Abstract: Asymmetries of rotatable members such as aircraft propellers can be balanced out by the careful addition of counterweights at proper positions on the members. An accelerometer coupled to the mounting of an unbalanced rotating member produces a signal containing a component indicative of the unbalance. This signal would be useful for helping in the balancing process but it may also contain noise components from which the useful component has to be extracted without too much alteration. A fixed frequency band-pass filter could be used but the results then have to be manually corrected if the actual frequency varies from that associated with the filter or a manually turnable filter could be used but then of course the filter has to be tuned for each test.