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National Advisory Committee for Aeronautics, Technical Notes - Special Propeller Protractor

Lack of equipment suitable for measuring airplane propellers
at flying fields and storage depots prevents the accumulation of
reliable information on the effect of storage and service on the
warping and change of blade angles in propellers. Such informa—
tion would be of much value in perfecting specifications for mans
ufacture and storage, and it is thought that a single instrument
for this purpose would be appreciated.

With praCtioally all protractors of any degree of accuracy
it is necessary to clamp the prOpeller being measured to a plane
surface and measure all angles with reference to this surface.
This requires a heavy plane table and equipment which is expen—
sive and is not available at most flying fields. Furthermore,
propellers cannot be measured by this type of protractor without
removing than from the shaft of the engine. To overcome these
difficulties a special protractor Was designed and built at the‘
Forest Products Laboratory, Madison, Wisconsin, with a view to—
ward supplying a simple, inexpensive, practical, portable instrur
ment for making these measurements under practically all condi—
tions and without the use of.auxiliary equipment. The work was
made possible through the cooperation of the U. S. Navy Department
by whom funds for its design and manufacture were furnished.

The protractor consists of a base plate A, and two movable
circles B and C. A rack is cut into the circumference of each
of the movable circles, and they may be rotated by means of pine
ions at D and F. The letter E indicates a screw clamp which
looks the circle B to the base plate A when desired, and G
is a spring pin which locks circle B to circle 0 in a posi7
tion where the zero points on the two movable circles are direct—
ly opposite. H is a 60—second level bulb rigidly fixed to cir—
cle G, with its axis perpendicular to a line connecting the
center and zero graduation of this circle. K is a level bulb
hinged to plate A and provided with a stop which places the
axis of the bulb perpendicular to plate A when it is open.