In preparation for the final races, McLaren have developed another iteration of their F-duct rear wing. The new version places the stalling slot onto the rear face of the main plane of the rear wing, where the previous versions had all placed the slot on the rear face of the flap. This is a subtle change and effects the way the wing stalls to create improve aero efficiency (i.e. more straight-line speed, or more downforce for a given top speed).
F-ducts work as they reduce the drag created by the rear at speed, this drag limit’s the top speed the car can achieve for a downforce level. The more downforce the wing makes, the more drag is created and hence the lower the top speed. Although a larger wing creates more frontal area and hence presents more of an obstruction to the airflow, it is in fact the drag induced the unseen air spilling off the wing that’s creates most of the rear wings drag. In fact an F1 wing despite looking so streamlined creates more drag than a solid block of the same dimensions. This is because an F1 wing is so highly loaded as it strives to create huge amounts of downforce from such a small surface area, that the air coming off the wing creates an invisible extension to the wings frontal area. Created by both the airflow rising all but vertically off the centre part of the rear wing and then the even more draggy vortices spiralling off the wing tips. These vortices are often seen in wet conditions and used to be seen as a sign of an efficient wing, but are in fact hugely detrimental to the downforce\drag coefficient of a rear wing. This is why we see such efforts to reduce wing angles near the endplates and team make the slits in the endplates, as these are all aimed at reducing these vortices.
An ideal situation would be a wing with steep angles of attack for downforce in the corners, where drag is of little consequence. Then a nice flat wing for the straights, where less drag improves top speed and downforce is not required to give the car grip. Without being legally able to move the wing itself(albeit this will allowed in 2011) there has no mechanism to create this effect in F1.
Teams have known for a long time that stalling the rear wing drastically reduces downforce and as a result reduces drag. This is because the large flow structures coming off the wing break up and shed the drag inducing effect they have. Many teams have tried to exploit the rules by flexing their rear wings to create just such an effect, but the FIA has outlawed this via a number of deflection tests and latterly the slot gap separator.
McLaren have now found that they can stall the rear wing, if they blow airflow out of a slot at right angles to the underside of the rear wing. But this in itself cannot be exploited unless there is a means to switch the airflow on and off. With the driver controlled F-duct, controlling the flow either to the stalling slot or to a neutral outlet, McLaren can achieve the ideal situation of a downforce wing setting for corners and low drag for the straights.
By the driver controlling a duct that affects the flow through a ‘fluid switch‘, which is a “V shaped duct behind the roll hoop, flow can either pass to the slot or a secondary duct exiting in the low pressure region well away from the upper rear wing.
When the F-Duct is disengaged air passes from the roll hoop inlet into the Fluid switch. From there the air flows both into the low level nuetral outlet and partly into the cockpit. When the driver covers this cockpit control duct, the change in back pressure makes fluid switch alter the direction of the roll hoop flow, to pass into the duct towards the rear wing.
When the driver engages the F-duct the airflow alters inside the fluid switch to send the air out of the stalling slot. This breaks up the vortices shed from the rear wing and reduces downforce and drag. McLaren initially had this full width slot towards the trailing edge of the flap, the airflow stalls quite late as it passes under the wing and the most likely effect of this is that airflow can reattach quickly when the duct is disengaged. Its also possible that a downside to this, as the wing stalls quite near the trailing edge there may still be some drag induced by the general upwash from under the wing.
When Sauber copied the F-duct at the 2010 Australian GP, they had their F-duct stall the wing via a stalling slot in the main plane of the rear wing. While Ferrari and Red Bull followed McLaren with a flap stalling F-duct, Force India, Renault and latterly Toro Rosso have gone the way of a main plane stalling solution. By stalling the wing much further upstream, its possible that the disruption to the airflow further reduces the upwash, in turn reducing drag even further. On the downside the wing may take longer to see the flow fully reattach when the duct is disengaged.
McLaren appear to have seen a benefit in the main plane blown effect. Although the solution has required new ducting and a new rear wing, it will only see at most three races before F-ducts are banned for 2011. Such is the cost of fighting for the championship this year.