 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. |
