The McLaren MP4-25 was a huge improvement of the previous year’s car and saw McLaren finish 2nd in the 2010 constructors’ championship. It featured an ingenious system called the F-duct, which allowed the driver to alter the airflow over the rear wing in order to gain a speed advantage in the straights.
The regulations
Article 3.15 of the technical regulations state that any aerodynamic device:
- Must comply with the rules relating to
bodywork - Must be rigidly secured to the entirely sprung part of
the car (rigidly secured means not having any degree of freedom) ; - Must
remain immobile in relation to the sprung part of the car.
The challenge which the teams face is generating high downforce to help in the corners, whilst reducing drag to aid top speed in the straights. Before we attempt to understand the F-duct design, firstly we must grasp the basic concept of downforce generation and wings.
The basics
A wing on a Formula One car generates downforce due to its shape. The air flowing underneath the wing has to travel further, therefore accelerating. The acceleration of airflow causes a low pressure to be created. The airflow over the wing is slower in relation to below and will therefore have a higher pressure. This difference in pressure creates a force pushing the car onto the track, known as downforce.
The ‘steepness’ of the wing is known as the angle of attack, the angle between the chord line and direction of airflow. This concept is illustrated below:
By increasing angle of attack, the downforce generated will be increased. However, too high and the wing will ‘stall’. This means that the flow seperates from the surface of the wing and causes a dramatic loss of downforce.
Multi-element wings
To combat this issue, multi-element wings are used, which allow some of the high pressure air above the wing to pass through to below – preventing flow seperation. This allows a steeper angle of attack and therefore more downforce can be generated. However, this comes with a drag penalty.
But, if the flow can be stalled, the drag produced decreases along with downforce. On a straight, downforce is less important and stalling the wing would be advantageous. Previously, teams have used flexible wings to close the slot gap (the small gap between wing elements) under high aerodynamic load – at high speeds. However, the FIA deemed this illegal as it was a moveable aerodynamic device.
The F-duct
McLaren’s ingenious solution to the issue was the F-duct. The driver was able to redirect air over the rear wing on straights, in order to stall it. Air entered the car at the front, with a vent in the cockpit to prevent the air reaching the wing. But, on straights, the driver could cover the vent with their leg – allowing the air to reach the rear wing. This design made them faster on the straights as it reduced the drag generated. It wasn’t considered a moveable aerodynamic device, as it was controlled by the driver, so therefore was deemed legal by the FIA.
The system was incredibly hard for other teams to copy as the cockpit and monocoque have to be homologated and the extensive safety tests make changes expensive. However, it was banned in 2011.