2017 New generation of WRC cars has introduced a bunch of aerodynamic modifications that deserve a deep review, in addition to the brand new rear wings and diffusers already evaluated. The introduction of dive planes, skirts, front splitter and original air cooling inlets and vents are some of the new features to consider.
All cars have introduced dive planes on the sides of the front bumper. Shaped as inverted wings, they are used to send airflow upwards, contributing to generate downforce at car front. As a consequence, cars are more stable and understeering is reduced, partly compensating the high amount of downforce generated at the car rear with this new configuration. Toyota has only installed one, real, small dive plane, while the other three manufacturers have placed two of them. Hyundai placed them the most separated, and it is the only one to include an external lip on the top one. The new Ford Fiesta incorporates the longest dive planes, which cover from car side to front air intake. In all cases they are used to channel the air flow upwards over front wheel arch, thus resulting in a smoother flow.
Ford and, to a lesser extent Toyota, have placed also dive planes on their cars flanks, to contribute sending the air flow over rear wheel arches, thus preventing air from flowing underneath the car, which would have had an adverse impact on downforce.
Also, side skirts have been placed in all cars, leading to a bigger car surface. They minimize the entrance of high-pressure air underneath the car, and, combined with wider and deeper front splitters, they ensure reduced but faster air flow below the car. Higher velocity means lower pressure underneath the car, while higher pressure is reached on top of the car due to the splitter, so higher downforce is obtained by the pressure difference. The bigger the area the car covers, the higher the low-pressure area and, as a result, higher downforce is obtained. Use of deep and wide diffusers also contributes to reducing pressure, as air accelerates under the car due to the expansion volume created by the diffuser. This effect, combined with the downforce supplied by the big, new rear wings, ensures a very significant increase of downforce at the car rear, which has to be compensated by the frontal dive planes. Note that Ford has also incorporated a small elevation on both sides of the front splitter (covered by Michelin sticker in the top picture).
Engine and brake cooling is very important for the new cars, so multiple air inlets and hood and side vents have been located to facilitate air flow to cool down engine and brakes. Hyundai has chosen to install NACA ducts on both sides of the car to collect fresh air for rear brakes cooling, although they have been blinded for the two winter rallies, due to low ambient temperatures. Surely they will be used since Mexico, as the ambient temperature will be much higher.
A NACA duct is an air inlet designed to have a minimum impact (in terms of drag) on airflow over the car, while some fresh air still enters into the car for cooling purposes. If not properly designed, they can generate vortices as the air enters, with a negative effect on the air boundary layer, which may result in undesired flow separation and a reduction of rear wing performance.
These inlets were already used in the test the team carried out last November in France.
Rear brakes air outlet have also been the object of deep analyses, in order to take advantage of hot air leaving behind the car. All manufacturers have incorporated louvres at such point so they can redirect air into the optimal direction to gain an additional aerodynamic advantage.