2004 LMP1 & 2 aero. rules

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Images copyright Frederic Le Floc'h (reproduced from the 1999 ACO Le Mans Annual), Franco Pizzagalli and Michael J. Fuller

Text copyright Michael J. Fuller
The ACO introduced the 2004 LMP 1 & 2 chassis rules in order to address some of the aerodynamic stability concerns that flat bottomed sports cars have encountered over the years.  In 2002 the FIA released a report called, "Aerodynamic Instability of Sportscars at Abnormal Yaw Angles".  The research was empirically compiled through scale wind tunnel testing of a 40% model on a rolling road wind tunnel.  The model was yawed through various angles relative to straight running and the research determined that the current flat bottomed prototype had a potentially dangerous instability when yawed past a certain degree.  The instability was such that the vehicle could easily become airborne.  Note that the yaw instability was a different phenomenon than that which was encountered by the Mercedes CLR at Le Mans in '99, the BMW LMR at Petit Le Mans in '00, and the Porsche GT98 at Petit Le Mans '98.  Those incidents, whereby the car flipped end-over-end, were caused by a combination of external conditions in conjunction with the large flat bottom area of the prototype.  The blow-over accidents, as well as the FIA's report, prompted further research at reducing aerodynamic instability and from that research came the core of the the 2004 aerodynamics modifications.
For new cars built to the 2004 LMP1 & 2 rules,  the ACO has eliminated the traditional flat bottom replacing it with a regulated tunnel system.  The idea was to reduce downforce generated outside of the car's wheelbase.  With the old flat bottom, the rear diffuser started at the rear wheel centerline.  For '04 the rear tunnels start 1000 mm in front of the rear wheels (yellow).  Therefore the primary suction peak generated by the tunnels is well within the car's wheel base and not at the rear axle centerline.  This, coupled with a reduction in rear overhang (to a maximum of 750 mm) as well as an increase in front over hang (to a maximum of 1000 mm) should reduce the pitch sensitivity of the cars and minimize the chance of a blow-over type accident.
Additionally the ACO is attempting to reduce the overall downforce generated by the cars in efforts to curb speeds.  One of the techniques used to achieve this is the mandate of a 20 mm plank on the bottom of the car (red plank in above Pizzagalli image).  The plank is intended to force an increase in ride heights and therefore reduces the effectiveness of the underfloor aerodynamics.  Secondly, the front splitter section forward of the front wheel centerline must be raised 50 mm over 1000 mm of width.  Again the aim is at slightly reducing the ability to produce downforce.  Lastly, the rear wing cord has shortened from 400 mm to 300 mm.  The shortening of the chord will reduce the efficiency of the airfoil.  The rear wing has also been moved closer to airflow disrupting structures such as the cockpit and/or roll over hoops by the reduction in rear overhang as detailed above and therefore will be operating in a less efficient flow field.  For '04 and beyond, open top cars will be required to provide roll over protection for the driver and theoretical passenger.  The obvious implications being increased drag for '04 open top LMPs over previous years.
To reduce yaw induced instability, the ACO have adopted a unique chamfered floor section.  In cross section, the bottom of the car is effectively wing-shaped.  Therefore, when the car is induced into a yaw situation, the underfloor will be encouraged to generate downforce where as in the past the top surfaces have generated lifting forces in excess of the flat bottom's downforce.  An additional yaw stabilizer will be large rear wing endplates.  The endplate rules specify a maximum size of 300 mm x 765 mm but also a minimum area of 1000 cm^3.
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ęCopyright 2004, Michael J. Fuller