Achieving draft-free, warm, and comfortable travel in a four-seat cabriolet has been a design and engineering challenge for as many decades as such cars have existed.
Compromise solutions have been tried, mainly via the implementation of draft deflectors, but these were often large, ungainly, and rear-visibility restricting. But now, arch technology innovator Mercedes-Benz has come up with an elegant aerodynamic solution for its new E-Class Cabriolet, which sets a new standard for comfortable four-seat, open-air motoring.
Called AIRCAP, it incorporates 20 patents, minimizes passenger-area drafts and turbulence, and is—literally—the most outstanding piece of novel technology on a car that incorporates a great deal.
Mercedes’ first efforts at achieving buffet-free, open-air motoring stretches back to 1990. But the designs created then were unacceptable for a premium cabriolet—probably for any cabriolet.
With advances in materials, electric motors, and automotive aerodynamic applications, Mercedes returned to the problem confident of success. That confidence was well placed, although it was to take 1000 h of wind-tunnel work and five years of design and development before an acceptable physical and aesthetic solution was achieved.
How acceptable that solution is will be proved by end-user response, but for driver and passenger the benefits are palpable, with the open-top environment becoming, as experienced by this AEI Editor, markedly quieter, warmer, and less turbulent.
The basic aerodynamic concept of AIRCAP was applied experimentally 20 years ago to the W124 E-Class Cabriolet. But the various static solutions tried looked too clumsy for production.
Bernd Plocher, who headed the team that created the new technology, said the project was resurrected in 2003: “The design breakthrough came in late 2005—a wind deflector (airfoil) and nylon net module that retracted into the windshield frame when not in use, complemented by an adjustable draft-stop between the rear head restraints. We built a conceptual model to demonstrate how the system functioned.”
Team member Daniel Seifert added: “Our job was to develop a system—basically a wing combined with a net—that could not be seen when folded closed but that could be extended sufficiently to create the required aerodynamic effect.”
Mercedes’ own wind tunnel and the acoustic tunnel at Stuttgart University were both used extensively in the lengthy development period. It was found to be necessary to have a rear deflector to achieve the correct balance.
The net has holes of different sizes to provide the necessary airflow without creating roar or hiss. “We started with simple net structures, but they were very loud,” said Seifert. Eventually, more than 200 net combinations were tested before a suitable configuration was determined. And a four-link kinematic system was needed to fold the system into the upper windshield.
The AIRCAP, which will be a standard fit for major markets but an extra cost option (at about €690) for others, extends by about 6 cm (2.4 in) when selected by the driver via a button. The windshield-mounted deflector elevates the airflow, with the net reducing negative pressure in the passenger area. The draft stop between the rear headrests reduces the backflow.
The system can be deployed at road speeds up to 160 km/h (99 mph)—but was tested to an airspeed of 210 km/h (130 mph)—and remain in place to the Cabriolet’s top speed. “We need about 5 N·m to deploy the system at high speed, but the space available for an electric motor to drive it is only about 2 cm,” stated Seifert.
The solution was a stepper motor from Swiss company Maxon, which develops micro-motors for aerospace and medical applications. The type fitted to the cabriolet has seven gears. It also drives a locking mechanism for the AIRCAP system, which is either fully retracted or fully deployed.
The rear draft stop has three positions linked to the AIRCAP windshield module: half extended when only the front seats are in use, fully extended when a rear seatbelt is buckled, or retracted when AIRCAP is deactivated. AIRCAP is linked to the car’s PreSafe system and folds in the event of a possible impact being detected.
The AIRCAP system incorporates 211 individual components and 32 types of material. Although designed specifically for cabriolet applications, it could be used on two-seat roadsters but at present has not been developed for possible use in relation to reduction of noise or the aerodynamic heterodyning effect for panoramic or regular sunroofs. Mercedes has a separate development program for those.
The new E-Class Cabriolet was designed and developed alongside the other versions in the range (coupe, sedan, station wagon). The cabriolet’s best Cd figure is 0.28, which compares to 0.24 for its coupe sibling.