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AERO AT 1000mph

The North American Eagle features a rounded, aeronautical fuselage under the car to allow shockwaves to travel off at oblique angles

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it had a large, flat underside, whereas the NAE has a rounded underside. Even Ron Ayers told me the rounded bottom would make it much easier for the shock waves to travel off at oblique angles. The rear of the vehicle will have the greatest propensity for lift, and will require some special engineering tricks to keep that part planted on the ground.’ Alluding to the salt spray phenomenon, Shadle added, ‘We expect some drag development along the fuselage but it is much more severe on salt than it is on dirt. The salt clings to anything it hits and builds up on the surface.’ Aerodynamic trimming will be done with adjustable front canard fins. The nose canards are 18in

(457mm) long and built in the same aerodynamic shape as the F-104 wing. The team currently runs them parallel to the ground plane but they are adjustable. Shadle: ‘As our CFD analysis is validated against physical runs at higher speeds, we will change the angle as a factor of speed. The angle can be changed based on input from the onboard data acquisition system.’ The team is relying a great

deal on computational fluid dynamics because, Shadle says, ‘It is so accurate, and it is easy to model various situations and then run the solutions in CFD. We will conduct [physical] runs with a lot of strain gauges, load cells, accelerometers and pressure sensors located in

strategic locations to validate the computer analysis.’

Sonic Wind Probably the smallest of the current contenders, Sonic Wind is rocket-only powered and has been sized purely to fit the driver, the motor and the 30in (762mm) diameter fuel tanks, the latter being the defining factor on the frontal area of the fuselage. Front and rear wheel tracks

Steve Fossett’s car has been rebuilt and re-wired from the ground up, with a longer wheelbase, wider track and lengthened wheel covers

are narrow, and part of designer and driver, Waldo Stakes’, design philosophy is that the aerodynamics will provide the vehicle’s stability. ‘The key to building an LSR vehicle is to build a “supersonically stable” vehicle from the start, and then accelerate it as fast as possible into the speed regime where it will become most stable. Then it is simply a question of balancing the vehicle’s weight against the lift and drag forces in order to achieve the speeds desired. ‘I have shaped the entire fuselage into a ‘bell’ shape [cross section] and air is denied access to the underside of the vehicle as much as possible. Any air that tumbles past the front wheels is tripped by a vertical von Karman supersonic ogive air dam with a blunt back end. This flow is then controlled by a tunnel under

the centreline of the car and vacuumed out the back by the rocket plume. ‘I have used a cruciform tail

to keep the rear end stable, this form of tail resisting movement better than any other design. And I also use twin canted out ‘bi-wedge’ design tails below the vehicle’s c of g. These generate shock waves that strike the ground on either side of the rear of the vehicle, and the pressure created by them holds the vehicle in roll. Most designers would use a widespread pair of wheels at the rear to do that job, but those wheels will generate shock waves that will choke off airflow under the rear of the vehicle and create rear lift.’ Sonic Wind has been shaped with the aid of CFD, but Stakes is more reserved about their value to the finished design: ‘CFD will give your ideas validity and corroborate your concepts, but then [you need to] build a metal model and ground plane fly it in a supersonic wind tunnel. Then you won’t be guessing, you’ll know without a doubt.’ So it remains to be seen who

will get there first, and whether we might then see the record changing hands soon after. It’s all going to be about risk management…

January 2012 • 71

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