Racecar Engineering – September 2019

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

TECHNOLOGY – LSR AERO STABILITY

In a perfect world the wheel downloads

would remain constant throughout the speed

range, but in reality this is not possible. Instead,

the wheel loads need to remain positive (to

avoid leaving the ground) and bounded (to

avoid structural overload of the chassis or

indeed the surface). The generic target we

identified for Bloodhound was to bound the

wheel loads to between 50 per cent and 150

per cent of the load due to gravity (from which

the above 10kN minimum is derived). This target

is simple to express but extraordinarily difficult

to achieve across a speed range from zero to

Mach 1.4 (1000+ mph). Nonetheless, it provided

the essential objective that drove the five years

and more of aerodynamic research that went

into establishing Bloodhound’s basic shape.

An alternative approach to bounding

wheel loads is to trim the vehicle using active

aerodynamic surfaces (such as winglets) to

adjust the vertical loads during the run. But

having assessed the complexity of a suitable

system to achieve this, and the huge negative

implications for safety if it malfunctioned in

any way at high speeds, we decided to take

the longer route for Bloodhound, and to find a

shape that would stay on the ground by itself

without active aero trimming.

Static stability

Now we will consider static stability in pitch.

Static stability is the tendency to return to the

neutral position when the vehicle is displaced.

Put simply, if a displacement caused the vehicle

to diverge, it is statically unstable, while if it

returns to the neutral position, it is said to be

statically stable. In theory, a vehicle will remain

on the ground if the total vertical down-load

(the sum of gravity, aerodynamic forces, pitch

due to thrust, pitch due to drag, etc.) acts

anywhere within the trapezium shape defined

by the wheel/ground contact points. In practice

more stringent criteria apply, and the centre of

download should act somewhere near the mid-

point of the wheelbase or problems of stability

and control are likely to result.

It is sometimes stated, erroneously, that it

is the aerodynamic centre that controls pitch

stability. The aerodynamic centre does indeed

have significance, but in connection with

dynamic stability (of which more later).

But it’s with stability in yaw where the

differences between automobile stability

theory and aircraft stability theory really

become apparent. An automobile is said to be

stable if the centre of download is behind the

centre of gravity as in Figure 1 (here the rear

axle is experiencing more download than the

front axle, generating more lateral grip and

ensuring the car has understeer).

An aircraft is considered stable if the yaw

centre of pressure is behind the centre of gravity

(like an arrow), as in Figure 2.

For a high-speed record car both are

significant, as automobile stability dominates

atlowvelocities(sincetheaerodynamicforces

willbesmall)butaircraftstabilityis massively

dominantathighvelocities.Andtherewillalso

beintermediatevelocities,likelytobeinthe

regionof200mphto400mph,wheretheywill

eachmakea significantcontribution.

However,therecouldbea problemwith

theautomobilecomponentofstabilityofjet

In a perfect world the wheel downloads would remain constant

throughout the speed range, but in reality this is not possible

Thrust Programme Ltd
Thrust SSC broke the Land Speed Record with 763mph in 1997, making it the first supersonic land vehicle

While Dieselmax was not jet-propelled it was

technically transonic – in that localised parts of it

experienced both supersonic and subsonic flows