4.5.2 Physical models
A physical model of an artefact or component is often built on a reduced scale, in size and/or by using materials that are cheaper and easier to manipulate than those intended for production. At this stage, we are not necessarily producing what you might think of as a prototype, but investigating particular aspects of the design. For instance, maybe we would produce a racing-bike frame to a new design but in a cheap material such as balsawood, in order to assess the air flow around it in a wind tunnel.
A good example of this is the way modelling was used in solving the problem of the steering for the Thrust SSC supersonic car (Figure 14), the first land vehicle to travel faster than the speed of sound (Mach 1), covering a kilometre in an amazing 2.9 seconds.
One of the first questions that needed to be asked, even before aerodynamic issues were considered, was 'can a rear steering mechanism work at speed?'.
Ron Ayers, the engineer in charge of aerodynamics on the car, explains:
[So] we had to steer with the rear wheels. As soon as we suggested that, all the experts on car dynamics waved their arms in the air and said it was unstable, it could never work, think of forklift trucks and shopping trolleys and other irrelevant comparisons. However, Professor Crolla at Leeds University did a theoretical study for us and said yes, it could work under certain circumstances and gave us those circumstances. There were, however, still plenty of critics who said it wouldn't work.
The rear-wheel steering Mini was built to demonstrate that you can steer a vehicle from the back [Figure 15]. Although it's a lot smaller than Thrust SSC, the wheel configuration and track, etc., is in fact a scaled-down version of the full-sized car. We drove that ancient Mini with this kind of extension out the back at 90 mph up and down the test track at MIRA and proved that it was very controllable – that we really could, with finger-tip control, keep it on the line.
Glynne [Bowsher, the mechanical and structural designer] designed it and built it with his brother-in-law. It's his brother-in-law's Mini, actually. I think they spent £300 and it cost three weeks and two near-divorces.
Figure 15 shows that the model looks nothing like the car that in 1997 made engineering history by smashing the barrier of sound! The Thrust team used mathematical and physical modelling intensively throughout the development of the car. Given the costs involved in trialling the final product, they needed to be as sure as possible that it was going to work. A model is built to prove it will work and to collect data or to test some aspect of design.
Identify which aspect of the Thrust SSC was not being addressed by the physical model using the adapted 'Mini'.
The most important aspect of the Thrust SSC not addressed by the physical model is the aerodynamics and how this affects the steering at full speed.
We will return to modelling at the end of this free course on the problem-solving process. When we look at it again, we will present two contrasting cases. The first is a familiar product, namely the bicycle, and will show the merits of careful 'hand calculations'. The second, probably less familiar, is an acceleration sensor for triggering vehicle airbag systems. It will illustrate the use of a more complicated, computer-based model.