Why doesn't the rear window get wet

[Varn]

I was driving Benzilla the other day in the rain. One thing that was curious was that the back window stayed completely dry. About like my VW with the rear window shade.

Is the air flow fairly smooth coming off the roof?

Here is a little detail that is hardly noticed. It is a small plastic panel that sits about half way between the rain gutter and the sheet metal. It is the same color as the paint. Hardly an inch long and running the height of the rear pillar.

[aerohead]

Once the air gets to the point of maximum greenhouse cross-section it is 'leeward' of the body,no longer attacking, and it can't 'push' the water as it would in the forebody.
If you were already in motion when the rain began,the water would be managed by the drip rail system (hidden in modern cars since Hucho's work) or blown over the captured-vortex and the captured-vortex atop the trunklid would help to centrifuge the rainwater away,with only moving air licking the surface of the backlight.
If you started out in a rain,this vorticity would eventually dry the backlight if your velocity were sufficient enough to outrun the momentum of the water.
If you drive a square-back, the wake turbulence will transport water pumped by the tires up from the road surface to totally obscure your view.Something which the re-attached flow on the 300's boot trailing edge prevents.

[Varn]

Hi Aerohead, You were the one that I had hoped would respond. I can usually follow your posts pretty well but I have read what you wrote several times and frankly I am missing something.

I was in motion when the rain started. Usually (on other cars) there is a turbulence at the back that makes the back window wet in a random fashion. Where as the rear window on the mercedes was completely dry.

Do you think the 85 w123 mercedes was designed with the knowledge about Hucho? The car was first produced in the mid 70's I believe.

Originally Posted by aerohead

Once the air gets to the point of maximum greenhouse cross-section it is 'leeward' of the body,no longer attacking, and it can't 'push' the water as it would in the forebody.
If you were already in motion when the rain began,the water would be managed by the drip rail system (hidden in modern cars since Hucho's work) or blown over the captured-vortex and the captured-vortex atop the trunklid would help to centrifuge the rainwater away,with only moving air licking the surface of the backlight.
If you started out in a rain,this vorticity would eventually dry the backlight if your velocity were sufficient enough to outrun the momentum of the water.
If you drive a square-back, the wake turbulence will transport water pumped by the tires up from the road surface to totally obscure your view.Something which the re-attached flow on the 300's boot trailing edge prevents.

[aerohead]

Originally Posted by Varn

Hi Aerohead, You were the one that I had hoped would respond. I can usually follow your posts pretty well but I have read what you wrote several times and frankly I am missing something.

I was in motion when the rain started. Usually (on other cars) there is a turbulence at the back that makes the back window wet in a random fashion. Where as the rear window on the mercedes was completely dry.

Do you think the 85 w123 mercedes was designed with the knowledge about Hucho? The car was first produced in the mid 70's I believe.

We had a 300 TD in the family and it was Mercedes 'coming-out' car for aero.Their print media advertising went to special pains to illustrate all the wind tunnel development work which went into the production car.Right down to the non-fouling taillight lenses.
With the higher and longer boot,the trailing edge of the trunklid would prevent any splash or spray from the wake to migrate up and over to the backlight.
The transverse locked-vortex atop the trunklid would spin-dry the water out of the air before it could circulate back to the glass,leaving only the spinning air to lick the surface.
The water is 833X as dense as the air and it's inertia-momentum won't allow it the aerobatic freedom the air enjoys.It's simply benched on the sidelines .A good thing!