In 2010 the key technical development was the F-Duct, a legal driver controlled system that stalled the rear wing for more top speed. During the course of the season, as more of the system was uncovered by prying cameras in the pit garages, I attempted to cover the workings of the F-Duct in several posts. But just a couple of years later I was able to buy a Force India F-Duct assembly from one of the teams licensed parts sellers. With this complete F-Duct and some background from people at the team involved with the project, we are now able to explain the solution in more detail.
History
For many years the concept of reducing rear wing drag at speed had been a covert aim of many F1 teams. Rear wings produce a lot of drag and reducing this at higher speeds, when more downforce is not required, means the car can achieve a higher top speed. With bans on moveable aero, the teams had to rely on flexible wing elements, either reducing the wings angle of attack or closing slot gaps to stall the wing. Being visible from trackside or from onboard cameras, large gains from this flexibility was eventually stamped out by FIA deflection tests. But the aim of downforce producing wings for corners and low drag wings for straight bring a non linear downforce curve with speed, remained an objective for the aero depts.
The F-Ducts first test at Valencia 2010
During the first pre-season tests for the 2010 the appearance of the McLaren caught many people’s eyes. The MP4-25 had a number of details that each could be put down to simple explanations; the duct on the nose could be for driver cooling, the roll hoop inlet could be for oil cooling and the slot in the rear wing might just be tape. Some teams noticed it break cover at the first winter test, others were a bit slower on the uptake.
McLaren’s initial knee operated control duct
I too was initially sceptical that anything else was going on. With Media banned from the pit lane for this test, none of us could get a close look at the car. When stories of a rear wing being stalled by the driver pressing his knee against a duct surfaced, I found it hard to take the rumours seriously. The knee control was later switched to hand control of the F-Duct, but this seemed so unlikely that teams hadn’t been thinking along these lines before the F-Duct, although I was reminded it’s not a new idea in motorsport to use the drivers anatomy to influence part of the car.
A classic example of this was Ferrari noticed that Michael Schumacher tipping his head to the side. When he did this it created an unwanted area of airflow separation along the engine cover, upsetting flow to the rear wing. But fortuitously this movement also increased pressure in the airbox for more speed. Thus his famous tipped head along the straight actually drag reduction and engine power!
Of course, what since became known as the F-Duct dominated technical development for the year, by achieving the aerodynamicists’ nirvana of having non linear downforce with speed. Being able to run larger wings for the corners and then lose their drag once the car was in a straight-line had obvious performance benefits.
Understanding that it was legal at the first race was important. FIF1 made an immediate response to the innovation, being particularly quick following their experience in 2009, when the teams that were quickest to copy the double diffuser had the greatest success with it.
Surprisingly the F-Duct was a relatively cheap development. The cost of the parts was minimal, the costs were mainly the DOWind TunnelCFD time to perfect the design. But for this minimal investment, the F-Duct has high return in terms of the performance it yielded.
Sauber were the first team to bring development parts to a race in a bid to catch up with the McLaren development. By the Spanish GP many teams had their first F-Ducts and the part remained on most cars throughout the season.
With the safety impact of the driver taking his hand off the wheel for the entire straight and even some fast turns, the FIA rightly agreed to rule many aspects of these systems illegal at the end of the year.
In their place was the legal and FIA controlled Drag Reduction System (DRS), which remains on the cars to this day.
Anatomy
As with many F1 parts the F-Duct is a relatively simple solution, with just four ducts emanating from an “X†shaped junction, allied to a slot machined into the rear wing. By altering the flow through the ducts via the switch, the slot in the rear wing would blow and stall the rear wing, reducing its drag.
It’s this junction of the ducts that’s key to making the F-Duct work, this is the fluidic switch. The switch changes the path of the airflow through the ducts, by means on a controlling duct. The fluidic switch is analogous to an electrical transistor, simply a switch operated by a control wire. The flow through the fluidic switch is controlled by simple aerodynamicfluid theory and is employed is other industries. Although the effect of blowing or altering the boundary layer was well understood, fluidic switches were new to F1 as far as I know.
The Fluid Switch up close
Feeding the switch was the high pressure feed duct, this was typically set inside the roll hoop or in FIF1’s case from two inlets either side of the roll hoop. Trailing the switch are two ducts, one neutral duct which curves down to exit under the Y75 winglet (monkey seat). The other duct is the stalling duct; this passes straight out behind the switch to attach to the rear wing. Key in the systems operation is the neutral duct being larger in cross section than the stalling duct. Lastly the driver control of the switch is achieved by a control duct, this has its own high pressure feed, the duct then passes into the cockpit where is has an opening, then the duct routes up the bottom of the switch.
