GT4 Wing Development pt2

GT4 Wing Development pt2

March 23, 2020

We developed the wing for the GT4 with much thought, and we experimented with many designs before arriving at our final destination. The final form sometimes hides all the work and sweat of the journey but it is the reason why many things end up the way they are. As promised, let's continue our journey diving into the next topic for the GT4 wing development. Gurney flaps have been proven to increase the downforce of wings. To put it simply, they alter the effective shape of the wing making it look larger in the flow. Let's take a look at the OEM wing streamline below.

The colors show the pressure distribution and the streamline is representative of the flow direction of a particle. We see that there are smooth lines running above and below the wing. Now take a look at the same wing but with the gurney flap added in the picture below.

As you can see, the pressure above the wing has increased and the area of low pressure under the wing has decreased as well. This already indicates an increase in downforce overall but what your eyes should notice are the vortices generated right at the trailing edge of the wing. This is due to the air separating from the wing due to the gurney flap. Although a vortex is generating on this edge, it is quite small and the flow closes up again further downstream. This in effect makes the chord (length front to back) longer and thus makes the wing more effective. Now the wing size has not changed much but how effective were the gurney flaps on the OEM wing? 

1deg: +20lbs/ +6lbs drag (vs OEM Wing at 1deg AOA)

3deg: +23.5lbs/ +8lb drag

5deg: +28lbs/+10.5lb drag

7deg: +31.5lbs/ +13lbs drag

9deg: +35lbs / +15lbs drag

11deg: +40lbs / +17.5lbs drag

So that's actually quite impressive. Keep in mind all the numbers above are compared to the OEM wing at 1deg AOA. This means the 40lbs of downforce that we see is when we increase the AOA to 11deg AND also add a gurney flap. Gurney flaps are quite effective but notice that they also increase drag significantly. This drag comes from the vortices generated by the flap right beind the wing. Notice that the drag does not increase at the same rate as downforce. It appears to increase more significantly as the AOA goes up. We can see that the lift to drag ratio starts to drop and our setup becomes less efficient as we increase the AOA too much. So what happens when we increase the height of the wing with the gurney flap?

5AOA, +100mm height, no gurney vs. 5AOA, +100mm height, gurney

+20lbs/+8lbs drag

So adding a gurney flap adds about 20lbs at 80mph compared to the OEM wing and seems to add roughly the same increase of downforce (+50%) at different heights and angles. For our ultimate setup with the stock wing, we would probably aim for ~5AOA, increase the wing height (4"), and a gurney flap for a net total gain of:

+33lbs + 11lbs drag ( +79% downforce / +220% drag)

This isn't too shabby. We estimate ~169lbs downforce & 36lbs drag @ 120mph. For reference the OEM Wing generates ~95lbs downforce & 11lbs of drag @ 120mph. I think that you can feel some difference at these high speeds but at medium high speed turns ~80mph I dont know if you can notice 33lbs of additional downforce in the rear of the car. Will it decrease your laptimes and increase your cornering speeds?

Our goal is to provide our customers with a significant performance increase and I argue that it will be very difficult to quantify a reduction in time with all the driver variances, weather, and tire conditions. Don't worry, we do have a solution coming soon! Our next post will be on swan necks and standard wing mounts!

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