This Memorandum gives a tentative method for predicting the contribution to the rolling moment derivative due to sideslip resulting from the interference effect of a fin on a tailplane, at subsonic speeds. The method given is applicable to aircraft geometries where the tail assembly consists of a single vertical fin located on the top of the aircraft rear-body in the plane of symmetry, with the tailplane mounted either on the fin itself or on the rear body.

The contribution arises because the fin induces an antisymmetrical spanwise loading distribution across the tailplane which generates a rolling moment about the tailplane centre-line chord. The total contribution of the tail assembly of an aircraft to the rolling moment derivative due to sideslip is obtained by adding this contribution due to interference effect to the contributions provided by the fin (using ESDU 82010 or Aero C.01.01.01), and the tailplane planform and dihedral (obtained from ESDU 80033 and Aero A.06.01.03 respectively).

The contribution due to the interference effect of a fin on a tailplane is often small enough, when compared to the fin contribution, for it to be ignored when estimating the rolling moment derivative due to sideslip for an aircraft, but it is of more importance structurally when considering bending moments at the tailplane root.