Influence of Aerodynamics on the Fatal Crash in Le Mans

EASC 2009
4th European Automotive Simulation Conference
Munich, Germany
6-7 July 2009
Influence of Aerodynamics on the Fatal Crash in
Le Mans 1955
Peter Gullberg, Lennart Löfdahl and Zhiling Qiu
Department of Applied Mechanics
Chalmers University of Technology, 412 96 Göteborg, Sweden
ABSTRACT
In the 1955 Le Mans race one of the worst crashes in motor racing history occurred and this
accident changed the face of motor racing for decades to come. However, still fifty years
after the fatal accident a number of questions remained unsolved. One open issue is the
influence of aerodynamics on the scenario, since the Mercedes-Benz 300 SLR involved in
the crash was equipped with an air-brake.
In a recent work [1], it was shown that the air-brake in operation generates a significant drag
increase, but also under certain conditions a down force on the vehicle. In the current work,
CFD is utilized as a tool for the investigation of the aerodynamic aspects of the accident.
More advanced parameters like the pitch angles are computed, and a simple model for the
flight path is derived. It is found that the pitch angles, which were largely affected by the air
brake, had a significant influence on the length of the flight path.
1. INTRODUCTION
The 1955 Le Mans 24 hour race changed the world of motor sport entirely, and many
persons have the opinion that this accident moved the sport from innocence to moderny.
After roughly two and a half hours of the race the largest accident in motor racing history
occurred; two cars crashed into each other just outside the former pilots and motor dealers
stands. The crash was a race incidence, however, also a consequence of the combination of
fast and slow cars moving on the same track as well as traffic in and out of the pit lane. A fast
Mercedes-Benz 300 SLR clipped the rear part of an Austin Healy 100 when the Austin Healy
100 did an evasive maneuver to avoid a collision with Jaguar who was going for a pit stop.
The weighty Mercedes, driven by Pierre Bouillon (a Frenchmen who raced under the name
“Pierre Levegh”) caught fire and was catapulted into the crowd of spectators, while the Austin
Healy driven by the Englishman Lance Macklin could stay on the track. Unfortunately, the
Mercedes-Benz went up in the air and at the end of its flights path it hit a concrete tunnel,
and spitted up in parts which killed more than 80 people. A brief summary of the race and the
accident scenario may be found on the web sites;
http://www.youtube.com/watch?v=FXtb5eDUuQw
http://www.youtube.com/watch?v=IuKP-rNyiOQ
Through the years this fatal accident has been much discussed, analyzed thoroughly and
numerous investigations have been accomplished in order to sort out the real accident
scenario. In 2005, fifty years after the accident, a vast amount of papers and books were
published to elucidate the catastrophe further and also put it into a modern perspective. To
mention a few good publications; Hilton [2] and Bonte [3] summarize the course of events in an
excellent way and point out several interesting political observations which not were so
obvious to anyone in the mid-fifties. It should be remembered that the event took place only
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4th European Automotive Simulation Conference
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ten years after the end of the Second World War, and at that time England, Germany and
France were nations under strong pressure to recover from the war damages. These political
effects are most interesting, but will not be discussed further in this paper. More relevant
issues for the present paper are the large number of technical questions and unclear
statements which still remain, and a few of these constitute the kernel of this paper.
The aerodynamic performance and documentations of the Le Mans cars used in the fifties is,
with today’s measure, rather inadequate. From the archives it may, however, be concluded
that the Mercedes-Benz 300 SLR used during the end of the “Silberpfeile-era”, was a most
interesting car. It had an extremely powerful drive line, good road holding, but a weak point
on the brakes since the cars were equipped with drum brakes which on long distance races
like Le Mans had an obvious tendency to fade. In order to match the braking force of their
main competitors, the Jaguar D-type that was utilizing disc brakes, Mercedes-Benz had
developed an air-brake system which, on this particular model, was located just behind the
driver. This device was complex but most efficient. Mike Hawthorn [4] wrote that;”…he (Juan
Manuel Fangio) could leave his braking (on the Mercedes) just about as late as I could on
the disc-braked Jaguar…”. Similar statements are found in Ludvigsen [5]. In addition, the
Mercedes works drivers at that time, Juan Manuel Fangio and Stirling Moss, were joined in
the statement that this version of Mercedes-Benz 300 SLR, “had a much better cornering
with the air-brake in operation”, see Moss and Nye [6] and the aforementioned short movies
for further illustrations.
