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Old 07-20-2009, 01:40 PM   #2 (permalink)
orange4boy
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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?
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Last edited by orange4boy; 07-25-2009 at 01:44 AM.. Reason: added aerohead's intro.
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