On the subject of drag, I believe it manifests itself in three somewhat unrelated forms, one only of which is related to angle of attack/lift.
It cannot be analytically useful to consider only three forces, thrust, weight and was it TR?, but it is useful to consider thrust overcoming drag, producing speed through the air, thus acting on the airfoil and producing lift to overcome the weight, while unfortunately generating various drag forces which in turn reduce the resultant force available to produce speed through the air.
Equally unfortunatey none of these effects is linear so the equations relating to the above simplified consideration of flight are both multivariable and non linear.