At first glance, a boomerang might appear flat to the unsuspecting eye. But alas, for a boomerang to return to your hand, it must actually act a bit like a helicopter rotor, with one side angled up one way and the other the other way, so that when it spins it creates lift.
But when you think of a helicopter, you think of something going up and down, not around in a circle. What you don't consider is that lateral movement of the boomerang cause air to flow past one side of the boomerang faster than the other as it rotates, create unequal lift, causing it to turn (this means two things: one, boomerangs are thrown vertically/almost vertically as opposed to horizontally, because that would cause more of a loop-de-loop, and it also displays one of the major shortcomings of single-rotor helicopters: they suffer from this same issue). It continues to turn as it flies, eventually creating the loop we all know and love.
So if you're making a boomerang, keep this in mind: angle the fins. And for legal purposes I do not support the use of boomerangs as a projectile weapon. Thank you and goodnight.