The results you have got are perfectly in line with the theory. As angle of attack is increased, the downforce increases. Indeed, this is the reason you have greater lift, from Newton's third law. However, due to the finite aspect ratio of the wing, some amount of air leaks from the bottom surface to the top surface. This causes the lift vector to tilt slightly backwards and hence an extra component of drag due to the lift itself. This is the lift induced drag. As 'alpha' or the angle of attack is increased, the lift will increase and so will the drag. But this increase in lift does not happen always. Due to viscosity, after some angle of attack, the flow will separate from the surface and you will have a sudden loss of lift. This condition is termed as stall. Hence, just before stall, you will have your maximum lift. But, once you hit the stall angle, your lift will decrease drastically. One of the simplest ways to detect stall and find the stall angle is to put pressure probes on the surface of the airfoil. Go on increasing the angle of attack and observe the corresponding readings of the pitot tubes. At a certain angle of attack, you will find that the pressure beyond a certain point on the surface of the airfoil doesn't change at all if you increase the angle of attack further. This is because the flow has separated and hence the constancy of pressure. You can now easily see that the above angle of attack is the angle of stall.
This was the method we followed in our propulsion lab to study the pressure distribution on an airfoil. It is a very neat and simple method. You just need a couple of pitot tubes and a U-tube manometer to read the pressure at different points on the surface of the aerofoil.
                                                 Sanjiv
                                               III rd year,
                                            Aerospace Engg.
                                              IIT BOMBAY

If you are still into the formular car.

Also consider the that vortext creates at the rear of the car...  As the result of this some forward force is apply to the car...  The force is small compare the the drag force though.

Best wish on your research.

The front wings will perform very much different than standard airfoil data for wing sections would indicate.  This is because they operate extremely close to the ground, rather than in a free field of air.

You will learn it in aircraft performance. Anyways: when you divide lift to drag it's the aerodynamic efficiency, everything will cancel except lift coefficient on drag coefficient. Make a partial derivative of the aerodynamic efficiency to lift coefficient equal to zero and you will find Cl and Cd optimum.

Bye.

Hello,

Reading through the messages in response to your question, I found many explanations and suggestions. It is very clear that they come from the aeronautical field.
I am myself an aernautical engineer, worked in this field, and listened to all of the good teachings on the subject. Since when I moved to an F1 team and worked on the aerodynamics I had to "tune" or adapt the knowledge.
Things are happening a bit differently there.
First af all be aware that the best front wing (FW) that you can design can lend the car an average of bad overall aerodynamic behaviour. Every single, small detail of the FW influences the rest of the car. I have many examples of really small details that made the car gain/lose tens of kg of downforce (or CL points, if you prefer).
Bear in mind that:
1. the FW works in ground effect, hence it's a good idea to look at the system wing-ground as an expansion channel BUT
2. the boundary layer on the wing is totally different than the one on the floor !
3. The greatest part of the downforce is better being generated by the mainplane rather than the flap, since the last one deflects the flow upwards a lot. The low pressure generated by the ground effect from the mainplane doesn't generate that much flow deflection.
4. It's hard to find the stall angle and I have never reached it testing on the car because the loss suffered by the rest of the car increasing the flap angle is very big. This prevents from increasing the flap angle. Consider that the rear wing (RW) behaviour depends on the FW flap angle and the flow under the floor (extremely important) is affected as well.
The way I would suggest you to follow for the study of a FW is the use of cfd. You would need the model of the rest of the car as well. I am talking of 3D modeling, because the vortices around the endplates and their interaction with the tires and the brake ducts are a nightmare.
I know it's not very encouraging, but this is the vision of an "aeronautical" who came to F1 and found out that the aerodynamics of these cars is incredibly more complex than what looks like from outside.