The Aeroplane Speaks. Fifth Edition

The above method is rather futile, as it does not test for the balance of centrifugal force, which comes into play as soon as the propeller revolves. It can be tested as follows: The propeller must be in a horizontal position, and then weighed at fixed points, such as A, B, C, D, E, and F, and the weights noted. The points A, B, and C must, of course, be at the same fixed distances from the centre of the propeller as the points D, E, and F. Now reverse the propeller and weigh at each point again. Note the results. The first series of weights should correspond to the second series, thus: Weight A should equal weight F. Weight B should equal weight E. Weight C should equal weight D. There is no standard practice as to the degree of error permissible, but if there are any appreciable differences the propeller is unfit for use. 5. SURFACE AREA.--The surface area of the blades should be equal. Test with calipers thus: The distance A--B should equal K--L. The distance C--D should equal I--J. The distance E--F should equal G--H. The points between which the distances are taken must, of course, be at the same distance from the centre in the case of each blade. There is no standard practice as to the degree of error permissible. If, however, there is an error of over 1/8 inch, the propeller is really unfit for use. 6. CAMBER.--The camber (curvature) of the blades should be (1) equal, (2) decrease evenly towards the tips of the blades, and (3) the greatest depth of the curve should, at any point of the blade, be approximately at the same percentage of the chord from the leading edge as at other points.