Right, time to start the design of the rear wing. The first step was to pick the best aerofoil profile to use and this is where the work I did for the Aston Martin project came in. A lot of time was spent looking at some of the most common motorsport wing profiles until I narrowed it down to the top two most efficient ones: the Eppler 664 & Benzing BE 122-125. I then swept the wings through a range of attack angles in CFD and looked at the effect on overall aerofoil efficiency (lift / drag). As you can see, there isn't much between the peak efficiency values for the two profiles, but the Benzing profile comes out on top - just! Furthermore, it's also more efficient across all the different attack angles, which provides another performance gain. As the car pitches under braking and acceleration the attack angle of the wing also changes accordingly. Despite there not being much difference between the peak efficiency of the two profiles, the Benzing profile will clearly perform better under dynamic conditions, which also have to be taken into consideration. For these reasons I decided to reuse this profile for the Z3 project. Perhaps I should have looked at creating my own aerofoil profiles, but I just didn't have the time!
I then added a small Gurney flap to the trailing edge of the aerofoil (5% chord length). These are a great addition to any aero device as they increase downforce significantly with only a very small addition in drag. Basically, the flap creates an area of low pressure at the tip of the wing which attracts the airflow on the lower wing surface, helping it remain attached for longer. This allows the wing to run at steeper attack angles which would otherwise cause flow separation and stall the wing.
Rear Wing Design
With the aerofoil profile chosen I was then able to start on the wing design and the first thing to decide was where I actually wanted it to be. Going back to the baseline CFD analysis, I took a section down the car's centreline and looked at the airflow over the rear of the car. From this I can see where the fast flowing and uniform flow is (yellow arrows), which is the ideal place for the wing and not the slow airflow as shown by the blue and green arrows! More importantly though, I can also see the angle of the oncoming airflow. As we saw in the first picture the Benzing profile works best at an attack angle of about 2 degrees. It's important then to rotate the wing so that it's working at its peak efficiency angle relative to the oncoming airflow and not the ground!
To get the most efficiency out of the wing I also wanted to incorporate a profile twist across the span. The outer ends of the rear wing that are not behind the cabin will be exposed to airflow that is not coming down the back of the roof, but in fact is more like "freestream" air. I took another section down the car, but this time roughly in line with the shoulders of the rear quarter panels. You can see that the airflow is now roughly parallel to the ground. To get the necessary twist I again rotated the wing profile so that it's still at roughly 2 degrees to the airflow.
The final addition to the rear wing is to fit the end-plates which help reduce the undesired effect of wing-tip vortices. The purpose of our rear wing is to create downforce by increasing static pressure on the upper surface and reducing pressure on the lower surface. What happens at the tips of the wing however, is that the high pressure is "attracted" to the lower pressure and will spill over the edges of the wing towards the lower surface. It's this rotation of the flow combined with the forward momentum of the car that creates the vortices shown.
Why's this a problem though? As well as increasing drag, the pressure differential at the edges of the wing (and hence downforce) is reduced and we end up with "medium" downforce- reducing the effective length of the wing and therefore the efficiency. These losses can be reduced by fitting end-plates as shown. For the moment I've gone with very simple flat end-plates as I've got to keep cost in mind, as well as the design time spent chasing every 10th- otherwise this car may never to see the track! I might come back to this later though, we'll see....
With the rear wing designed the next step is to work out how to attach it to the car and keep it there without it breaking off! I'll cover pylon design in my next post.