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Formula One
VIDEO: How Do F-Ducts Work?
The rollout of the F-Duct is reshaping Formula One...
Racecar Engineering  | http://www.racecar-engineering.com  |  Posted July 17, 2010   Charlotte, NC
The above video shows a lift generating wing stalling but the basic theory is the same for a downforce generating race car wing. (Image: Racecar Engineering)
Article written by Lawrence Butcher, Racecar Engineering

When McLaren's F-Duct system first appeared in pre-season testing it was hailed by many a a true stroke of genius, a classic example of out-thinking the regulations. With the basic idea being that the driver is able to alter the airflow over the rear wing, without infringing regulation 3.15 (below), and in doing so gain a speed advantage on straights.



3.15 Aerodynamic influence: With the exception of the cover described in Article 6.5.2 (when used in the pit lane), the driver adjustable bodywork described in Article 3.18 and the ducts described in Article 11.4, any specific part of the car influencing its aerodynamic performance:

- 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

This speed advantage appears to have given the team the upper hand at the Shanghai circuit, Racecar decided to investigate the theory behind the new system.

Why is the F-Duct beneficial?

Basic wing theory

First we need to look at some basic aerodynamic theory regarding wing profiles and lift/drag ratios. At the simplest level a wing generates downforce due to its profile accelerating airflow on its lower surface in relation to the flow over the top surface. If flow is accelerated pressure drops, with the result being a pressure differential between the upper and lower surface of the wing and thus a net downward force.
An additional pair of slot gaps in the upper rear wing element are fed by airflow from the duct that exits from the Shark Fin engine cover. (Image: Craig Scarborough)

Flaps and slot gaps

If the angle of attack of a wing is increased it can ultimately 'stall' due to flow separation along the trailing edge, with a resultant loss in downforce and consequently aerodynamic grip.

To get around this problem, dual element or slot-gap wings are used, these allow for some of the high pressure flow from the top surface of the wing to bleed to the lower surface of the wing. This increases the speed of the flow under the wing, increasing downforce and reducing the boundary flow separation.

If you look at a modern F1 rear wing you can see this concept taken to the extreme, with multi-element wings creating huge amounts of downforce, the downside being a significant drag penalty. However if the flow over the 'flap' section of the wing can be stalled, the lift/drag ratio worsens, but the overall result is a massive drop in the coefficient of lift, resulting in a net reduction in drag, hence the benefits in relation to top speed. It should however be noted that it is only stalling the trailing edge flow that is beneficial as opposed to stalling the entire wing.

Early solutions

Previously teams had contrived to create flexible wing sections the allowed the 'slot gap' to close up under high aerodynamic loads, once this became evident to the governing bodies it was rapidly outlawed. Wings are now subject to static load tests to ensure that they cannot flex. So if a team were able to achieve a similar effect within the regulations, considerable straight-line performance gains could be made. Racecarcar spoke to a source in F1 to find out exactly how significant these gains could be.

'If you stall the flap on an F1-wing (in the wind tunnel) then the drag drops enough to calculate that the top-speed of the car could be 3-5kph faster (we did this ten years ago) but the trick is doing it in a way that's legal (well, not illegal). Wind tunnel engineers can do this by altering the slot-gap geometry and/or changing parts to simulate flexing-on-the-track. It's very easy to demonstrate in a wind tunnel - just very difficult to engineer it so that it's not illegal."
Photographs of the Mclaren cockpit show a clear channel running alongside the driver. (Image: Craig Scarborough)

McLaren's solution

McLaren appear to have found a very neat solution for redirecting the airflow over the rear wing and consequently allowing the flap to stall. Whilst they have been very tight lipped about the system, it is most likely that the conduit from the front to rear of the car has a vent in the cockpit that can be blocked by the drivers left leg, which is not in use on long straights. Blocking the vent could direct enough airflow through the conduit to disrupt the flow over the rear flap and induce a stall. This approach is ingenious for two key reasons:

- By using the drivers leg to direct the flow, the regulations are not contravened regarding movable areodynamic devices.

- By incorporating the design into the monocoque it becomes very difficult for other teams to copy the device, due to the fact monocoques have to be homologated and changes are very expensive to make.

While the exact workings of the system are impossible to judge, the above explanation is the most likely. McLaren have managed to get a jump on their competition and a number of teams have already tested their own interpretations of the system, although whether these will integrate as efficiently with their existing aero packages remains to be seen.

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