Being the geek that I am, I sometimes like to sit down and watch Discovery Channel’s “How Stuff’s Made”. Last week, the program was on tortilla chips, combat knives and mattresses. Anyhow, this blog post will be like that, but CUER themed, and the “Stuff” in question is the canopy.
Aerodynamics is not a precise science. If it were, then all F1 cars would look and perform the same (given the same engine etc), and there would be no need for the teams to spend millions of pounds on wind tunnel testing and computational fluid dynamics (CFD), and Adrian Newey would be out of a job. However, it isn’t, and that makes designing an aerodynamic piece of bodywork somewhat of an art form, where the intuition of the designer can make a great deal of difference. Just like a good artist knows how to paint a house to make it look, well, like a house, so a good designer knows how to draw a form that is aerodynamic.
So, when CUER recognised through our own wind tunnel tests that the 2009 canopy wasn’t very good (it was shedding two big vortices), a fourth year project student turned solar car designer sat down with the computer equivalent of a drawing board and sketched out some curves based on his own intuition and knowledge, and thus was born the first iteration of the 2011 canopy.
As pleased as no doubt he must have been with his work, you can’t just sketch some curves and call that a new design. The canopy was then honed through a series of wind tunnel tests and also with the use of CFD. With each run, things were learnt about the airflow over the bodywork. Patches of flow separation were found and the geometry would be subtly modified to try and correct it on the next run.
This iterative process underlies all aerodynamic development work, from the wings of 747s to the diffuser of a F1 car. Essentially, the process can continue indefinitely, ideally converging on a “perfect” design, but in the real world, time and budget constraints decide where the cut-off is.
This canopy was designed over the course of a fourth year project in a wind tunnel, and then integrated with the existing upper body geometry using a CFD based approach before a finished version of the canopy was assembled in Solidworks, our CAD package provided by Dassault Systems. It is purported to reduce the total drag of the car by nearly 10% compared to the previous canopy.
The canopy material was to be polycarbonate, chosen for its impact resistance (good for deflecting stray kangaroos) as well as its transparency. So, the next step of the production process was to make a mould for the vacuum forming.
The mould making is a two part process. First, a female mould is milled from modelling board, which is basically Wood+, which has very uniform density and stable material properties over a range of temperatures, allowing it to be easily milled to any shape.
In the past, these shapes would have been made by hand, no doubt by a very skilled carpenter. However, because Technology Is Awesome, all we have to do now is feed the CAD file into a computer connected to a big machine and it will cut the exact shape out that we need. This work was done by Jaguar Land Rover, who also generously provided the expensive modelling board needed for the process.
You’ll notice it comes in two halves; this is because we chose to hinge the front half of the canopy, whilst the rear half stays attached to the car, so we vacuum form the two halves separately. Due to the material property of polycarbonate, forming it directly onto Wood+ results in a translucent finish, which is not very conducive to being able to see properly. Therefore, we use a casting resin (ALWA Mould D), which was generously provided by JohnBurn. This resin is poured into the mould, which then sets and takes the exact form of the mould.
Once the resin sets, we then have a male mould and this is what we use to vacuum form the polycarbonate.
Et voila! We end up with a nice shiny new canopy. All that remains now is to attach it to the hinges and nest it into the upper shell and it will be done!