The hollow rear wing with the stalling duct feeding into it
Copyright: Morio 7 October 2010, ”Suzuka Circuit”
The rear wing profile is hollow, the stalling duct attaches to the wing and the airflow can pass out through a thin slot spanning nearly the entire the width of the wing. The slot itself is at an oblique angle to the wings surface, rather than tangential to it, again this is key in to making the system work.
McLaren had the F-Duct idea in development for a few years. The decision to fit it to the MP4-25 meant that the packaging could be worked out, to fully integrate it into the design of the car. Thus McLarens version was neat and efficient. Other teams were stuck in a period of homologated monocoques and crash structures; simply making holes in the tub was not possible. So the teams playing catch up had to find packaging solutions to get the high pressure feed and control duct to pass through the tub.
Also we can see there’s a pressure sensor fitted to this F-Duct, this would be relatively easy to integrate into the cars loom and telemetry system.
How it works
The high pressure flow from the roll hoop inlets pass into the fluid switch, this flow passes smoothly through the switch, tending to follow the floor of the switch via the coanda effect and out of the neutral duct. Air tends to flow into the neutral duct, rather than the stalling duct, as it has a larger cross section and a lower pressure area at its exit. Meanwhile flow passing into the control duct also follows the path of least resistance and vents into the cockpit. At this point no (or very little) air passes into the stalling duct and the flow around the rear wing remains attached to create downforce.
When the driver wants to reduce downforce and drag, his hand covers the control duct, the flow that was passing into cockpit, now passes up the control duct and into the bottom of the fluid switch. This trips up the boundary layer flow inside the switch and the splitter at the back of the switch helps send more flow into the stalling duct. This flow passes into the wing and out of the slot. This also affects the boundary flow, the flow under the wing separates from its surface and the flow stalls. This reduces downforce and unlike a stalled aircraft wing also reduces drag. As an F1 wing is so highly loaded, the majority of its drag is from trailing vortices, stalling the flow under the wing reduces these vortices and their inherent drag.
F-duct in Practice
Although simple in concept the reality was more difficult.  The biggest issue was sealing of the assembly due to its being constructed of single side moulded composites and added to a finished car. There were also issues of repeatability of operation due to the size of the control duct of the fluid switch. I was told about one occasion where a leaking assembly actually worked and a sealed one didn’t because the control flow was too weak.
These issues resolved, the F-Duct did offer a real performance gain. Today it’s probably easier to compare the F-Duct to the current DRS. In the best cases it probably had about 65% of the effect of the DRS. Presumably because you can’t lose the form drag of the flap as effectively. DRS gains about 10-15Km/h and 0.5-0.7s/lap.
The other big factor in the exploitation of the F-Duct was the driver, I’m told the better drivers got the most from the system and were able to cope with the unusual aspects of driving with it. The FIF1 system didn’t require the hand to be fully off the wheel to activate and was ‘fail safe’ as opposed to some that required the driver to seal the activation duct in corners and open it on straights!
My greatest memory of the f-duct and one that perfectly encapsulates its purpose was at a wet Spa Grand Prix. In the race the McLaren drivers would exit a turn; spiralling vapour trails (tip vortices) were forming at the rear wing tips. As the car ran out to the exit kerbs and straightened up, the driver must have covered the control duct to stall the rear wing and these vapour trails stopped instantly. Showing how effective the F-Duct was at reducing the drag inducing tip vortices.
Over the winter new rules banned driver controlled aero (excluding DRS), slots in the rear wing and bodywork connecting to the rear wing. Although these approaches banned the driver operated F-Duct, the fluid switch and stalling concepts remain valid and the current un-raced generation of Drag Reducing Devices (commonly termed DRD or passive DRS) feature many of the components and concepts of the F-Duct. Hopefully later this year we will see these DRD’s get to race.
The F-Ducts twin inlets flanking the airbox inlet
The duct work up close
The control duct, more pipework from the cockpit joins to this fitting
The stalling duct outlet fitting
The neutral duct outlets
An F-Duct trial fitted to Seat Leon!
+Info: scarbsf1.com
