ESDU Aero C.04.01.08 gives the rate of change of control hinge moment with deflection of a sealed tab in subsonic flow. A graph plots that quantity against the ratio of tab chord to local wing chord for various values of local forward balance (the ratio of control chord forward of the hinge line to control chord aft of the hinge line). The ordinate includes a factor that allows for a tab of a different span from that of the control, for the effects of sweep, the Prandtl-Glauert compressibility correction, and a parameter that allows for section shape and is plotted against trailing-edge angle in a second graph. The data apply to controls on wings, tailplanes and rudders with aerofoil sections with maximum thickness at about 30 per cent of chord at incidences up to 5 degrees and for control and tab deflections up to 10 degrees. Transition is near the leading edge. Tab gaps up to 0.001 of wing chord at tab mid span are treated as effectively sealed. The values predicted are expected to be accurate within 10 per cent.