How the downforce calculator works
Downforce is the dynamic pressure of the air times the wing area times the lift coefficient: F = ½ · ρ · V² · S · CL. Double the speed and the downforce grows four times, so the calculator plots the whole speed range, not a single point. Drag follows the same formula with the drag coefficient, and the power the wing costs is the drag times the speed.
The coefficient is where most calculators go wrong. A value read from an airfoil polar belongs to an infinitely long section. A real wing has tips: air spills around them, part of the lift is lost and induced drag appears. Load your polar and the calculator applies the finite-wing correction for your span, chord and endplates before it computes the forces.
The Reynolds number must match
An airfoil polar is valid at the Reynolds number it was computed for. The calculator compares it with the Reynolds number of your wing at the chosen speed and tells you when a new simulation is worth running.
A rear wing sits in disturbed air
The wing on a car meets the air that leaves the roof, not the free stream. When you know the local dynamic pressure is lower, scale it with the local flow factor. Leave 1.0 for a wing in clean air.
Why is my downforce lower than the airfoil numbers suggest?
Three reasons add up: the finite span lowers the lift of the wing compared with the section, the polar may come from a different Reynolds number, and on a car the wing works in the slower air behind the body.
How much power does a wing cost?
The engine has to push the drag force at the car speed, so the power is drag times speed. At 200 km/h a wing with 300 N of drag costs about 17 kW.