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AERO AT 1000mph

the same as if it were circular, then additional shock formation and wave drag will hopefully be avoided. Conversely, if the shape is thinned at some point then it may be necessary to add cross section elsewhere locally so a satisfactory longitudinal distribution of cross-sectional area is maintained. These principles have been applied in supersonic aeronautics for a long time, but land speed record vehicles differ from aircraft designs in three key ways – they have no large lifting surfaces (although trim devices might generate some lift), they run on wheels and, of course, they also run on the ground.

GROUND EFFECT Ground proximity is put to good use on conventional racecars by accelerating air between the body and the ground, and the resulting low pressure can be managed to generate a substantial portion of a racecar’s total downforce. In the context of land speed record vehicles, not only is substantial downforce unnecessary, likely causing an unwanted increase in rolling resistance, the very fact that there is a small gap beneath the vehicle can cause other

passing beneath the vehicle have exerted sufficient lift to raise the rear wheels off the ground. Thrust SSC dynamically adjusted the vehicle’s pitch angle to try to maintain reasonably consistent wheel loads at these speeds, and most of the current designs carry design features and / or controls, such as active canard wings, to trim the vehicle and maintain a consistent balance. And then there’s the influence

of the vehicle on the ground itself. Thrust SSC encountered a totally unexpected problem when it approached Mach 1 and the bow shock wave ‘fluidised’ the desert surface, generating greatly increased rolling resistance (and wheel tracks that disappeared beneath the surface by 150mm). The challenges then are considerable, so let’s look to see what each of the teams is doing aerodynamically to combat these.

Aussie Invader 5R The basic shape of the rocket only-powered AI 5R reflects the thinking of the team’s well known co-designer and consultant engineer, Englishman John Ackroyd of Thrust 2 and SSC fame. ‘The transonic and shock wave effects may be

“ length-to-diameter ratio is also a key weapon “

unwanted aerodynamic effects. Even in the subsonic speed

region, ignoring compressibility effects, airflow entering this gap is liable to be accelerated and create reduced pressures, so ground clearance and pitch angle are even more important. And, as such, nose height and shape are critical, since this affects how much air passes under the vehicle. As Bloodhound SSC’s chief of aerodynamics, Ron Ayers, explains in ‘Bloodhound TV episode 5’ one aspect to be avoided is the positive pressure that might occur in the convergence zone under a raised nose. Lowering the nose and using a modified nose shape can overcome this. Historically, there have also

been problems in the transonic region where shock waves

reduced by a slender fuselage producing a small cross-sectional area, a ‘v’ profile underbelly for shock deflection, a high ground clearance to achieve reduced [underbody] pressure and a rear wheel strut / support design producing zero to minimal lift.’ Team principal and driver,

Rosco McGlashan OAM, is unequivocal about the aerodynamics of his charge: ‘Aussie Invader is the most aerodynamically efficient shape ever conceived for a car. We have a single five-inch axle that, when compared with any other outrigger car, is very slick. Most run four or five two-inch round struts to locate the rear wheels – in other words, twice the area and a lot more drag. Our rear axle will have a leading and trailing fairing with a minimum amount of width

Figure 1: the Sears Haack concept, with its gradual and uniformly changing cross-sectional area, would not create additional shockwaves along its body

Supersonic aeronautical engineering is an obvious source of data at speeds approaching 1000mph but, though some principles can be applied, they are fundamentally different beasts to land speed record vehicles

Shock waves occur where there are changes in cross-sectional area. Wave drag, which occurs as a result of this is a major obstacle at supersonic speeds

to prevent ground-generated pressure from creating lift. Rear wheel fairings are presently being investigated.’ Aussie Invader’s v-shaped belly is based on minimising the effect of the shock waves on the underside, allowing it to reflect off the ground and dissipate past and around the fuselage without causing unwanted lift. An extensive CFD programme has been undertaken at Western Australia’s new

Pawsey Centre, from where Dr James Jewkes of the Curtin University Fluid Dynamic Research Group commented: ‘We are using the well-regarded OpenFOAM CFD library, and our methodology reflects that used by any modern motorsport team, using a validated (and sanity checked!) baseline, then gradual modifications to the design compared with the original baseline data. Our design goal is primarily to look at how we can

January 2012 • 67

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