What are the large flat structures on the sides of the F-4 Phantom directly in front of the engine air intakes? They are more or less square, and may be hinged vertically.
A splitter plate is a component in some jet aircraft, used to control the airflow into the engine. Where the engine air intake is mounted partway back along the fuselage or under the wing, the splitter plate diverts the boundary layer away from the engine intake. It is a form of boundary layer control.
From Youtube: an F4