I read somewhere on the internet that dihedral decreases wing lift,so my question is will dihedral decrease the lift of an airplane wing since the wings are attached to the fuselage at an angle ? If yes,will anhedral also decrease the lift produced by the wings? If dihedral/anhedral decrease wing lift,is the decrease in lift negligible im comparison to thesame wing(with thesame area,aspect ratio) but without dihedral?
The effect of dihedral on lift can be obtained applying high school trigonometry:
$$ L=L_0 \, cos \,\theta $$
where $L_0$ is the lift of the original flat wing and $\theta$ is the dihedral angle. This holds true for negative values of $\theta$ as well.
Since the cosine can be approximated by unity for small angles, the net effect is negligible in most practical cases, e.g. $ cos(5^o)= 0.996$