Cabrio Windscreen Tabs
What are the black plastic tabs for at the top of the windscreen of a cabrio actually for?
You may have noticed the oddly placed black plastic add-ons at the top of the windscreen on
smart cabrios and wondered what they were. Here is the single tab on the fortwo 450 cabrio.
The twin tabs on the Roadster.
And the single tab on the 451 cabrio.
You have either seen them and ignored them or thought that maybe they have something to
do with reducing air that comes into the cabin when the roof is open. You'd be right but...
How Can That Small Tab Do Anything?
To really appreciate how effective that tab is, you need to drive a smart cabrio without a tab.
You may as well not have a windscreen in considering the amount of air in your face.
Obviously there is a whole load of physics going on here.
The Coanda Effect
The Coanda effect is where airflow attaches itself to a nearby surface and will
follow that surface even if it curves away from the direction of the original flow.
This is hard to demonstate as you can't see air unless you add smoke. However, water reacts
in the same way as air. A quick experiment in my kitchen using a tap and the back of a spoon...
...and you can clearly see the water bending around the spoon as it follows the curve.
Without the tab at the top of the windscreen you get the air flow bending down off
of the top ridge of the windscreen, straight into the cabin and straight into your face.
However, with the tab in place you can kick the air up enough to reduce the Coanda effect.
This disturbance also causes a wake behind it which, using the Bernoulli effect,
collects other streams of air and pulls them up and away from the cabin too.
It's not perfect though. The high pressure air on the front of the car will always try
its best to move to an area of low pressure, in this case the cabin. So you always
get a small amount of through flow of air in the cabin.
So, The Coanda Effect Is The Enemy?
Not always, it has a lot of good uses like keeping planes in the sky, race cars on the ground and goes a long way to making the Dyson Air Multiplier work. It can create lift and downward forces.
By using the Coanda effect with an upward sweeping plane like the rear of a Lotus Elise you
can use the forces of the clinging air to pull the car to the floor plus it also reduces the venturi
effect behind the car. It increases air pressure behind the car creating less drag.