03-09-2014, 10:33 AM
Aerodynamics of Race Cars
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INTRODUCTION
Automotive racing must have started at the turn of the twentieth century when the first
two automobiles pulled one beside the other. From that first moment on the sport
consistently grew, not always following the evolutionary trends of the automotive
industry. For example, contemporary race cars have components such as inverted
wings and protruding angular plates, which seem unpractical, and are hence unusable
by the automotive industry. Those involved with the sport insist that motor racing is
a “pure sport” with its own set of rules that need not benefit the general automotive
industry. Such opinions paved the way to numerous forms of racing. In some racing
categories the vehicles resemble production sedans, and in others they look more like
fighter airplanes, not to mention the various tracks that range from paved/unpaved to
straight, oval, or regular road courses. In all forms of racing, however, aerodynamics
eventually surfaced as a significant design parameter, and by the end of the first
100 years of automobiles, all race car designs have some level of aerodynamic element.
Although the foundations of aerodynamics were formulated over the past 200 years,
not all principles were immediately utilized for race car design. Naturally, the desire
for low drag was recognized first and Hucho (1998, p. 14–15) describes one of the
first streamlined race cars (the 1899 Camille Jenatzy) to break the 100 kilometer/hour
(km/h) “barrier.” This electric-powered racer had a long cigar shape in an effort to
reduce aerodynamic drag. The rapidly developing automotive industry followed and
one of the most significant designs of that era is the 1924 Tropfenwagen (“droplet
shape” in German) described by Hucho (1998, p. 18–19). This automobile’s shape was
dominated by the airfoil shape (particularly from the top view) and recent tests in the
Volkswagen wind tunnel showed a drag coefficient of CD = 0.28, which is outstanding
even by today’s standards
HOW DOWNFORCE IS CREATED
Race car design was historically always influenced by streamlining the vehicle body,
particularly when the focus was on reducing high-speed air resistance. This trend
continued well into the middle of the 1960s, implying that aerodynamic vehicles are
also aesthetically attractive, an image that was somewhat altered by the discovery of
aerodynamic downforce and its effect on race car performance. The foremost and
simplest approach to generate downforce was to add inverted wings to the existing
race cars. However, this newly discovered advantage was not free of complications.
For example, the aerodynamic downforce increases with the square of the vehicle’s
speed whereas tires depend far less on speed. Consequently, if the inverted wings are
attached to the vehicle then the suspension spring rate must be stiffened to allow
for the additional high-speed loads. Variable downforce-generating devices followed,
mostly based on reducing wing or flap angle of attack at higher speeds
Race Car Wings
Airplane wing design matured by the middle of the twentieth century and it was only
natural that race car designers borrowed successful airplane wing profiles to use on
their vehicles. However, this approach was not entirely successful due to the inherent
differences between these two applications. The difficulties in this technology transfer
were highlighted by Katz (1994) and his findings can be summarized as follows:
A race car lifting surface design is different from a typical airplane wing design
because (a) a race car’s front wings operate within strong ground effect, (b) open-wheel
race car rear wings have very small aspect ratio, and © there are strong interactions
between the wings and other vehicle components (e.g., body, wheels, or other wings).
These arguments are discussed in more detail in the following paragraphs
Wind Tunnel Methods
During the 1960s, just when the significance of aerodynamics for race car design
was realized, wind tunnel methodology was already mature and widely used by the
aerospace industry. It was only logical that wind tunnel testing became an integral
part of all race car development projects, as well. Small-scale tests (e.g., Katz 1985a)
helped in investigating basic ideas prior to building the vehicle, and validations were
performed later on the track with the actual race car. However, wind tunnel testing of
a race car posed several difficulties when using traditional aeronautical wind tunnel
facilities. The first major problem was due to the small clearance between the vehicle
underbody and the stationary floor of the test section (the second problem related
to how to mount the rotating wheels). Existing wind tunnel correction methods (see
Barlow et al. 1999, ch. 9–11) could not correct for the additional shear layer created
CONCLUDING REMARKS
The complexity of automobile and race car aerodynamics is comparable to airplane
aerodynamics and is not limited to drag reduction only. The generation of downforce
and its effect on lateral stability has a major effect on race car performance, particularly
when high-speed turns are involved. In the process of designing and refining current
race car shapes, all aerospace-type design tools are used. Because of effects such as
flow separations, vortex flows, or boundary-layer transition, the flow over most types
of race cars is not always easily predictable. Due to the competitive nature of this sport
and the short design cycles, engineering decisions must rely on combined information
from track, wind tunnel, and CFD tests