Aerodynamics and Air Routing
Aerodynamics and air routing
During development of the new vehicle it became obvious that further enhancement of the aerodynamic properties of the 911 Carrera was required. The objective in doing so was to take account of the increased performance by reducing the aerodynamic lifting forces and at the same time to further reduce the drag coefficient (despite the higher cooling air requirement for brake and engine cooling).
The result was a significant improvement in aerodynamic coefficients in comparison with the previous model. The drag coefficient was reduced to Cw = 0.28 (previous model 0.30) and the lift coefficients at the front and rear axle were each reduced by 0.01 to Cav = 0.05 und Cah = 0.02 respectively.
These values cannot be matched by any of our international competitors.
The main challenges facing the aerodynamics engineers of the 911 Carrera were:
- The larger frontal area, caused by flared wheel housings and wider tyres in particular
- The lower lift coefficients and hence the increased wheel load
- The engine's higher cooling air requirement
- The higher cooling air requirement of the brakes due to the increase in performance
- The more efficient cooling of the gearbox, and of the 6-speed manual gearbox in particular, due to the higher engine output
Optimisation of the outer skin
The main starting points when it came to optimising the outer skin, apart from the front and rear of the vehicle, were the wings and front side sections. Particular attention had to be paid to aerodynamic integration of the flared wheel housings.
The sweep and the radii of the front apron have been optimised to produce an aerodynamically efficient air flow at the front end and significantly better prerequisites for cooling air flow without affecting the styling of the outline. The lateral openings on the front apron required to supply cool air to the radiators have also been carefully optimised and integrated in the front end concept. The lateral front apron radii and wheel houses are designed to shield the front wheels from the air flow in an optimum manner and to provide much greater wheel housing ventilation. These measures reduce drag and, by providing greater wheel housing ventilation, also reduce lift at the front axle.
The shape of the A-pillar has also been optimised in terms of flow characteristics and is now more vaulted. This reduces resistance and cuts down on wind noises at high speeds for noticeably more comfort on long journeys. The door mirror has been redesigned and is now connected to the mirror triangle via a new double arm. The mirror housing and the air duct at the mirror and the side windows have been optimised for minimum resistance and maximum protection against drops of water landing on the mirror glass and the side windows. In addition, a new hydrophobic surface coating which virtually eliminates soiling of the side windows is being used for the first time on the front side windows to keep them free of dirt.
In the rear area, the flared wheel housings and the rear spoiler as well as the rear and rear centre sections have been optimised. As on the front, the radii of the rear side sections have been designed to produce an aerodynamically efficient air flow at the rear wheels and the rear end, which improves resistance and reduces lift at the rear axle.
The redesigned rear spoiler has a carefully optimised edge that produces a defined air flow breakaway when the spoiler is extended. In combination with the optimised extension height of the rear spoiler, this ensures that the rear spoiler functions at the optimum aerodynamic operating point and that the desired rear axle lift with the optimum resistance advantage is achieved. On top of this, slotted openings have been integrated in the upper shell of the rear spoiler and, in combination with the underlying engine compartment scavenging blower, optimised to ensure adequate cooling and ventilation of the engine compartment at all times.
Optimisation of the cooling-air guide
The main objective when designing the cooling-air guide is to ensure the necessary cooling air requirement for the engine and brakes in all operating states of the engine. The general idea is to achieve a flow with as little resistance as possible to minimise the effect of the cooling-air flow on the resistance coefficient of the vehicle as a whole. The effect of the flow on the lifting forces must also be minimised or used as cleverly as possible to achieve the objectives for lifting forces and lift balance.
In addition to using larger and more efficient radiators, the optimised cooling-air guide is a significant factor in the increased cooling performance. As on the previous model, the new cooling-air guide is completely closed to avoid leakages. At the same time, targeted guidance of the cooling air ensures an optimum air flow for the radiators. When designing the cooling-air guidance, particular attention was paid to keeping the air ducts short and the deflection as low as possible. The exhaust air flow from the radiators is therefore now expelled laterally into the wheel housing liners instead of vertically downwards in front of the front wheels. This reduces flow loss for the cooling-air guidance system while lateral expulsion of the air reduces the lift at the front axle. Another benefit of lateral expulsion is that it results in much less dust being raised when the radiator fan is running on dusty surfaces. Ventilation flaps have been inserted into the corners of the square fan frames to further increase air throughput during journeys. These ventilation flaps open above speeds of approx. 70 km/h to facilitate an additional flow of cool air.
All in all and despite the increased cooling-air throughput, the new resistance-optimised air routing concept has enabled the air flow resistance to be limited to approx. 1.5 % of the total resistance - an extremely low level in comparison with the competition. Requirement-driven supply of cool air has enabled significantly smaller air inlet openings in the front end in comparison with the competition.
The improved driving performance has also resulted in increased requirements for brake cooling. An optimised brake air deflector has been developed for the 911 Carrera which ensures more efficient deflection of the air routed through the brake air ducts on the underbody to the brake discs and significantly better brake cooling as a result of the higher air throughput.