2011 Mercedes-Benz Cabriolet Showcases New AirCap Technology

STUTTGART, Germany — When Mercedes-Benz unwraps its tasty new 2011 E-Class Cabriolet at the 2010 Detroit Auto Show next month, the big news will be its place in the automotive firmament as perhaps the first soft top that can be driven comfortably with the lid down all year long.

This Sir Thomas More of a four-seat convertible owes its versatility to a remarkable invention called the AirCap, a feature that took a team of Daimler engineers four full years to develop.

They garnered 20 patents along the way, but perhaps more rewarding once the car hits the streets late next summer will be the thanks of men and women around the globe who can finally ride in an open car without worry about frazzled hairdos and frozen faces.

The AirCap, an aluminum, mesh and plastic air diverter blade that rises from the top of the windshield frame with the press of a center-console-mounted button, is an engineer's way of fooling the laws of aerodynamics into thinking that the E's smooth, low-profile ragtop is still in place and that all those heads of hair aren't there for turbulence to tussle.

The device, which forms a broad and glossy black frame at the top of the E-Class Cabriolet's curved windshield when stowed, rises up to 2.4 inches above the top of the glass and flips into a horizontal position when deployed.

It works in conjunction with a height-adjustable three-position mesh draft-stop mounted between the rear seat head restraints to alter the flow of air over and around the cabin when the top is down.

The effect, best experienced at speeds of 30-150 mph, with the side windows up and top down (it wouldn't make sense to deploy if the top was up, and it won't operate in that circumstance anyhow), is wondrous and threefold:

It deflects the wind that would otherwise flow over the top of the windshield and pour down into the cabin, especially affecting those in the backseats.

The deflected air, which takes on the approximate curvature of the convertible top (a curve that's visible in computer simulations and in a plume of smoke that defines the flow in wind tunnel tests) helps keep warm air inside the cabin rather than forcing it out with an in-rush of chilled outside air.

The relative absence of turbulence and wind rush inside the open cabin results in a much quieter top-down ride, making conversation — even cell-phone calls — possible at speeds of up to 75 mph.

These, by the way, aren't just claims made by Mercedes but actual observations made while sitting in the open cabin of a pre-production E-Class Cabriolet at Daimler's 70-year-old wind tunnel facility at the mother plant in Stuttgart's Unterturkheim district.

With wind tunnel and aerodynamics manager Teddy Woll at the controls, the E-Class Cabriolet's top stowed under the rear deck and the side windows up, the air flow from the giant fan gradually rose to a speed of 140 km/h, equivalent to driving along a U.S. highway at about 85 mph.

In the backseat, which is elevated a good 2 inches above the front seats for improved visibility, the top of my head was about 3 or 4 inches above the top of the windshield frame and caught the breeze quite well.

With the AirCap stowed away, I felt a lot of turbulence around the top and back of my head at 100 km/h (62 mph) escalating to hair-straightening and forehead-chilling force at 120 km/h (74 mph) and to an uncomfortable cheek-flapping, eye-burning gale at 140 km/h.

But when the deflector was deployed, the wind became a breeze and the turbulence all but disappeared, even at the top wind speed.

The cabin also became noticeably quieter. While at 140 km/h it still will require a good set of lungs to make yourself heard, but conversation was possible at 120 km/h and below.

Chill from the wind disappeared — except around the top of my head at 140 km/h — and it was easy to believe Herr Doktor Woll's claim that the car would be comfortable at speed on a cold winter's day with the top down, heater on and AirCap deployed.

Woll says Mercedes is calling it the "warm air bath" effect.

From the front seats, with the top of my head only about an inch above the windshield frame when the AirCap is tucked away, the ride was comfortable at both 100 and 120 km/h and was just barely breezy at 140.

Noise levels, though, made conversation difficult except at 100 km/h wind speed.

Deploying the AirCap worked even better than it had when I was in the backseat. Conversation was possible without yelling at 100 and 120 km/h, virtually eliminating any impression of wind disturbance.

Daimler hasn't priced the E-Cab yet, and no one would give us an idea of how much the AirCap — a piece of standard equipment — would contribute to the price.

But consider that it consists of 211 individual parts, about a third of them going into motion when the switch is pressed, and you can expect it won't be a cheap addition to the E-Class' already impressive list of features.

Inside Line says: Mercedes-Benz has gone the distance to make sure its new E-Class Cabriolet is every bit as enjoyable during open-air motoring as it is with its roof snugly in place. — John O'Dell, Senior Editor, Green Car Advisor