A casting core (200) for a twisted gas turbine engine blade, including: an airfoil portion (202) having: an airfoil base end (208), an airfoil tip end (210), a concave side exterior surface (212), a convex side exterior surface (214), a leading edge (204), and a trailing edge (206). The airfoil portion is twisted in a radial direction from the airfoil base end to the airfoil tip end. The airfoil portion includes a first void (220) between the concave side exterior surface and the convex side exterior surface and extending radially to define the shape of a rib of an airfoil to be cast around the core. A first leading edge surface and a first trailing edge surface of the void are twisted from the airfoil base end to the airfoil tip end.