Travel of the center of pressure of airfoils transversely to the air stream Page: 3 of 20
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N.A.C.A. Technical Memorandum No. 530
of the pressure distribution on the wing surfaces (the method
employed by American and English investigators in both model
and free-flight tests), but determined directly the moment of
the mean force about an axis passing through the middle of the
span parallel to the direction of flight. The results thus ob-
tained can be immediately used in strength calculations of
wings and give values comparable to those obtained under the
customary assumptions of an elliptical, rectangular, trapezoidal
or other form of pressure decrease toward the wing tips. They
also indicate the lateral travel of the center of pressure ac-
companying changes in the angle of attack. Lastly, they show
how this lateral travel of the center of pressure is affected
by changes in the aspect ratio, plan form, and thickness of the
wing near the tips.
The proper performance of these tests necessitated the con-
struction of special apparatus. As already mentioned, the ob-
ject was to determine the point of application of the air force
to each half of the wing with respect to an axis passing through
the middle of the span parallel to the chord. Figure 1 shows
the arrangement of the apparatus. It consists of a hanging
weight P connected with the wing model by two fine steel wires
1 and 2. This weight enables the application of a variable hor-
izontal force to the model in the direction of the arrow q by
shifting the support T. The magnitude of the pull Q1 can be
determined by the formula Q = Qm/ ' - m, when Q, i and m
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Katzmayr, Richard. Travel of the center of pressure of airfoils transversely to the air stream, report, September 1, 1929; (digital.library.unt.edu/ark:/67531/metadc277471/m1/3/: accessed February 22, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.