Aerodynamic Streamlining Template: Part-C

[ERTW]

Kamperbob, you are correct. I wanted to investigate the prismatic shape because firstly I don't see anybody using the revolved shape. The template works for a revolved shape, or even a high aspect ratio wing. However, cars are narrow boxes. Flow around such a body is very different. That's why I'm modeling a prism. Perhaps Aerohead can comment.

You could argue that the box fish is the slipperiest shape found in nature. Boxes are also much more pragmatic than footballs; they offer more usable space and are easier to build than compound curves.

Aerohead, I put the template up against a 2010 GMC Sierra, and found that it only goes out to 45%. What is the minimum Cd at that point?

My template approximation is good to about 60%. The zebra stripes should be concentric circles - as they are at the peak. You can see the broad space and the reverse stripe on the tail which confirms a discontinuity.

[aerohead]

Originally Posted by ERTW

Re: the template - where are the vertical % to match the horizontal %? In other words, what are the X and Y coordinates for the template? Does the spline have an equation?

Doesn't the data show that an ellipsoid has virtually the same Cd as the teardrop?

What's the Cd down to 55% template? This is where I see a pickup terminating. It'd be handy to determine point of diminishing returns.

I've observed snow on my car. I hypothesize that sharp corners discourage atmospheric pressure side air from moving towards the lower pressure area above the car, and reduces vortices. If no vortices are created above the roof, then it's even less likely imo to create vortices at the trunk edge. I think that a polyellipsoid reduces the lateral pressure gradient - which also prevents vortices.

ERTW,I've never finished the thread so,so far there is no data for what would be happening in plan view.I hope to complete it within my lifetime.
As it is based on a streamline body of revolution,the 'Template',in theory would begin plan taper at same point as the roofline.
Of course it would make the vehicle too wide.
I'm compiling plan views of vehicles to post at some date,so members can get a sense of what has already been done with this 'pumpkin seed ' form.
As to your 55% question.If your vehicle was completely cleaned up,then at 55% of 'Template' you'd have a potential for around Cd 0.16.
To fully exploit the 'Template' with a pickup would require that the truck box roll into tumblehome as it progresses rearward,allowing the elliptical cross-section to evolve.
I was in the process of doing this to the Dodge pickup when the T-100 showed up in my driveway.So this project has languished since 2005.KamperBob is the only person who has seen it.
The boat hull "poco-loco" which replaced the original aeroshell has both elevation and plan taper,mimicing the 'Template' best I could.She produces no vorticity and by itself is good for 13% better mpg (hwy).
With the full kit on the T-100,by the delta-fuel economy/delta-drag coefficient method,she's estimated at Cd 0.18.
There is a tuft-testing photo of it at the "Full-Boat-Tail with Gap-Fillers for T-100 Pickup' thread.

[aerohead]

Originally Posted by ERTW

I couldn't wait.

I scaled the template and input it into a 3D program. It's a few percent squatter, and I'm happy with the resolution and accuracy (especially since most forms will be truncated where it's a better approximation).

So...now I have to figure out how to CFD test it.

I'd love to make up a bed cap for my dad's Sierra 1500.

ERTW,I need to get you to an image of Paul Jaray's original pumpkin seed car tested by Wolfgang Klemperer in 1921.It is on the internet at GOOGLE.
It is fully contoured and is almost exactly like the aerofoil-profile radome atop the US NAVY Grumman E-1 (WF-2) Tracer.
In plan view the 'Template' will have plan-taper and be continuously rounded over like a stretched version of the 1938 Schlor car.

[aerohead]

Originally Posted by ERTW

Kamperbob, you are correct. I wanted to investigate the prismatic shape because firstly I don't see anybody using the revolved shape. The template works for a revolved shape, or even a high aspect ratio wing. However, cars are narrow boxes. Flow around such a body is very different. That's why I'm modeling a prism. Perhaps Aerohead can comment.

You could argue that the box fish is the slipperiest shape found in nature. Boxes are also much more pragmatic than footballs; they offer more usable space and are easier to build than compound curves.

Aerohead, I put the template up against a 2010 GMC Sierra, and found that it only goes out to 45%. What is the minimum Cd at that point?

My template approximation is good to about 60%. The zebra stripes should be concentric circles - as they are at the peak. You can see the broad space and the reverse stripe on the tail which confirms a discontinuity.

