ESDU 81013 gives a semi-empirical method for predicting the changes in sideforce and yawing moment derivatives due to sideslip that occur when wing trailing-edge flaps are deployed for landing and take-off. The method applies to wing-body-tail combinations (with or without nacelles) at low subsonic speeds provided that there is a linear variation of sideforce and yawing moment coefficients with sideslip and of lift coefficient with angle of attack. During the development of the method it was found that wind-tunnel data showed that small changes in model geometry could result in significant changes in the magnitudes and the behaviour with angle of attack of both derivatives. These variations were often large compared to the magnitudes of the derivatives and made it difficult to establish an accurate empirical method in terms of simple geometric parameters and flap characteristics. The overall accuracy of the method produced has been limited by this scatter in the experimental data. Despite the sometimes large differences between the predicted and experimental values it is considered that the method provides reasonable first approximations to the derivatives suitable for use in initial project studies of aircraft lateral stability during landing and take-off when high-lift devices are deployed.