ESDU Aero C.04.01.01 derives the slope of the hinge moment coefficient curve against incidence for a special series of aerofoils having straight upper and lower surfaces over the rear 10 per cent of chord with a trailing-edge half angle with tangent equal to the aerofoil thickness/chord ratio. The results have then to be corrected for the effect of trailing-edge angle. For the special series a graph plots curves of the theoretical rate of change of hinge moment coefficient with incidence for inviscid flow against control chord to aerofoil chord ratio for various values of thickness/chord ratio. A second graph gives curves that allow the theoretical value to be corrected for the effects of viscosity since they give the ratio of the slope of the hinge moment coefficient curve with incidence in incompressible flow to that theoretical value. The curves are plotted against control chord to aerofoil chord ratio. Each curve applies to a value of the ratio of the lift-curve slope in incompressible flow to that in inviscid flow and that ratio may be obtained from ESDU Aero W.01.01.05. The hinge moment coefficient curve slope is finally corrected for the effect of trailing-edge angle, if it is not as defined for the special series of aerofoils, using an equation that involves the lift-curve slopes in viscous and inviscid flow, the thickness/chord ratio, and the trailing-edge angle defined by lines joining points on the aerofoil surface at 95 and 99 per cent of chord. The method applies for sealed gaps or gaps not exceeding 0.002 times chord and when the hinge moment coefficient curve with incidence is linear. It is accurate to 0.05/radian. ESDU 89009 may be used to correct for finite aspect ratio effects.