• Version
• 122 Downloads
• 999.28 KB File Size
• 1 File Count
• April 2, 2017 Create Date
• April 2, 2017 Last Updated

Variation of Peak Pitching Moment Coefficients for Six Airfoils as Affected by Compressibility

SUMMARY

Pressure-distribution tests of six NASA 16-series
propeller sections with 1-foot chords were conducted in
the NAGA 8-foot high—speed tunnel to determine the
compressibility effects on peak section pitching—moment
coefficients. The data are presented as curves of peak
section pitching-moment coefficient against Mach number,
thickness ratio, and camber.

The peak pitching-moment coefficients were found to
occur in the regions of positive and negative stall. For
these conditions, especially for the thicker airfoils
and in the region of positive stall, the critical speed
occurred at Mach numbers as low as 0.50 and marked
changes of the peak moment coefficient occurred at
Mach numbers as -1ow as 0. 55. Increases in thickness and
camber were found to accentuate the compressibility
effects on peak moment coefficient.

INTRODUCTION

The problem of the excessive twisting moments
developed by prOpeller blades and the consequent failure
of pitch-control mechanisms have aroused interest in the
factors contributing to these twisting moments. One of
these factors is the aerodynamic pitching moment of the
.propeller-blade sections.

The conditions encountered on the blades for normal
prepeller operation are bracketed between positive and
negative stall. The condition of positive stall is
associated with take-off, climb, and pull-out; and
negative stall might be associated with dive and dive
entry.
2 "-—-_ 1mm ACR No. 11.317

The general effect of compressibility on the pitching-
moment coefficient is shown in references 1 to 3, but the
range of angle of attack tested is not sufficient to
include the conditions for maximum mbments. Some moment
data are available for sections desi ed to delay adverse
compressibility effects (reference h , but again these
data are limited to conditions below the points where the
maximum values, both positive and negative, of the moment
coefficient are reached. Further limitations of the
tests of reference A are'that the Reynolds numbers are
lower than for the present tests and the tunnel-wall
effects resulting from the larger ratio of model size to
tunnel size are larger. Because of the importance of
compressibility effects on airfoil characteristics, a
detailed investigation of these effects is being conducted
by the NACA. The present report includes a part of the
data obtained from tests'conducted in the NACA 8-foot
high-speed tunnel on several airfoils covering repre-
sentative ranges of thickness and camber. The data
obtained in the present investigation constitute an
extension of the results of reference h, and part of the
data were obtained to study the effects of the differences
in the test conditions previously noted.

Use of l—foot-chord models gave practically full-
scale Reynolds numbers and reduced tunnel-wall effects.
Use of pressure-distribution measurements in the central
spanwise region of the models, which spanned the tunnel,
gave practically two-dimensional results. Particular
emphasis was placed on pressure-distribution tests rather
than force tests because the type of phenomenon that
occurs is more clearly illustrated. The Mach number
range extended from 0.12 to 0.68.