Figure 1: Smoke visualizations in the Daimler-Benz tunnel, from Motor Klassik [7]
All three Mercedes-Benz 300 SLR used in Le Mans 1955 were equipped with air-brake. One
remaining question from the race event is whether Pierre Levegh was able to raise his airbrake before the crash. No one will ever know; so in this paper it is assumed that the air
brake was in full operation just before and during the course of the accident. Another key
issue is how the aerodynamics of the whole vehicle changes when the air-brake was in
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operation; even though a heavy vehicle like the Mercedes-Benz 300 SLR creates a
substantial momentum it is difficult to explain how it could fly such a long distance as it did
before it hit the concrete barricade?
Reliable aerodynamic data on old racing cars are, due to the experience of the authors, very
difficult to find, however, in the late eighties, smoke visualizations and drag coefficients on
the Mercedes 300 SLR were published in the German motor magazine, Motor Klassik [7], see
figure 1. A drag coefficient of 0.437 with the air-brake down, and 1.090 with the brake in
operation were measured. It was claimed that the data were from measurements in the
Daimler-Benz Wind tunnel in Unterturkheim, 1978 and 1986, respectively. Relatively good
agreement between these reported experimental data, and computations were found and
discussed in a recent paper by Gullberg and Löfdahl [1]. Figure 2 shows the computed total
pressure distribution of the Mercedes Benz 300 SLR on the ground and with the air-brake in
operation from the Gullberg and Löfdahl’s work [1].
Figure 2: Total pressure, wing high vehicle on ground
The objective of the present work is to continue to use CFD in order to elucidate some unclear questions of the accident scenario in the 1955 Le Mans race. This paper relies on, and
is an extension of the previously reported work by Gullberg and Löfdahl [1], however, in the
present case focus is extended from drag and lift coefficients at zero pitch angle to the
computation of the aerodynamic coefficients at different pitch angles and other quantities
necessary for the determination of the flight path. To estimate this path, a number of
assumptions on initial and boundary conditions have been made, and a highly simplified
model for the flight path has been derived and used. The assumptions made are discussed
in detail in the next sections, but here it could be remarked that the objective of the flight path
calculations was not to determine an exact flight path in terms of height and length but rather
to find the influence from aerodynamic forces on the flight length of the vehicle. In addition, a
common goal of this paper and the previous work of Gullberg and Löfdahl [1] are to establish
CFD as a tool for the reconstruction of un-explained race accidents.
Like many other non-commercial projects this project also suffers of limited recourses in
terms of computer as well as man time. Hence, it is important to keep this in mind when
evaluating results and conclusions made, since better resolutions, more sophisticated flight
models and additional adequate input data might change some statements. At this point it is
natural as well to remark that no information or data used in this paper have been supplied
by Daimler Benz. So all data utilized have been found in the open motor sport literature, and
estimates of inertial moments etc. are based on the laws of classical mechanics.
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2. METHODOLOGY
2.1 Generation of Surface Geometry
To get a representative geometry of the Mercedes-Benz 300 SLR, a 1:24 die-cast model of
the 1955 Le Mans car of Fangio / Moss was purchased from the model maker, Pauls Model
Art, who is well known for their high level of details and representative models; see Figures 3
and 4 for further details.
Figure 3: 1:24 model of the Fangio/Moss Mercedes-Benz 300 SLR
Figure 4: Side view of the model with detail features
This model car was laser scanned with and without the air brake in operation, and the
obtained surface data was prepared in ANSA version 12.1.3 to generate a dense and clean
surface model suitable for CFD simulations. To make realistic computations a driver CAD
model was prepared in ANSA, and located in the driver position inside the cockpit, Ludvigsen
[5]
. The corresponding surface model is shown in Figure 5.
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4th European Automotive Simulation Conference
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6-7 July 2009
Figure
e 5: Surface
e model for CFD simula
ations.