At 45% you'd be in between Cd 0.209,and 0.177.

[ERTW]

This is the 1939 Schlor car aka pillbug. Schlörwagen "Pillbug" - ApteraForum.com - Unofficial Aptera Car Forum

A little irony about the Schlor car...if we had a plan view, Solidworks will convert 2D images into 3D at the click of a button...d'oh!

And I believe I found the E1 Tracer. I wouldn't recognize what you're calling the pumpkin seed car if I saw it - google doesn't show me cars when I type that in.

I've found a bunch of SAE technical papers. I'll pick them up at U of T soon. I found a few of Morelli's old papers too.

A comment about the Aptera: the 1998 GM Precept got down to 0.153 Cd in just a few hours of wind tunnel testing (0.168 final shape). That's on par with the "ideal" shaped Aptera. imo the Precept is a fairly normal, and nice looking vehicle. Either Aptera screwed up the shape, or there's more to ground plane aero than we know.

[NeilBlanchard]

The Hucho book has the plan view of the Schlörwagen:



By the way, I started doing a SketchUp model with this image and I am almost certain that the 1275mm height should go to the underside of the chassis, not the ground. That is the way it scales, anyway.

Here's my collection of the best Schlörwagen pictures:

Schlor Pillbug pictures by NeilBlanchard - Photobucket

Going by the tuft test in the wind tunnel, there would be less drag if the tail was broader and flattened, like a beaver tail, and/or if there were rear wheel strakes.

[racerc2000]

would this be good or bad? or is my positioning off?

I tested it against the base and the SC honda element 09-11 model years. the differences are small but they are there.

but I guess it is self explanatory for the CD being high but not as high as it could be.

makes me wonder if doing some minor bodywork to the roof to move the 0% to the door line would have a nice improvement?

[ERTW]

Aerohead, I used an equation based spline for the back. It makes it really easy to shape the curve as I like.

As a confirmation of the hard work you've put in, the revolved solid shows smooth contours all the way out to the tail. So the boxed shape requires a different approach as I'd hypothesized. I hope to have some CFD analysis done this weekend.

[aerohead]

Originally Posted by ERTW

This is the 1939 Schlor car aka pillbug. Schlörwagen "Pillbug" - ApteraForum.com - Unofficial Aptera Car Forum

A little irony about the Schlor car...if we had a plan view, Solidworks will convert 2D images into 3D at the click of a button...d'oh!

And I believe I found the E1 Tracer. I wouldn't recognize what you're calling the pumpkin seed car if I saw it - google doesn't show me cars when I type that in.

I've found a bunch of SAE technical papers. I'll pick them up at U of T soon. I found a few of Morelli's old papers too.

A comment about the Aptera: the 1998 GM Precept got down to 0.153 Cd in just a few hours of wind tunnel testing (0.168 final shape). That's on par with the "ideal" shaped Aptera. imo the Precept is a fairly normal, and nice looking vehicle. Either Aptera screwed up the shape, or there's more to ground plane aero than we know.

Jaray's pumpkin seed is probably at 'wind tunnel' or 'windkanal' search words with GOOGLE.
On the Grumman Tracer,the pumpkin seed is on top of the plane,housing the radar.
Some fella has a website for CdAs and includes Aptera.He's showing Cd 0.11 for their final design.The X-Prize was supposed to investigate all this sort of stuff.
Hucho has Morelli's form at Cd 0.05 in ground proximity,with no wheels.
When Morelli put wheels on his 'Banana' car the drag jumped from Cd 0.161,to Cd 0.35.They had to do a lot of chiseling to get it back down to 0.201.
Have fun at UT Austin,that ought to be a hoot!WHOOPS! I guess that would University of Toronto,not Texas.

[aerohead]

Originally Posted by NeilBlanchard

The Hucho book has the plan view of the Schlörwagen:



By the way, I started doing a SketchUp model with this image and I am almost certain that the 1275mm height should go to the underside of the chassis, not the ground. That is the way it scales, anyway.

Here's my collection of the best Schlörwagen pictures:

Schlor Pillbug pictures by NeilBlanchard - Photobucket

Going by the tuft test in the wind tunnel, there would be less drag if the tail was broader and flattened, like a beaver tail, and/or if there were rear wheel strakes.

Neil, have you seen the image of the Schlor car model under green light? It tells a lot about the centerline flow in elevation.