Racecar Engineering – September 2019

SEPTEMBER 2019 http://www.racecar-engineering.com 81

damping should cater for these problems,

but it is also nice to know that these possible

modes of instability are not being forced. We

understand that heave has been experienced

by Formula 1 cars, but although porpoising is a

theoretical possibility we do not know if it has

ever been recorded in practice.

Heave instability is controlled by the heave

derivative (which is the measure of how vertical

aerodynamic load changes with vertical chassis

movement), which we have computed using

our CFD capabilities. In the case of Bloodhound,

the heave derivative is actually extremely small,

so forcing would not occur.

Porpoising is controlled by the pitch

derivative (the measure of how vertical load

changes with pitch angle). For Bloodhound,

our CFD shows that the point of action of lift

increment due to a change of pitch attitude

(that is, the aerodynamic centre) is well behind

the centre of gravity and so is stabilising.

Although there is a need to fit front winglets to

cope with any possible front-end trim problems,

they must not be larger than is necessary as this

could lead towards porpoising instability.

It is also interesting to note that for an

aeroplane, having a positive static margin

(with the aerodynamic centre behind the

centre of gravity) provides static stability but

on the suspension-constrained Bloodhound, it

provides dynamic stability.

Car or plane?

In the introduction to this piece the question was

posed as to whether, at supersonic velocities,

Bloodhound will behave like a high-speed car

or like a very low flying aircraft. It is neither of

these. At high velocity, the desert surface will

fluidise under wheel contact so lateral wheel

forces are expected to be minimal. Steering will

be provided by the aerodynamic forces on the

steered front wheels, while the fin provides the

yaw stability. Thus, the dynamic yaw response

of Bloodhound will probably feel like that of a

supersonic hovercraft, or a supersonic hydrofoil, if

such things existed. As they certainly do not, we

will have to explore the handling characteristics

in step-by-step testing, with gradual speed

increases to limit the magnitude of any changes,

just as we did with Thrust SSC.

Provided we remain focussed on

maintaining our key requirements of positive

and bounded wheel downloads, and a positive

and bounded yaw static margin, we can ensure

that the car will remain firmly on the ground

and pointing in the right direction.

This in turn will enable us safely to explore

(and refine) the performance and handling

characteristics of our supersonic record car.

In this respect, any suitably cautious attempt

to set a new world Land Speed Record

combines the best of experimental research

with the ultimate in motor racing.

The cockpit in which driver and co-author of this feature Andy Green will sit. At high speeds steering will be via aerodynamic effects on the front wheels, rather than lateral forces