|Aero controller design, showing the motor controller and arduino board|
|Aero controller, showing custom PCB and heatsink for motor controller|
The first thing to do is to define the constants that will be used. In this case, we need to know the application points for the aero loading, the rear wing drag (which creates a moment that tends to unload the front wheels and load the rears- the front wing drag is insignificant due to the magnitude and application point), and a lot of calibration constants.
I redid all the CFD to reconfirm the previous results (half car, lots of cells), and ran simulations for different wing angles on the rear wing second element. From this data, I chose a range of wing angles that I could achieve with the actuator and created a fit based on the data for both aero balance (F/R) and aero load.
|Aero balance shifts forwards with less wing angle (F/R), and aero load decreases with less wing angle (downforce is negative load in this convention)|
Next, I used the tire data we have to create a load sensitivity fit for our tires. Though the load data only goes from 50-350lbf, I needed to extrapolate a bit farther due to the effects of weight transfer. Is it perfect? Hardly, but it's good enough for these purposes (in fact, this is a 1D fit here, as it only takes into account load, in this case at 0 inclination. But I'm going to ignore camber for simplicity's sake in this analysis, since in the end I'm looking for a model grounded in physical reasoning, not a perfect model). I did some work with using neural networks to create a tire model last year, and this fit seems to be in line with the results from that project.
|One corner of the non-aero wheel load spreadsheet|
|Cornering capability: light red is what the vehicle can't achieve, dark red is where we lift two wheels.|
|The black box encircles what portion of this we will probably use, and the light green is the approximate performance envelop of the car without aero|
|One corner of the aero wheel load spreadsheet|
|Aero lateral performance envelope, 20mph on left, 60mph on right (yeah, that's a lotta g's!)|
How bad is the balance when the wing is all the way up? Pretty bad, but that just means we have to make sure the wing gets to its desired position before we need the extra grip on the rear end, which means somewhat more conservative tuning.
|Static aero balance|
|Aero limiting balance - Wheel loads F/R, 20mph on left, 60mph on right|
What the hardware for the controller will do is obtain wing position from a sensor (linear pot), and try and hit a target wing angle based on its onboard sensors. The microcontroller will process the information, send a position command to the motor controller, which will handle all the feedback and PID control. I think there will still be a lot of hands-on testing and tuning to make this work right, but a little physical intuition never hurts.