Racecar Engineering – September 2019

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

TECHNOLOGY – LSR AERO STABILITY

to the high thrust line of the jet. According to
the theory of stability for cars, this is clearly
destabilising. At M = 0.62 the rocket ignites and
its lower thrust-line restores the (download-
related) stability for the rest of the acceleration.

Instability theory
The question arises, does this theoretical
instability matter? At low Mach numbers, say
below about 0.2, the fin will not be providing
much positive stability so one might expect
the automobile instability to cause steering
problems. However, we have personally met or
communicated with drivers or designers from
most of the jet/rocket powered cars since they
began breaking records in the 1960s and none
of them referred to problems of directional
instability on deserts (with one exception;
Thrust 2 did suffer serious steering problems
when its metal wheels were badly affected by
conditions on a rutted Bonneville salt surface,
but no steering problems were subsequently
experienced on Black Rock Desert’s more
compliant dry mud lake bed). We infer from
this that the rate of divergence of these very
long vehicles is sufficiently slow that the driver’s
reflexes can correct for any steering deviation –
probably without the driver even realising that
the car is unstable. Thus, we appear to be in the
region described as ‘unstable but controllable’.
A bicycle is another vehicle that is unstable but
stillcontrollableevenatlowspeeds.

But will yaw instability present a difficulty

for Bloodhound in this velocity range? With a

wheelbase of 8.9 metres there should not be

a significant problem. But the first runs into

this velocity range will still be treated with

appropriate caution – just in case.

As the velocity increases, the aerodynamic

component of yaw stability becomes

increasingly dominant. At this point, the key

parameter for yaw is the yaw static margin, i.e.

the distance between the centre of gravity and

the aerodynamic centre in yaw. It is essential

that the yaw static margin is positive as speed

increases – if the static margin is negative, the

vehicle will try to swap ends, an effect that

would increase with speed until it overwhelms

all other forces and the driver loses control.

What is the ideal value for yaw static

margin? For Dieselmax we aimed for two to

three per cent of the wheelbase. This worked

well in a gusty crosswind and is also the target

figure for Bloodhound. But this may not be

easy to achieve since yaw static margin varies

significantly during the course of a run due

to the fuel consumption moving the centre

of gravity and the yaw centre of pressure

varying with Mach number. Experience will

show us what works best.

The above analysis makes no reference to

the type of surface the vehicle is traversing, but

solid wheels on a desert will have very different

characteristicsfromrubbertyresona road.For

instance, the unyielding shape of the aluminium

wheel must sink into the desert surface.

Experiments have shown that Bloodhound

wheels will sink about 7mm to 8mm into

the Hakskeen desert surface (the South

African venue for the attempt) at low velocity.

Computations show that at 1000mph the

wheels will ‘plane’ at a depth of about 3mm, so

surface compliance will provide little assistance

to the suspension. Rolling resistance will be

some two to three times greater than for rubber

on tarmac throughout the velocity range.

Fluid deserts

The lateral constraint on a solid wheel by the

desert is still a matter of conjecture, but clearly it

will be much less than for a rubber tyre/tarmac

contact. Crude experiments at low velocity

show that the desert structure breaks away for

any lateral load greater than about 0.35g to 0.4g.

At still lower sideloads, it is not clear whether

lateral constraint will exhibit the characteristics

of viscous friction or of some other compliance

law. As the record car accelerates towards

the 600mph mark the desert fluidises at the

wheel contact points, which clearly will reduce

the lateral grip still further, so the handling

characteristics will be constantly changing.

Rubber tyres on tarmac generate increased

lateral grip as the download increases. It is

quite possible that solid metal wheels running

ona desertracetrackwillalsocreate higher

Automobile stability dominates at the lower velocities, but

aircraft stability is massively dominant at higher velocities

For runway testing Bloodhound has used regular rubber tyres but for the high-speed desert tests and the Land Speed Record attempt itself solid aluminium wheels will be fitted