a Kammback.

To play devil's advocate, it could be that the big car-makers want to maximize rear windshield slope for other reasons, like visibility. It could be possible that they sacrificed more Cd in getting these curves than they gained by keeping the tail area down. (Earlier I suggested they may be making space for a trunk, but actually all 3 of these cars are hatchbacks.)

It just makes sense to me that the *optimal* solution should change a little if you add a constraint on a parameter.
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Last edited by christofoo; 08-28-2012 at 04:29 PM..
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Old 08-28-2012, 10:28 AM   #34 (permalink)
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Phil's template is a great tool. For us EcoModders it's an excellent starting point for improving upon stock econoboxes. Applying it to flagship FE vehicles with wind tunnel backing should be done with a grain of salt...
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Old 08-28-2012, 04:50 PM   #35 (permalink)
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Quote:
Originally Posted by KamperBob View Post
Phil's template is a great tool. For us EcoModders it's an excellent starting point for improving upon stock econoboxes. Applying it to flagship FE vehicles with wind tunnel backing should be done with a grain of salt...
So... wind tunnel backing increases or decreases the number of grains of salt?

I had a feeling someone would say this. Putting words in your mouth, I think the message is that these FE flagships may have some tested and optimized features that wouldn't be present on aging econoboxes but which improve the designer's ability to contract the roofline curve, and which would be impractical for hackers like us to analyze or properly mimic. For example, front end, esp. the windshield angles, or positioning of the radio antenna for vortex generation. The template, presumably, is a simpler shape.

My "eyeball gauge" still wants to say that it's just a roofline, how complicated can it be?

Short of CFD, all we can do is argue about it and remain uncertain.

Last edited by christofoo; 08-28-2012 at 04:57 PM..
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Old 08-28-2012, 07:56 PM   #36 (permalink)
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Hucho tells us that the premise of streamlining is to move the separation point rearward,minimizing the turbulent wake, or eliminate the separation altogether.
If you wanted to ere on the side of safety,you'd stick close to the Template as it in indicated for use.
A caveat would be Buchheim's 23-degree roofline.He's a PhD and had access to a world-class tunnel at VW,and his research indicated that rear slope tangent angles could be pushed to 23-degrees,rather than W.A.Mair's 22-degree limit (which is what the 'Template' is based upon),given the proper preliminary curvature.
I believe you can see Buchheim's profile with the latest iteration of VW's 1-liter car.
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General Motors has had very stringent requirements for outward vision (which I understand having driven the 1st-gen Insight and recent Prius) and I believe this has certainly been part of their design criteria.
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Old 08-28-2012, 08:38 PM   #37 (permalink)
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You should use some updated air-foils instead of the 1930's Dymaxion cross section.

http://www.ae.illinois.edu/m-selig/a..._database.html

naca66-18 ( laminar flow airf0il)


http://www.desktop.aero/appliedaero/...ksections.html



I am searching the database for a thick airfoil:






Oh this applet is cool:

http://www.ppart.de/aerodynamics/profiles/NACA4.html
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Last edited by arcosine; 08-28-2012 at 09:15 PM..
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Old 08-29-2012, 06:50 PM   #38 (permalink)
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air-foils

Quote:
Originally Posted by arcosine View Post
You should use some updated air-foils instead of the 1930's Dymaxion cross section.
Abbott and Von Doenhoff warn us about attempting to use 2-dimensional flow sections for 3-dimensional flow in ground proximity.Hucho warns against it as well.
All the performance curves for wing sections are for 'flight conditions,' at altitude,in laminar air,and flight Reynolds number.The tabulated data is invalid for operation in ground proximity.
The other thing about 'laminar' sections,is that the air that a road vehicle encounters is already turbulent and will transition over to a fully turbulent boundary layer at about 20-mph.So unless you limit your speed to below 20-mph,and drive only on windless days,without any other traffic around you,there's no chance that vehicle will ever see laminar boundary flow.Even if it does,you still lose,as skin friction is such a minor constituent of overall aero drag,and the large fineness ratios of the 'laminar' bodies would have high drag if built tall enough for a conventional door-slammer automobile.
Even for Aptera,and all the trouble those folks went to, to capitalize on Dr.Alberto Morelli's (NACA 1948 mathematical model) low drag form,the Aptera failed to achieve as low a drag as Jaray's 92-yr-old pumpkin seed.
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Old 08-30-2012, 04:54 PM   #39 (permalink)
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You can't just use pieces of an airfoil arbitrarily, there has to be continuity in the differentials.

Anyway, this was an interesting page:

Dark Roasted Blend: Aerodynamic Marvels
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Old 08-30-2012, 06:44 PM   #40 (permalink)
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pieces

Quote:
Originally Posted by arcosine View Post
You can't just use pieces of an airfoil arbitrarily, there has to be continuity in the differentials.

Anyway, this was an interesting page:

Dark Roasted Blend: Aerodynamic Marvels
Yeah,you might have to test everything again in ground-effect to adjust for the turbulence and also the effective bluntness caused by the ground plane.
----------------------------------------------------------------------------
Looked at the article.It's good.And I'm glad they digressed to Rumpler's Tropfenwagen of 1919.
They probably should have shown the Stanley Brothers canoe-based land speed record car of 1906.basjoos was the first to post that one,and we think it's the earliest streamliner reported so far.

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