IN a paper on airscrew‐blade vibration, published in the Journal of the Royal Aeronautical Society lor September, 1937, the author gave brief descriptions of different designs of the same general typo of mechanism for exciting harmonic torques in airscrew drives for experimental purposes. 1n these exciters, harmonic inertia torque is produced by rotating a set of identical unbalanced pinions mounted in a Gage and arranged symmetrically about a shaft axis. The pinion axes are parallel to the shaft axis and the pinions are driven either by a sun gear or by an annulus gear. The gear meshing is such that the direction of unbalance for all the pinions of the set is radially outwards at the same instant: when the unbalance is radially outwards or inwards there is momentarily no torque effect.