Control Surface Gap Drag

The parasite drag also includes extra drag due to gaps at the control surfaces. This is best estimated based on experimental data. The drag depends on the detailed design of the controls, but for the purposes of this course we take the drag increment to be:
f_gaps = .0002 Cos² Sweep S_affected,
where S_affected is the area of the wing, horizontal tail, or vertical tail affected by control surfaces. This is typically about 0.3 S_wing, 1.0 S_horiz, and 0.9 S_vert, but the detailed layout should be used. The correction for sweep is included since the component of the dynamic pressure normal to the gap is the significant term.