carefully read through some of the JWAE MIRA wind tunnel test data on a Porsche 917 that John Horsman shared with me. Surprisingly, the optimum down force-to-drag ratio occurred at around 5 degrees incidence. This is much lower than I would have guessed. JWAE ran their 917s on some very fast tracks, such as Le Mans and Spa, where its top speed was close to 240 mph so drag was important as well as down force. Another effect of increasing rear down force is that it creates a lifting moment on the front due to the leverage of the rear spoiler being behind the rear tires. As down force increased, it became more sensitive to yaw angle as well.
My research turned up a paper that stated a Gurney flap was most effective (down force-to drag ratio once again) at between 1% and 4% of cord- again, less than I thought.
I finished the spoiler this afternoon and laid it in the rear body to see how wide it looked. Both the body and the spoiler are upside down, just lying in there.
Regards, Neil Tucson, AZ