ESDU 80034 gives a semi-empirical method for predicting (Lv)f, the change in rolling moment derivative due to sideslip that occurs when wing trailing-edge flaps are deployed for landing or take-off. The method is applicable at incidences where there is a linear variation of rolling moment coefficient with angle of sideslip and of lift coefficient with angle of attack. The method covers wings of aspect ratios from 3 to 12 and half-chord sweep angles from -10 to 45 degrees, with plain, split or slotted flaps.