Project: making a permanent Metro Kammback extension

[garys_1k]

Originally Posted by MetroMPG

Video is the plan: "web cam on a stick!" (Duct taped to the side of the car.)

I hope that stick doesn't kick up enough vortices on its own that it messes up the flow over the body. Maybe you should use a chase car and video from that.

[Cd]

Oh boy, oh boy,oh boy.

Can't wait to see this !!

One of the guys here ( millenniumtree ) suggested using audio tape insted of yarn .
My previous tuft tests showed that sewing yarn seems to cause vortexes for the tufts further downwind. Also, the yarn tends to stick to itself and those surrounding it at as you get up to speed, thereby ruining the test.
I had better luck with thick cotton thread.
I must give millenniumtree credit for the audio tape idea - it fits completely flush with the body of the car, yet flutters like mad in a turbulent area,
Just out of curiousity, I cut up some 2'1/2 " strips of tape from a video cassette and applied then to my car as i took a short drive. They seem to work just as well, yet are easier to see....but not on a black car like you have.
As I'm sure you remember, audio cassette film mainly comes in a dark brow color, but I remember tapes that had the film being white as well.

[Cd]

Originally Posted by garys_1k

I hope that stick doesn't kick up enough vortices on its own that it messes up the flow over the body. Maybe you should use a chase car and video from that.

As long as the stick is not in the airflow area that he is filming he should be fine.
Trying to get a chase car together is a real pain because it requires three people to do it right : two drivers and a camera-man.
Another benefit of using a webcam is that they are so small that they can be used to do tuft testing underneath the car. ( Or even in the wheel wells if the camera is small enough. )

[garys_1k]

Yeah, I thought about that later. If he has the camera on the left side of the car and monitors the tufts mainly on the right side roof, no problem.

[cfg83]

.Cd -

Originally Posted by .Cd

...

Just out of curiousity, I cut up some 2'1/2 " strips of tape from a video cassette and applied then to my car as i took a short drive. They seem to work just as well, yet are easier to see....but not on a black car like you have.
As I'm sure you remember, audio cassette film mainly comes in a dark brow color, but I remember tapes that had the film being white as well.

This is cool. I was thinking of X-Mas tree tinsel, but that's crinkly. Audio/Video tape is smooth, so I think it is better for staying flush to the car.

CarloSW2

[88CRX]

how about a VW Beetle? it seems like the original VW would have a low Cd.

[Cd]

Originally Posted by cfg83

.Cd -



This is cool. I was thinking of X-Mas tree tinsel, but that's crinkly. Audio/Video tape is smooth, so I think it is better for staying flush to the car.

CarloSW2

Let me add a disclaimer : I tested only a few strips at the base and middle of my winshield and drove for all of a mile and a half. I placed some of the strips directly behind my winshield wiper and noticed severe flutter once I reached anything past ten or so MPH. This is good for indicating that there is drag in this region, but I wonder if the cotton thread would show the direction of the vortexes better.
Sorry - I was keeping my eyes on the road in front of me vs. on the test.

After running prior tuft tests with the car, I saw the direction in which the flow traveled along the windshield.
(As you know, sometimes air will travel in directions that you would never imagine it would. Sometimes it will even reverse itself.)
To test how these strips would handle this kind of situation, I intentionally placed some of the strips perpendiculal to the airflow.
( I placed them vertically where I knew from the previous testing that the airflow was horizonatal.) I found that the strips fold over themselves and accurately show the true direction.

Still, I wanted to add that I did a really quick, sloppy test. The experts ( used to ) use yarn. Apparently there is a reason they dont use the tape.

[Cd]

The AeroCivic looks even better when you compare the entire profile of the tail :

( A perfect match in fact ! )

[MetroMPG]

Originally Posted by garys_1k

Yeah, I thought about that later. If he has the camera on the left side of the car and monitors the tufts mainly on the right side roof, no problem.

That's pretty much what I did - set up from above, to the side and slightly behind the area in question:



I just got in from some runs and got some cool pics and video. I'll post later.

I won't keep you in suspense though: there appears to be attached flow on the top, but not on the sides (on the sides, I suspected I'd gone too far inside that ideal "tear drop").

[MetroMPG]

I'm revising my evaluation about the "side panel": the turbulence is from the outside mirror! I forgot to fold it in when I ran the tests (I usually fold it back at highway speeds).

I did a few more runs comparing mirror deployed to mirror folded to window up/down, the difference is clear. I'm putting together a video now to post on YouTube...

There's still a little wiggle in the yarn, but none more than what I'm seeing on the side glass ahead of the Kamm back (got some shots of that too).