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National Advisory Committee for Aeronautics, Report - Aerodynamic Characteristics of Circular Arc Airfoils at High Speeds

The aerodynamic characteristics of eight circular-arc
airfoils at speeds of 0.5, 0.65, 0.8, 0.95, and 1.08 times
the speed of sound hare been determined in an open-jet
air stream 2 inches in diameter, using models of 13-inch
chord. The lower surface of each airfoil was plane; the
upper surface was cylindrical. As compared with the
measurements described in N. A. 0. A. Technical
Report No. 31.9 (Reference 1), the circular-arc airfoile
at speeds of 0.95 and 1.08 times the speed of sound are
more efiicient than airfoils of the R. A. F. or C’larh: Y
families. At a speed of 0.5 times the speed of sound,
the thick circular-arc sections are extremely inefiicient,
but thin sections compare farorably with those of the
R. A. F. family. A moderate rounding of the sharp
edges changes the characteristics rery little and is in
many instances beneficial. The results indicate that the
section of the blades of propellers intended for use at
high tip—speeds should be of the circular-arc form for
the outer part of the blade and should be changed gradually
to the R. A. F. or Clark onrm as the hub is approached.
In the course of the measurements of the aerody-
namic characteristics of the 24 airfoil sections described
in Technical Report No. 319 (Reference 1) of the Na-
tional Advisory Committee for Aeronautics, the authors
found that it was advantageous to move the maximum
ordinate of the section further back from the leading
edge as the speed was increased. A segment of a
circular cylinder with the maximum ordinate at 50
per cent of the chord was found to be more efficient
at high speeds than airfoil sections of the R. A. F.
or Clark Y families of the same thickness. It was
deemed desirable to determine whether this increased
efliciency at high speed is characteristic of circular-
arc sections of all the thickness ratios from 0.08 to 0.20
times the chord and to determine the effect of rounding
the sharp edges, which would be necessary in practice.
These were the objects of the work now to be described.
The measurements were carried out at the National
Bureau of Standards with the cooperation and financial
assistance of the National Advisory Committee for
Aeronautics.