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National Advisory Committee for Aeronautics, Report - Free Space Oscillating Pressures Near the Tips of Rotating Propellers

The theory is giren for calculating the free-space oscillating
pressures associated with a rotating propeller, at any point in
space. Because of its complexity this analysis is concenient
only for use in the critical region near the propeller tips where
the assumptions used by Gutin to simplify his final equations
are not cal-id. Good agreement was found between analytical
and erperimental results in the tip hfach number range 0.45 to
1.00 where static tests were conducted. Charts based on er—peri—
mental data are included for the fundamental frequencies of
two-, three-, four-, fire—, six-, and eight-blade propellers and for
a range of tip clearances from 0.04 to 0.30 times the propeller
diameter. If the power coefi‘icient, tip Jfach number, and the
tip clearance are known for a giren propeller, the designer may
determine from these charts the average maximum free-space
oscillating pressure in the critical region near the plane of rota—
tion. A section of the present report is deroted to the fuselage
response to these oscillating pressures and indicates some of
the factors, to be considered in soloing the problems of fuselage
ribration and noise.

Pressures in the region ahead of the plane of rotation tended
to be out of phase with those. behind it. A raiector in the pres--
sure field increased pressures in the plane of its surface by an
amount which depended on its shape; a flat surface caused a
doubling of the free-space z‘alues. Blade plan form is shown
not to be a significant parameter. The nondimensional param-
eter. tip clearance divided by propeller diameter, however, is
shown to be significant. As the tip clearance was decreased,
pressures in a region about as wide as one propeller radius were
greatly increased. At a constant power the pressure ampli-
tudes of the lower harmonics tended to decrease and the higher
harmonics tended to increase with an increase in tip JIach
number. The fundamental frequency of pressure produced
by a four-blade propeller was essentially independent of tip
.llach number in the useful tip JIach number range. At tip
Jfach numbers near 1.00, the pressure amplitudes were not
appreciably reduced by increasing the number of blades; how—
erer, the resulting higher frequencies of the impinging pressures
were beneficial in greatly reducing the vibration amplitude of
the wall.