How to AeroMod "Paddle" Mirrors

[trebuchet03]

It's cool and all, and I give you props for going forward with prototyping....

Alas, it's not so much the leading edge that is the source of our aero frustration - it's the trailing edge. If you can figure out how to make a clear bubble to gently taper the rear side AND still keep the functionality of the mirror....

Remember, the pointy end points backwards

[wyatt]

gascort: I know these plastic bags to be water tight, but their potential downfall would be that they are super cheap and super light. I think within 1000 miles they would need replaced simply due to wind related ware, not to mention the sun they would have to endure.

diesel john: Very true... I will have to look into doing a water drop test on the car. I have done it for the windshield, and it goes to almost horizontal at the lower corners. I placed the tip at that point, but it may not be optimal. Since the passenger's side is still stock I may have to look at it on that side.
trebuchet03: I actually read about something like this once. A guy that makes carbon fiber mirrors that have a great leading edge and a partial boat-tail. The boat tail doesn't restrict sight but isn't clear, it's just a start. The leading edge reduces the drag on them by 30%, and the start on the back by another 25%. If I was to make some out of fiberglass I would try to do the same, start a boat tail. If I can come across a good, cheap, clear plastic to work with, I may just build a full boat tail on them to see if there is an improvement.

[saltoricco]

Originally Posted by wyatt

If I can come across a good, cheap, clear plastic to work with, I may just build a full boat tail on them to see if there is an improvement.

You can build such a transparent tail using PETG:

http://www.youtube.com/watch?v=aCXrXKllzSI

.

Holger

[wyatt]

I have removed the plastic bag, it made a lot of noise and was starting to inflate pretty bad (and I did a very good job at sealing it up). I have been looking around, and it is possible that my shape isn't optimal...

a cone with a low half angle has better aero than a high half angle...
My half angle was around 15 deg, and was a wedge with a half cone on the end. Cd around 0.5? Better than the stock mirrors I would guess.

but from what I have seen for Cd, a half sphere or cupped half sphere does a better job than a cone...

and an aerofoil has a much lower Cd than those!
So I may be making my mirror more "aerofoil-like" by rounding the front out and tapering the rear. I will have to see about coming by some clear plexi for cheap or learn some more about the PETG (or similar) forming.

[wyatt]

The plastic bag was inflating it's self, so I wound up taking it off, it seemed to work much more quietly without the flapping bag...
The exposed cardboard wound up rained on a couple times, it actually held up ok. I would say about two more rainy days and it would have fallen off... but it survived two rainy days. Using the rain and my dirty car to my advantage, I drove a few miles and pulled off to check out the flow around the cardboard and around the other mirror (without any mods). The flow around the cardboard looked flawless, but when looking at the other mirror, it looked like the tip should be lowered about 1 or 1 1/2 inches, so I took that into account for future builds.
Before the cardboard was lost to rain, I got some campaign signs (by asking a few candidates and waiting until the day after elections to collect them), and have since built the drivers side out of coroplast, with only white exposed. It sure gets lots of attention for being so small... I will have to post some new pictures once I get both of them in place. I have been told I should build a big nose cone (by fellow co-workers)... I think they were just joking, but it could happen after installing an undertray (coroplast permitting).

[groar]

wyatt, great experiment. Keep us informed of your results please

Thanks to your pictures I just discovered this site :
Aerospaceweb.org | Ask Us - Aerodynamics Questions
(search for "drag coefficient for a cylinder" to find the pictures)
there is also explanations for golf balls' dimples and vortex generators/devices.
bookmarked

The differences between the different shapes are interesting. In your case you should also make your mirrors look like the #2 and not the #3, ie forward the outer parts of yours mirrors toward the back of the car (hope someone understands me ).

Denis.

[wyatt]

I think I would try to make it more parabolic... but coroplast is not exactly the ideal thing to be working with for complex shapes...
I think I understand what you are saying... to paraphrase:
I should be trying to get the lower Cd of .38 for the "hemispherical cup" rather than the .42 for the "hemisphere" by possibly extending the trailing edge beyond the end of the mirror.
An improvement on this would be to taper the over-extension to start a boat tail. I haven't gotten around to trying to quantify my results. These aren't super hard to take off or put on, but I really don't want to try doing a-b-a testing until I have two of them in place that I am happy with.

[NeilBlanchard]

Hi,

Here's a direct link the above reference:
Aerospaceweb.org | Ask Us - Drag of Cylinders & Cones

[Christ]

I'm not 100% sure if this was already mentioned here, but looking from the front of the vehicle at your mirror "adjustment" (cover), I'm thinking that the "knife edge" portion of it (the flat frontal area) should actually be cut diagonally from the inner-most point to the outer-most point on the mirror, then tapered back to a point...

This has two effects:

It cuts frontal area considerably.

The shape is considerably less deflective than a flat edge facing into the wind.

[wyatt]

Basically I think the steeper angle without any means of smoothing the flow to a more parallel with the car type situation will cause a greater cross sectional area, not by actual area, but by the air moving around. I have no proof of this, and if I had a way to test, I would probably give it a shot both ways.