The answer is to use geometry. Measure the actual blade width, or the chord length of a cambered blade.  Then measure the vertical height from the bottom edge of the blade to the top edge of the blade.  Setting the impeller on a flat surface, with the bottom edge of the blade resting on the surface should give a good measurement of blade height.  The vertical height divided by the actual blade width will be the sine of the blade angle.  Calculating the arcsine of the length ratio will give the blade angle.

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