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aerohead 07-18-2009 04:47 PM

Aerodynamic Streamlining Template: Part-C
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orange4boy 07-20-2009 02:40 PM

Aerodynamic Streamlining Template: Part-A
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Hello all,Al is scanning materials and we hope to post,although he has "real work to do." In the meantime,I want to bring everyone up to speed on this tool.

Premise: The template is designed to be a quick and dirty reference for anyone contemplating aft-body streamlining to an existing vehicle or from a "body in white."

By properly scaling an image of an real or imaginary vehicle and placing the image under the template such that the "high-point" or,point of maximum roof camber of the vehicle under study matches that of the template,then the template will define an architecture for a roofline,free of flow separation,for any length you care to make it.This is equally valid for the sides of the vehicle as well.

Your drag reduction will be a function of your new wake area, as compared to original wake area.Example: you extend the roof and sides back such that your new wake is 10% less than the original,then you've just cut your drag by 10%,and increased mpg by 5% at 55-mph,and 6% at 70-mph.That's it!
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Background: For a "streamlined car",we can turn to the man who first used the term in 1922,Paul Jaray.Jaray was an aeronautical engineer with the Zeppelin Werke,involved in airship design which includes "Los Angeles" built for the U.S.under the Versailles Treaty.

( from Hucho ) It was Jaray who recognized that the flow around a body of revolution ( spheres,cones,ellipsoids,cylinders,etc.) of very low drag in free air,is not axially symmetrical close to the ground.As a result,the drag increases. Where ground clearance approaches zero,the optimum shape for a low drag is a half-body,which forms a complete body of revolution together with it's mirror-image produced through reflection from the roadway.THIS IS EXTREMELY IMPORTANT!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! !!!!

Refer to Figure 4.118.

Also from Hucho,is a table which shows one of the lowest Cd belonging to a "streamline body" of Length/Diameter ratio of 2.5:1,@ Cd0.04.This image is from Hoerner's book of 1965,which is also in his 1951 book,and comes from : Goethert,Drag Measurements on Streamline Bodies in the High-Speed Tunnel of the Deutsche Versuchsanstalt fur Luftfahrt,Berlin,Zentrale fur Technisch-Wissenschaftliches Berichts 1944,p.377. I've taken this profile as the "minimum" for use in the template as it is drag-free(profile) and has the minimum wetted area to also minimize friction drag. The linear progression combines mirror-image of the 2.5:1 streamline body to create the "pumpkin-seed" Jarayesque form above the ground,with curves just "slow" enough to guarantee arrached flow.

You will see Fig.4.119 from Hucho which illustrates a 1966 VW Beetle in "mirror" and will notice that from the graph,at super-critical flow,the ellipsoid of lowest drag also occurs at a fineness-ratio of 2.5:1

From Marchaj' book I've included a table of "sections" which includes a drag table showing form-drag and friction drag combining for "Total drag". You will see that a drag minimum occurs for wing-loke structures at a fineness ration(aspect-ratio/thickness-ratio) of 3.92:1

I've included some tables from Fluids texts which shows some forms and their respective drag coefficients in both 2-and 3-dimensional flow,at varying Reynolds Number.

I placed Sunraycer under the template,it was a perfect match.Bill Watson's airship "White-Dwarf" of 1984 is about 2.57:1 ratio. Watson worked with Burt Hibbs,aerodynamacist for Sunraycer. Both of AeroVironment. This shape seems to end up on some of the most efficient vehicles known in the world,I believe it to be a shoe -in for aero-modding.Those with more advanced aerodynamic toolbags will will no doubt venture out into some of the more exotic shapes,but for amateurs,I believe this form can serve us well.

Good design and engineering calls for the bare minimum to get the job done.Using the minimum will guarantee lowest weight,most efficient use of material and energy and lowest economic and environmental impact.

The above is the intro the the templates which were posted separately by aerohead.

Thanks for the charts and template. Good timing for me because I'm going to start my kamm-back soon. I have several neophyte questions that have not been answered so far (that I could find).

1. Does the optimal fineness ratio change with speed? I am hoping that if I make mine lower, that is to make the slope steeper, but I stay below say 55mph, will the flow stay attached? My aim is to reduce the size of the tail, and I could live with the slower speeds to get the wake and tail as small as possible. I don't think the Egg will ever go "super-critical". Sounds like you would need a flux capacitor for that.

2. Does the kamm-back or boattail have to be closed off to get the best result? How much difference is there between a closed off and an open ended tail?

theycallmeebryan 07-20-2009 02:58 PM

Keep in mind, the right chart that shows the .25 t/c being optimal is at a reynolds number of 400,000, much much lower than a car traveling at 55mph.

Orangboy, for your vehicle, you want to experiment with shapes at RE of around 11,200,000, considering your vehicle is 15 feet long stock, and i'd expect atleast 5 feet to be added for a boat tail

RE = 9360 * SPEED(in mph)*DIMENSION(in feet)

Using a program called DesignFoil, i have found that the best shape for a vehicle traveling 60mph has the location of minumum pressure located 70% the length of the cord(from front to back). The shape is NACA 67-025.

