TO overcome the discomfort, nerve strain and fatigue to both passengers and crew on multi‐engined aircraft resulting from the noise and vibrations of engines running at slightly varying speeds, the need of an accurate, easily read indication of such variations is readily apparent. A direct comparison of r.p.m. indicators is ineffective because the permissible tolerance on each indicator might result in a total difference of the order of 50 r.p.m. even though the speed of the engines under comparison is the same. The development of the 3‐phase electric tachometer indicators overcame this difficulty by comparing the relative frequencies of two or more tachometer generators (not the voltages), before this has been translated into an indicated r.p.m., by means of a small synchronous motor operating a magnetic drag unit; the desired result is obtained even though the possibility of permissible errors in a yet serviceable indicator has not been obviated. These two frequencies arc used to operate what is, in reality, a beat‐frequency indicator with a pointer rotating in proportion to the difference in speed. As the difference in speed of the two engines results in a difference in the frequencies developed by the two tachometer generators, so the pointer of the synchroscope will rotate either clockwise or anti‐clockwise according to which engine is running faster. When both engines are running at the same speed the frequencies will be alike and the pointer will remain stationary. Any difference in voltage output of the two generators will not affect this final indication, but will have some bearing on the speed difference at which the pointer begins to rotate where this rate of rotation is directly proportionate to the difference in engine speed. However, as a large variation between engine speeds will not cause any pointer rotation at all, but merely be shown as a flickering of the pointer, this is of no material importance.
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