[世界賽車比賽] F Ducts: How do they work?

carwing

McLaren have found a clever loop hole in the 2010 regulations allowingthem to stall the rear wing at high speed, Racecar looks at how theymay have achieved this, and why it provides an advantage
When McLaren's F-Duct system first appeared in pre-season testing itwas hailed by many a a true stroke of genius, a classic example ofout-thinking the regulations. With the basic idea being that the driveris able to alter the airflow over the rear wing, without infringingregulation 3.15 (below), and in doing so gain a speed advantage onstraights.
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.

Thisspeed advantage appears to have given the team the upper hand at theShanghai circuit, Racecar decided to investigate the theory behind thenew system.
Why is the F-Duct beneficial? Basic wing theory
Firstwe need to look at some basic aerodynamic theory regarding wingprofiles and lift/drag ratios. At the simplest level a wing generatesdownforce due to its profile accelerating airflow on its lower surfacein relation to the flow over the top surface. If flow is acceleratedpressure drops, with the result being a pressure differential betweenthe upper and lower surface of the wing and thus a net downward force,as illustrated below.

Flaps and slot gaps
Ifthe angle of attack of a wing is increased it can ultimately 'stall'due to flow separation along the trailing edge, with a resultant lossin downforce and consequently aerodynamic grip.
The above video shows a lift generating wing stalling, however thebasic theory is the same for a downforce generating racecar wing.
Toget around this problem, dual element or slot-gap wings are used, theseallow for some of the high pressure flow from the top surface of thewing to bleed to the lower surface of the wing. This increases thespeed of the flow under the wing, increasing downforce and reducing theboundary flow separation. (See below)


Ifyou look at a modern F1 rear wing you can see this concept taken to theextreme, with multi-element wings creating huge amounts of downforce,the downside being a significant drag penalty. However if the flow overthe 'flap' section of the wing can be stalled, the lift/drag ratioworsens, but the overall result is a massive drop in the coefficient oflift, resulting in a net reduction in drag, hence the benefits inrelation to top speed. It should however be noted that it is onlystalling the trailing edge flow that is beneficial as opposed tostalling the entire wing.
Early solutions
Previouslyteams had contrived to create flexible wing sections the allowed the'slot gap' to close up under high aerodynamic loads, once this becameevident to the governing bodies it was rapidly outlawed. Wings are nowsubject to static load tests to ensure that they cannot flex. So if ateam were able to achieve a similar effect within the regulations,considerable straight-line performance gains could be made. Racecarcarspoke to a source in F1 to find out exactly how significant these gainscould be.  
'If you stall the flap on an F1-wing (in the windtunnel) then the drag drops enough to calculate that the top-speed ofthe car could be 3-5kph faster (we did this ten years ago) but thetrick is doing it in a way that's legal (well, not illegal). Windtunnel engineers can do this by altering the slot-gap geometry and/orchanging parts to simulate flexing-on-the-track. It's very easy todemonstrate in a wind tunnel - just very difficult to engineer it sothat it's not illegal."
McLaren's solution McLarenappear to have found a very neat solution for redirecting the airflowover the rear wing and consequently allowing the flap to stall. Whilstthey have been very tight lipped about the system, it is most likelythat the conduit from the front to rear of the car has a vent in thecockpit that can be blocked by the drivers left leg, which is not inuse on long straights. Blocking the vent could direct enough airflowthrough the conduit to disrupt the flow over the rear flap and induce astall. 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.
:Byincorporating the design into the monocoque it becomes very difficultfor other teams to copy the device, due to the fact monocoques have tobe homologated and changes are very expensive to make.
Below are some images of the most probable routing for the system:
(Illustrations by Craig Scarborough)


Photographs of the Mclaren cockpit show a clear channel running alongside the driver.

Additionalpair of slot gaps in the upper rear wing element are fed by airflowfrom the duct that exits from the 'Shark Fin' enigne cover.

Illustration of the most likely routing for the duct.
Whilstthe exact workings of the system are impossible to judge, the aboveexplanation is the most likely. McLaren have managed to get a jump ontheir competition and a number of teams have already tested their owninterpretations of the system, although whether these will integrate asefficiently with their existing aero packages remains to be seen.