It's an interesting problem. You'll have to use a force for the air friction. The basic formula for that is:
Where b is the strength of the friction.
The amount of air friction (or drag) would vary according to the angle of the frisbe relative to it's direction of motion. The force would be greater when the frisbe is perpendicular than when it's parallel to the motion.
So we could write it like this:
Well, actually if we want to use the apply force at angle action we can do this:
Force: b*sin(anglediff(frisbe.angle, angle(0,0,frisbe.physics.veloctyx,frisbe.physics.velocityy)))*distance(0,0,frisbe.physics.veloctyx,frisbe.physics.velocityy)
Also I did something similar before with falling leaves which you can find with a forum search. It could be useful.
That still probably won't like a frisbe would move so maybe you could simulate the gyro effect of spinning objects be using a variable to specify the spinning speed of the frisbe and make it harder to tumble when it's spinning. Another is modeling a better physical shape for the drag, since with the above it would be treated as a flat disc.
Anyways that's just some ideas of the stuff you could do. You maybe could even do something simpilar that looks better.