WHEN an airscrew leaves the factory, where it has been brought within close limits of perfect force, moment, and aerodynamic balance, the problem of maintaining it in smooth operation throughout its life is only partly solved. The precision of balance which should be met for optimum performance is higher than any piece of mechanism can preserve indefinitely even though it be judged perfect by ordinary standards. Surface abrasion of the blades, material lost in smoothing and polishing, changes in bearing clearance, even though insignificant from the standpoint of mechanical operation; these as well as wear on the centring surfaces of the airscrew shaft and cones may easily conspire to produce unbalance readily detectable in the aeroplane cabin.
Muellert, R.K. (1942), "The Balancing of Airscrews: Part IV.—The Testing for and Correction of, Unbalance in Airscrews in Service in the Field", Aircraft Engineering and Aerospace Technology, Vol. 14 No. 1, pp. 9-12. https://doi.org/10.1108/eb030859
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