ESDU 78013 gives a method for estimating the rearward shift of aerodynamic centre by treating the nacelle as an annular wing of diameter equal to the nacelle spanwise width. A graph gives the lift-curve slope for an annular wing as a function of its width/length ratio@ the lift is assumed to act at the nacelle lip@ and the lift contribution of the pylon is approximated by treating the two exposed stubs as a single low aspect ratio rectangular wing. Factors allow for the effects of nacelle-pylon-fuselage interference and wing downwash. The method is validated by comparing predictions with wind-tunnel data taken from the literature mainly for free-flow nacelles where mass flow ratios would be appropriate to cruise. It is found to predict the shift to within 0.02 of mean aerodynamic chord. It is emphasised that care should be taken in applying the method to configurations outside the geometry for which it was validated and that it should not be applied to cases where pylon span is large compared with nacelle width or where the nacelle entry is close to the wing trailing-edge. The wing lift-curve slope@ also required in the method@ is obtained from ESDU 70011 and the aerodynamic centre of the wing-fuselage combination@ to which the rearward shift is applied@ is obtained from ESDU 76015.