Project: Geo Metro boat tail prototype - 15% MPG improvement @ 90 kph / 56 mph

[Frank Lee]

I think it's safe to say some of these angles we take for granted can be cheated somewhat (made steeper) if other elements are in place. As evidence I offer the way the upper rear of the EV1 sits under the aero template. With the good Cd it has, I'd think the flow back there is good.

[NeilBlanchard]

Hi,

I think if we consider the 3D flow, then the angles can be steeper than the 2D template. If the air closes in from the sides, it may be filling in where the flow down from the top cannot?

[MetroMPG]

Originally Posted by MadisonMPG

MetroMPG, can you post the top right picture (the original) that is in post 1?

There are a couple of top views in this post.

Originally Posted by Frank Lee

I think it's safe to say some of these angles we take for granted can be cheated somewhat (made steeper) if other elements are in place. As evidence I offer the way the upper rear of the EV1 sits under the aero template. With the good Cd it has, I'd think the flow back there is good.

I'd be inclined to agree. 0.195 (or 0.21 or whatever GM wants to claim now) is too good to have separation there.

However, just today Phil mentioned that he thinks there is separation on the rear glass:

Originally Posted by aerohead
Where the wind tunnel smoke trace over the backlite of the stock EV1 clearly separates

I'd bet against separation though. I wish we had the ear of someone who worked on the project. (FYI we have screen grabs of the EV1 smoke test in the wind tunnel here.)

Originally Posted by NeilBlanchard

I think if we consider the 3D flow, then the angles can be steeper than the 2D template. If the air closes in from the sides, it may be filling in where the flow down from the top cannot?

I would have thought so as well, but the template is supposed to apply in 3D. It's half of a body of rotation.

[Frank Lee]

Maybe my interpretive skills are faulty, but it looks to me like the smoke tracer off the back of the Sunraycer was no better than the one off the EV1. For darn sure the Sunraycer doesn't suffer separation back there.

[Cd]

Originally Posted by Frank Lee

Maybe my interpretive skills are faulty, but it looks to me like the smoke tracer off the back of the Sunraycer was no better than the one off the EV1. For darn sure the Sunraycer doesn't suffer separation back there.

This is not full scale, but note the smoke trail angle on the back of the Loremo model.

I've seen smoke test do some unexpected things.

Also, smoke testing doesn't really seem to show things like turbulence very well.
As an example, just look at the apparent lack of turbulence on the image of the convertible, which we all know is an aerodynamic mess

And note the distance of the smoke stream from the windshield which has attached flow. On some cars, the smoke stream hugs the hood and winshield, while others have a good 6" or so of distance.

I much prefer to see tuft testing done at the same time that smoke tests are done.

( Am I the only one sick of seeing SUVs and trucks that are shown with a smokestream over the top of them to give the impression that they are aero ? )

[Cd]

On the topic of rear slope angle on an automobile boattail, I was noticing the rear slope angles of the EV-1 racer and the Aerotech ( which seems to slope from 11* to around 17 * )





One more : note the "apparent" angle of the smoke stream on the virtual boat tail of my avatar. You can't always trust the smoke, but I find it interesting never the less.

[Bicycle Bob]

The clearance between the smoke stream and the windshield is a function of the original height of the smoke stream, not the body shape. To improve the appearance for advertising, the photographer can eliminate smoke that is showing the turbulence of a convertible, showing only higher streamlines. Also, long exposures will smear a lot of wiggles that would show up with tuft testing.

[aerohead]

Originally Posted by MetroMPG

There are a couple of top views in this post.



I'd be inclined to agree. 0.195 (or 0.21 or whatever GM wants to claim now) is too good to have separation there.

However, just today Phil mentioned that he thinks there is separation on the rear glass:

I'd bet against separation though. I wish we had the ear of someone who worked on the project. (FYI we have screen grabs of the EV1 smoke test in the wind tunnel here.)



I would have thought so as well, but the template is supposed to apply in 3D. It's half of a body of rotation.

I recently purchased a gizmo which converts photographic slides to digital.
I believe I have images of both the EV1 and Ultralite with smoke traces over them in the GM tunnel.
They both have clearly visible separated flow which never re-attaches.I believe they compromised aero for good rearward visibility.
I've also got a side shot of the EV1 at Ft. Stockton setting the LSR with the boat tail.
Roof-to-tail tip angle is 15-degrees instead of about 20.5-degrees.The boat tail angle itself is only 9-degrees
I'll try and get slides scanned for next weekend.

[Cd]

Phil, do you have a larger version of this picture too ?



The upwards slant to the back of that EV-1 'tail makes the car resemble Goldmembers' car.

[aerohead]

Originally Posted by Cd

Phil, do you have a larger version of this picture too ?



The upwards slant to the back of that EV-1 'tail makes the car resemble Goldmembers' car.

Cd,yeah,that's the photo.Mine is from the brochure and quite a bit larger although your's contains all the data.
I just laid straight edges over the image and took a protractor to it."It's shagadelic baby!"