Racecar Engineering – September 2019

SEPTEMBER 2019 http://www.racecar-engineering.com 77

Figure 1: The rear axle is experiencing more download than the front, generating more lateral grip and causing the ‘car’ to understeer

Figure 2: An ‘aircraft’ is considered to be stable if the yaw centre of pressure is behind the centre of gravity, like an arrow

Figure 3: The stability can be negative in the sense that the thrust-line of the jet engine is higher than the vehicle’s centre of gravity

powered cars. Namely, that the automobile
stability can be theoretically negative, so at first
sight they should be difficult to drive at low
velocities. The stability can be negative in the
sense that the thrust-line of the jet engine is
higher than the vehicle centre of gravity (since
much of the vehicle mass is in the chassis and
the wheels) thus generating a nose down
pitching moment, so giving more download
on the front wheel than on the rear. This
nose-down moment will be increased by the
influence of wheel drag on the surface making
the car even more unstable (Figure 3 ).
The graph (Figure 4) illustrates the influence
of the thrust line-of-action for Bloodhound.
It shows the computed downloads for the
front and rear pairs of wheels (in kilonewtons)
against Mach number as the car accelerates up
to a Mach number of 1.35. At Mach numbers
below 0.62, front wheels experience greater
Figure 4: Here the Y axis shows downloads in kN, X is Mach number. At Mach 0.62 the rocket in the tail of the car ignites downloads than the rear wheels due primarily