2.2 Gen
neration of a CFD Mod
del
The surrface geom
metry in Figure 5 was then triang
gulated in ANSA,
A
and after this Harpoon
H
version 3.1 was ussed to gene
erate a Hex--dominated volume me
esh. The tunnel was siimulated
ed three carr lengths ah
head of the vehicle and seven car lengths be
ehind it. Th
he height
extende
of the tunnel
t
was 2.5 car len
ngths for th
he cases with floor and 5 car len
ngths for th
he cases
where the car was placed in the
t free-stre
eam, total width
w
of the tunnel was 4 car lengtths. One
refinement zone of
o the mesh
h just around the veh
hicle and wake
w
was u
used, and a further
refinement around the air-bra
ake region. As usual, the finest mesh
m
cells a
are found cllosest to
the surfface in ord
der to repre
esent the boundary
b
layer properlly. A magn
nified picture of the
mesh iss seen in Figure
F
6, an
nd the total mesh size
es, dependin
ng on different configu
urations,
range frrom 11 to 16
6 million ce
ells.
Fiigure 6: A tyypical mesh
h density ussed
Finally, the mesh was importted into Flu
uent version 6.3.26 where
w
the ca
ases were studied.
These flow casess were set--up using a standard
d Fluent incompressib
ble solver with
w
the
realizab
ble k-epsilon
n turbulencce model. Each
E
case was
w solved at a veloccity of 200 kph
k
with
moving ground and
d stationaryy wheels ass boundary conditions
c
d to the a
due
assumption that the
driver was
w
braking
g the vehicle during th
he accident. Because of the rela
atively high
h ground
clearance of this vehicle, a flat underb
body was assumed
a
an
nd used in the compu
utations.
Configu
urations with
h different pitch
p
angless ranging frrom -20° to 20° in 5° sstep, and additional
pitch an
ngle of ± 30
0° and ± 60
0°, were sim
mulated. All configuratio
ons were ccomputed as
ssuming
that the
e air brake was in operation durring the acc
cident. The
e pitch anglle of the ve
ehicle is
defined in Figure 7,
7 and Table
e 1 shows th
he configura
ations studied.
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4th European Automotive Simulation Conference
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Figure 7: Vehicle pitch angle definition
Condition
Vehicle with wing high in vicinity of floor Vehicle with wing high in free-stream
Pitch angle
0°, ± 5°, ± 10°, ± 15°, ± 20°, ± 30°, ± 60°
Table 1: Configurations calculated in CFD
To normalize the forces, the wheelbase was used as reference length, see Figure 8, where
also the centre, located midway between the wheel axles, used for moment calculations is
shown. The frontal area of the vehicle with the airbrake in low position was used for
normalization and the numerical values are found in Table 2.
Figure 8: Reference length used is wheelbase and reference position for moment is midway
between the wheel axles.
Ref. area:
Ref. velocity
Ref. density
Ref. length
1.951
55.55
1.2
2.363
m2
m/s
kg/m3
m
Table 2: Input data used for normalization
3. FLIGHT PATH CALCULATION AND DISCUSSION
3.1 CFD Simulation Results
Some of the aerodynamic coefficients used are shown in Figure 9, and a cubic interpolation
is utilized. As expected, the aerodynamic drag force and moment rise significantly with the
increase of the absolute value of pitch angle. Moreover, the results show that the
aerodynamic lift increases when pitch angle reduces and vice versa, see Figure 7 and 9. At
large negative pitch angles, the rear aerodynamic lift coefficient increases significantly due to
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4th European Automotive Simulation Conference
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the blockage between the rear end of the vehicle and the ground as is shown in Figure 9.
The same effect is found on the front for positive pitch angles. Of course, these effects are
not present when the vehicle is off the ground.
Figure 9: Aerodynamic coefficient change with pitch angle
3.2 Calculation Model
In the model for the flight path, the vehicle is simplified as a rigid rod which represents the
wheelbase length and center of gravity (CG). The coordinate system used and the forces
acting on the vehicle are shown in Figure 10. Equation [1], [2] and [3] below describes the
motion of the model in terms of linear accelerations in X and Z direction, and the angular
acceleration around an axis through CG in Y direction.