However, this shape has a center of pressure that is out in front of the shape 8% of the chord length. What this means is its very susceptible to cross winds, and will get blown around.

When you move the max width forward a bit, the cd increase slightly, but the center of pressure moves backward drastically, meaning its better in crosswind. For example, going from NACA 67-025 to NACA 66-025 (70% to 60%), the COP is moved back to 30% of the length (IE, its now inside of the shape)

orange4boy 07-20-2009 03:14 PM

I just thought of some other questions I have not seen answered elsewhere.

Regarding the point of stagnation and the underbelly. Does the air under the car have a lower or higher pressure? I think it's a low pressure area but it's not obvious to me.

Is it better to have a sharply defined, upward sloping air dam parallel to the lowest points on the car or a bumper curved under the car from the stagnation point (like in the template?) the trailing edge of which terminates parallel to the lowest point under the car?


theycallmeebryan 07-20-2009 03:22 PM

From everything i have read, it seems that the optimal design is to have no airflow underneath the car. The flip side is that this is not really practical in a road car because there needs to be some sort of ground clearance for speed bumps, parking lot entrances (some are not level with road), etc. So the best answer i can give is that the least amount of airflow under the car is ideal.

theycallmeebryan 07-20-2009 03:39 PM

I would like to hear what aerohead thinks about what i have talked about so far.


orange4boy 07-20-2009 03:53 PM

I think you are right regarding the least airflow under a body is best but at some point assuming the same ground clearance are you not increasing frontal area if you block off all under flow. It could be this is an advantage anyhow. If the larger frontal area is offset by reduced drag. I have not heard this spelled out yet. Thoughts?

theycallmeebryan 07-20-2009 04:34 PM

Well, there has been some discussion about totally covering the front to eliminate all the air going under the car, even if the airdam is lower than the underbody. Its a common practice at the salt flats and in nascar.

orange4boy 07-20-2009 04:52 PM

B*tchin' Camaro!

So what you need is:
1) a stiff rubber lip at the front for aero
2) a stiff upper lip to deal with the guff from the unwashed masses.
3) a nascar decal.

Got it.

Bicycle Bob 07-20-2009 07:07 PM

Thanks for the source postings. The bible I've been going by is "Theory of Wing Sections" by Abbott and Von Doenhoff, which takes up the history from just after that first chart. Being concerned with wings, it makes sense for them to not go beyond 21% thick, as they want to leave something in hand for some angle of attack. That had persuaded me to go to 25% on a strut myself.

The NACA tests progressed to "laminar flow shapes" of which the 67-series is the most extreme. These are almost lab curiosities, first achieved in production on the P-51, using bondo over the flush rivets. No such shape can tolerate a blemish, let alone a wheel opening, without tripping the boundary layer into turbulence in a wake spreading 15 deg to each side. The "standard roughness" in these tests, which double the drag, consists of .011" grit, covering 5-10% of the area, just within 8% of the length from the leading edge. (like small bug strikes)

Here are a few selected data points from my book:
The lowest cd shown was for a shape that didn't make a stable wing, so there may be much room for exploration. That wing is the 0010-35, in the old system, 10% thick. At Re from 3 to 9 million, the cd is .003 near-polished, and .009 "rough."

The 2424 shape is similar to the 2412 It gets a cd of .008 smooth and .013 rough, at 24% thickness, while the 12% thick shape gets .0065 smooth and .010 rough.

Moving on to the later laminar shapes, which rely on perfect conditions to maintain a laminar boundary layer back as far as the second term in the identifier, usually the widest part of the shape. A 63-006 shape is a 6-series, (laminar) with 30% laminar flow, no camber for lift, and 6% thick. - ie: 100" long, 6" thick. The 67-025 is 70% laminar, no camber, 25% thick. The 66 and 67s are slipprier than the 63 and 64s, but who can wait for 70% of the way back for the first seam, wheel opening, or other feature?

Shape: cd around 6 mil Re. cd with standard roughness
63-006 .0043 .0087
63-021 .0055 .011
64-009 .0043 .0087
64-015 .0047 .0097
64-021 .0054 .018
66-006 .0032 .0085
66-021 .0041 .013

These all show a clear preference for the slender shape, despite the extra surface drag. The text helps illustrate the trends with increasing speed, and gives examples that improve, such as the 65-418, with less separation as you go faster. This is not uncommon, but the devil is in the details, and these shapes are only a rough indication about designing practical cars.

Re: underflow - it is good if done right, but blocking it off is a cheap fix that gives downforce, which is good for road racing, not cruising. Underflow is not really hard to visualize; it is like a duct, but with the boundary layer on the flat side moving and helping stabilize things. After F1 banned the vacuum cleaner cars, they did quite well with underbody venturi systems.

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