The Air Propeller, Its Strength and Correct Shape Page: 4 of 13
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As in a supporting wing, this force is somewhat inclined to
the rear and may be resolved into the two components lift and
drag, which, by summation over the entire blade produce the pro-
peller thrust T and the torque Q, respectively, the latter
setting up an equal and orosite turning moment to the engine
It is known that the center of pressure, at small angles of
attack, lies in advance of the first third of the wing section.
T deflects the blade in the axial' plane x - x, while Q acts
in a perpendicular direction thereto, Besides this, the resultant
force, R, imposes on the blade a torque in the direction of the
arrow. There remains for consideration, the influence of the cen-
trifugal force F. If the blade .were a straight rod, tension only
could be induced in it by the centrifugal force; aince, however,
this is not the case, bending stresses also will be introduced.
In orderto obtain tangible results, tests were carried out on
a propeller 4.9 nm (16 ft) in diameter, absorbing on the testing
stand 500 HP at N =-730 R.P.M. (See Fig. 3). The integral in the
expression F = /f dm r is best determined graphically. The
blade cross-sections S are measured by planimeter, and plotted
on the S-curve (Fig. 4). From this a curve of
mr =Sd r Xg .
is obtained,'mking the assumption that, = -- 0,7 where 7Y
is the specific gravity of the wood (walnut or ash). The area of
this curve then gives, to the appropriate scale, the total centrifu-
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Dietsius, H. The Air Propeller, Its Strength and Correct Shape, report, February 1923; (https://digital.library.unt.edu/ark:/67531/metadc53811/m1/4/: accessed June 20, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.