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Figure 10: Simplified model and free body diagram
ax =
1
( FD + N ⋅ f )
M
[1]
az =
1
( FLf + FLr + N − M ⋅ g )
M
[2]
θ&& =
1
{[ FLf ⋅ ( L − k ) − FLr ⋅ ( L + k ) − N ⋅ ( L + k + Lr )] ⋅ cos θ −
IY
[3]
[ FD ⋅ k + N ⋅ f ⋅ ( L + k + Lr )] ⋅ sin θ }
FD , FLf and FLr are the aerodynamic drag, front and rear aerodynamic lift force
respectively. The definitions of these forces can be seen in Equation [4], [5] and [6]. The
normal force N is created in case the vehicle rear end touches the ground during the flight.
Since velocity in Z direction of vehicle rear end is fairly small during the flight, the normal
force from ground is estimated into Equation [7].
1
FD = ⋅ CD ⋅ ρ ⋅ A ⋅ V 2
2
[4]
1
FLf = ⋅ CLf ⋅ ρ ⋅ A ⋅V 2
2
[5]
1
FLr = ⋅ CLr ⋅ ρ ⋅ A ⋅ V 2
2
[6]
N = M ⋅ g − FLf − FLr
[7]
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The values of the vehicle model properties used in the flight path calculation are shown in
Table 3, where the total mass of the vehicle includes the net mass of the vehicle (880 kg) [9]
and the assumed mass of driver (70 kg). The moment of inertia is estimated by assuming the
vehicle to be a regular box shape with the dimensions shown in Figure 11 and calculated
according to Equation [8]. However, given that the engine is located close to vehicle front
end and the gear box is near the vehicle rear end [10], the moment of inertia used for
calculating flight path shown in Figure 13 and 14 is estimated by using 120% of the value
from Equation [8] according to authors’ experience.
Figure 11: Moment of inertia calculation model
IY =
1
r
1
f
⋅ ( ⋅ M ) ⋅ (H 2 + 4 ⋅ f 2 ) + ⋅ ( ⋅ M ) ⋅ (H 2 + 4 ⋅ r 2 )
12 2 L
12 2 L
[8]
The vehicle weight distribution is set to be 58% on the front axle and 42% on the rear axle [11].
Hence, the value of k, see Figure 10, will be 0.189 m.
Total mass
Moment of inertia
k (distance between CG and midpoint)
f (friction coefficient)
g (gravity acceleration)
950
480.24
0.189
0.5
9.8
kg
kg ⋅ m2
m
kg/s2
Table 3: Input data used for simplified vehicle model properties
A Matlab Simulink model based on Equation [1], [2] and [3] was set up for the calculation of
vehicle motion in real time. The aerodynamic coefficients used in the model were taken from
the cubic interpolation; see Figure 9, in steps of one degree of the pitch angle. The
aerodynamic coefficients used for the flight path calculation were determined through a linear
interpolation between the condition of in-vicinity-of-floor and in-free-stream, according to
Equation [9] and [10] so the ground effect on vehicle aerodynamics disappears completely
when the height of vehicle lowest point reaches one meter from the ground. The initial motion
of the vehicle is defined in Figure 12, more details may be found in Bonté [3].
p=
1 − height of vehicle lowest point
(Length unit: m; Minimum: 0)
1
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Aero.Coeff . = p ⋅ Value-in-vicinity-of-floor + (1 − p ) ⋅ Value-in-free-stream
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[10]
Figure 12: Initial motion of the vehicle model
3.3 Calculation Results
In figure 13 and 14 one computed flight path with and without the influence of the
aerodynamic forces are shown. The flight path is most sensitive to the initial conditions, pitch
angle, take off angle and velocity. In the shown example the pitch and take off angle is the
same (-5 degrees) as is shown in figure 12. As is evident from the example shown in figure
13 and 14, the aerodynamic forces have a significant influence on the length of the vehicle
flight. Without any aerodynamic forces the vehicle model can only fly approximately 55
meters, but including aerodynamic effects this distance increases to at least 100 meters.
