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An aircraft flying control includes an accelerometer 2 arranged to detect vertical acceleration of the hinge line of a control surface 4 and to pass a signal proportional to said acceleration through signal shaping means 1, to servo‐mechanism 8 for operating the control surface, the signal shaping means being such that a hinge movement is applied to the surface equivalent to that which would be produced by mass‐balancing. In the electrical embodiment of FIG. 1, the circuit includes amplifiers 5 and 6, a pilot's control 9, and an electro‐mechanical device 7 shown in FIG. 2. A shunt wound d.c. motor 30 has a constant field excitation and its armature current is supplied by a thermionic valve 33, which has inputs from the amplifier 6 and from a tachometer generator 32 driven by motor 30. The load 31 of the motor may be a differential in the servo‐mechanism 8, or it may form part of the input linkage, error transmission linkage, or feed back linkage, e.g. by adjusting the length of a link. The signal shaping means 1 may be an inductance, resistance, capacity, capacity resistance, and inductance wired in series, the input being applied to the two terminals of the network, and the output being taken from one terminal and from a point between the two capacities. In a mechanical embodiment, the signal shaping device may comprise masses 40, 41 (FIG. 3) springs 42, 43, and 44, and dashpots 45 and 46 arranged as shown. The input is applied between the two masses, and the output obtained from mass 40.