Aerodynamics

In 2006-2007, the Alerion Supermileage team has redesigned an entire shell. Since our ancient shell had good aerodynamic results, we started our analysis by pointing out some defects of this design and proposed improvements to be done for the new one. The study led to several concepts for which flow modeling analysis were conducted.

The main concern in terms of aerodynamic considerations is the drag reduction. The skin friction drag term is hardly reducible, but the pressure drag term can be considerably decreased by avoiding flow separation. This is an aspect that was well achieved in last years designs. The main difference here is the frontal area which has been greatly reduced. We now have 0,285 m2 compared to 0,375 m2 for the 2006 prototype, an approximate 25 percent reduction. There are two main factors that made this reduction possible. First, we have inclined the front wheels 8 degrees toward the inside of the vehicle which gave us the opportunity to greatly reduce the needed space in the upper part of the shell. Second, we have chosen a self supporting shell concept that leaves less unused space. The self supporting shell gives much more room for the driver and we are then able to considerably reduce the frontal area. Both aspects can be seen in the image below where both prototypes are presented.

Flow modeling analysis allow us to estimate the drag coefficient as low as 0,10 against 0,14 for last year prototype. However, no experiment where led inside a wind tunnel to support those results.

Making Of

The designing process first requires to be well aware of the rules and regulations of competitions because they imply constraints which have to be respected. For instance, according to the SAE Supermileage the vehicle has to be stable when inclined 20 degrees sideways. On the other hand, according to the Shell Ecomarathon, the upper most part of the vehicle should not exceed in high 125 percent the distance between the two front wheels. Other constraints such as the space needed for the driver, the wheels and engine also had to be integrated adequately into a CAD model (ProEngineer Wildfire was used). These first steps are critical and allow us to save time later.

The next step consist in surfacing a shape that matches our needs as closely as possible, meaning that we have to respect the established constraints while keeping in mind our aerodynamics concerns. More than a hundred hours were spent on perfecting the shape over and over again.

Once the model is verified and approved, it is exported into MasterCam. This software is intended to generate what we call the G-Code. This latter controls a CNC equipment (five axis) which is able of machining into a piece of wood the exact shape with very low tolerancing. Because the shell was to large to be machined in one piece, it was separated in four. Fortunately, this made the transportation easier because the whole weight 385 kilograms. This part of the process took about three weeks mostly because we had to learn using software and equipments. Videos showing the machining CNC can be found in the media section. Thanks to Centre de Recherche sur le Bois who made the previous step possible.

Every following step was accomplished by our partner Kayaks St-Laurent who developed an expertise in manufacturing carbon and fiberglass kayaks.

All four pieces of the male mold are then fixed together. It is polished until the surface becomes glossy.

We then make a female mold out of fiberglass and gelcoat. The first shell built has an exhibiting purpose and was made out of fiberglass. In this specific case, the paint was first applied inside both upper and lower female mold. The picture below shows a step of this process.

Only then, the fiberglass is placed and definitely fixed with polyester-based resin. The two parts obtained after getting them out of the mold are then put together and that requires someone to go inside the shell and place a layer of fabric all around the joint. This latter, still visible from the outside, is polished until it is hardly noticeable. However, due to lack of time, we bypassed that step and placed a black tape over it. Here is the result.

The same process is used to mold the carbon fiber shell except that the paint is applied after. In fact, approximately three kilograms of paint was used for the fiberglass shell and much less for the final one. Finally, a rigid and light shell of approximately 11 kilograms is obtained from the process and ready to receive the other parts such as the steering system, firewall and engine.