With aerodynamic forces acting on the vehicle, it has a tendency to quickly increase its pitch
angle due to the large rotating moment generated by the aerodynamic lift force. The
aerodynamic lift force rises in proportion to the increasing pitch angle and due to this effect
the flight path is prolonged. The pitch angle reaches its peak value around the midway of the
flight path due to the increasing aerodynamic drag.
Figure 13: Flight path without aerodynamic forces
Figure 14: Flight path with aerodynamic forces (upper) and aerodynamic coefficient change
(below)
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In order to estimate sensitivity of the flight path calculations a brief sensitivity analysis was
conducted. Table 4 shows three different take off angles, 2.50°, 3.75° and 5.00° together with
three different values of the moment of inertia, 100 %, 120 % and 140 %. The flight path
model seems to be quite insensitive to variation in the latter quantity, but the take off angle
influences the flight length. However, the tendency shown in figure 13 and 14 remains i.e.
that the flight length is longer when the aerodynamic forces are included in the model.
Initial pitch angle
-2.50°
-3.75°
-5.00°
with aero. eff.
without aero. eff.
with aero. eff.
without aero. eff.
with aero. eff.
without aero. eff.
Moment of inertia
100 % 120 % 140 %
35 m
35 m
36 m
29 m
78 m
75 m
71 m
42 m
102 m 103 m 100 m
55 m
Table 4: Flight distance change with initial pitch angle and moment of inertia
4. CONCLUSION
In this work CFD has been used to establish some of the aerodynamic performance of a
Mercedes-Benz 300 SLR with the specification used in the 24 hours Le Mans race of 1955.
Raising the air-brake increases drag and pitch coefficient significantly and this certainly
influenced the car handling as has been described in the Gullberg and Löfdahl work [1]. In
addition, the current work confirms the general trend that the drag coefficient decreases as
the vehicle moves away from the ground, a well-known effect for slender bodies see for
instance Barnard [8].
The results also indicate that the pitch angle has significant influences on the aerodynamic
drag, lift and especially the rotating moment. The aerodynamic drag and moment arise
significantly with an increase of the absolute value of pitch angle.
The flight path calculations show that the aerodynamic influences of the vehicle with airbrake in operation are significant. Although the flight path estimations were based on a highly
simplified model a clear tendency of a much longer flight path was found when including
aerodynamic effects.
A full determination of the accident scenario of the 1955 Le Mans accident will, however,
require more sophisticated and detailed CFD studies to be conducted. So the current work
should be considered as an introductory part with the objective of forming a few cornerstones
for future computations.
ACKNOWLEDGE
The authors appreciate David Söderblom for supporting this work with Matlab model input
data and driver CAD model for the CFD simulations.
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REFERENCES
1. Gullburg, P. and Löfdahl, L., The Role of Aerodynamics in the 1955 Le Mans Crash,
SAE 2008-01-2996
2. Hilton, C., (2004), Le Mans´55, Breedon Books Publishing, ISBN 1 85983 441 8.
3. Bonté, M., (2005), 11 June 1955, B.A. Editions, ISBN 2-915744-01-7.
4. Hawthorn, M. (1958) Challenge Me The Race, London
5. Ludvigsen, K. (1971) The Mercedes-Benz Racing Cars, Bond/Pankhurst Book, ISBN 087880-0093-3.
6. Moss, S. & Nye,D., (1999) My cars, my career, Haynes publish., ISBN 1 85960 661 X.
7. Motor Klassik, “Das aktuelle Magazin fur alle Freunde klassicher Automobile” Issue #3
1987
8. Barnard, R.H., (2001), Road Vehicle Aerodynamic Design, Mechaero Publishing, ISBN
0-9540734-0-1
9. http://www.conceptcarz.com/vehicle/default.aspx?carID=2252&i=2#menu, available on April
2009
10. Engelen, G. and Riedner, M., (1999) Mercedes-Benz 300 SL, Vom rennsport Zur
Legende, ISBN 3-613-01268-5
11. http://www.classicandperformancecar.com/features/octane_features/222306/mercedes_300slr.ht
ml, available on May 2009
CONTACT
Peter Gullberg: [email protected]
Lennart Löfdahl: [email protected]
Zhiling Qiu: [email protected]
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