Snowbanks in the recent Rally Sweden has put once again in trouble the refined aerodynamic design of the 2017 WRC cars. All teams have suffered at different moments from losing different parts. Some of the most exposed, due to its location, are the dive planes. Pictures below show how all teams suffered from losing them during the rally.
M.Østberg/T.Eriksen, Citroën C3 WRC, Rally Sweden 2017, 6th – picture by Michelin
A.Mikkelsen/A.Jaeger, Hyundai i20 Coupé WRC, Rally Sweden 2017, 3rd – picture by Harald Illmer – iR7.at
S.Ogier/J.Ingrassia, Ford Fiesta WRC, Rally Sweden 2017, 10th – picture extracted from WRC+ images
O.Tanak/M.Jarveoja, Toyota Yaris WRC, Rally Sweden 2017, 9th – picture by Michelin
Although small in size, dive planes are essential for keeping the aerodynamic balance of the car, which translates into good car handling. Losing any of these planes can convert a car into an undrivable machine, as downforce generated in the rear of the car becomes unbalanced, and the car can easily lose cornering ability (understeering) and power (traction).
This is the reason why they were introduced for the first time in rally cars in 2017, in pursuit of aerodynamic balance to fully exploit all the downforce generated, and thanks to the higher design margins engineers found on the new regulations approved in 2016.
Also known as dive plates or canards, the name of dive planes comes from its resemblance with the winged appendages on submarines. They consist of winglets usually made with carbon fiber reinforced plastic, and mounted on both sides of the front bumper, just ahead of the front wheel arches. They cannot protrude outside the existing outer line of the car body.
The picture below shows the Monte Carlo 2018 dive planes configuration for all WRC teams.
Comparative of dive plane configuration at the start of season 2018 – pictures 1 and 2 by Nacho Mateo, 3 by @world and 4 by Honza Fronek
In all cases, they have the shape of an inverted airfoil, at considerable angles of attack: the higher the angle, the higher the downforce generated by pressure difference, but also the higher the drag. That’s why engineers use the lift to drag ratio, as a measure of the gain in lift related to the drag impact.
Most of the teams use two dive planes, except Hyundai, who moved from two to one by the end of last season. Apart from supplying some extra downforce, the second plane can be very useful in case of damage to the other one.
Also, most of them are isolated, while Fiesta’s upper and Citroen’s lower planes connect with the fender, leading to dead ends which probably contribute to increasing drag.
K.Al-Qassimi/C.Patterson, Citroën C3 WRC, Rally Catalunya 2017, 17th
They are similar in size in the case of Citroën and Hyundai, while Toyota uses longer planes. The upper dive plane of the Ford Fiesta WRC is the longest…while the lower is the smallest.
In some cases, they incorporate side lips (Toyota), to better channel airflow. Also note the presence of a small wing on top of the fender, in order to protect air exiting from wheels on top of the fender from airflow coming from dive planes.
Toyota Yaris WRC, Rallye Monte Carlo 2018 – picture by Ondra Beneda
Other configurations have been used in the past season by Hyundai and Toyota, going in opposite directions: Hyundai removed one of them, while Toyota added a second one. Also, Hyundai removed the side lips, while Toyota added them.
H.Paddon/S.Marshall, Hyundai i20 Coupé WRC, Rally Finland 2017, ret. – picture by Hyundai Motorsport
J.Hännninen/K.Lindström, Toyota Yaris WRC, Rally Catalunya 2017, 4th
Citroën tested a modification of the upper dive plane in September 2017 tests. It consisted of a prolonged plane, up to the top of the fender. Such a solution has not been seen in any other test session since then.
S.Loeb/D.Elena, Citroën C3 WRC, Test in Spain, September 2017 – picture extracted from Jaume Soler video
The main advantage of dive planes is that they can generate downforce on both the front and rear of the car. On the front of the car, and due to their airfoil shape, downforce is generated by pressure difference on both sides of the plane (high on the upper side, lower below the plane), although at a certain drag cost.
But the presence of the dive planes has an additional effect: they generate a line of vortices that travel alongside the car, acting as a barrier that prevents air from entering under the car. The lowest the amount of air below the car, as we already saw in our previous post on undercar aerodynamics, the higher the downforce at the car rear. But, for this to happen, dive planes have to be designed so they send the vortices line in parallel to side skirts, while the orientation of most of the current dive planes suggests the line is sent upwards, and this effect might be not achieved.
Other minor contributions of dive planes are that they help to convert turbulent airflow reaching the front of the car into laminar (which is more effective for downforce generation) or they redirect air into the top of the fenders or even into the rear wing (also generating additional downforce and reducing drag).
They can also have a negative impact, in case they are located too close to the front splitter, leading to the interaction of high and low-pressure areas, which makes lose efficiency to both.
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