Abstract: A calculating method to determine the clearance size of the rudder face along the extension direction of the swept-back wing for airplane is presented,which can be used for calculating the clearance size of the rudder face at limiting deflection angle,and point out that several parameters for determining the caearance is essential.Moreover,the wing face trimming is studied. The resulting method has been used in the designing and manufacturing of a type of unmanned plane. The wind tunnel experiment of the model plane and test flight of the sample plane shown that the rudder can be rotated easily within the limiting deflection angle,the wing face is very smooth along extensiondirection and string direction after trimming,the thickness of the rear edge of the wing section is reasonable.