• Version
• 108 Downloads
• 1.29 MB File Size
• 1 File Count
• January 20, 2017 Create Date
• January 20, 2017 Last Updated

National Advisory Committee for Aeronautics, Technical Notes - A Theoretical Study of the Effect of Forward Speed on the Free Space Sound Pressure Field Around Propellers

The sound-pressure field of a rotating propeller in forward flight
in free space is analyzed by replacing the normal-pressure distribution
over the propeller associated with thrust and torque by a distribution
of acoustic pressure doublets acting at the propeller disk and subject
to uniform rectilinear motion. The basic element used to synthesize the
field is the pressure field of a concentrated force moving uniformly at
subsonic speeds, for which an expression generalizing one of Horace Iamb‘s
for the fixed concentrated force is given. This result is presented both
for the moving and for the fixed observer. The strength of the doublet
distribution is related to-the thrust and torque distribution in a con-
venient but approximate way. The sound field is expressed by integration
over the propeller disk, and also by integration over an effective ring,
and is given both for the near pressure field and, in a simpler form,
for the far field. Known results for the zero-forward-speed case present
themselves in the special case of Mach number M = 0. Some illustrative
examples are calculated and discuSsed.

The rotating propeller is the source of an intense soundepressure
field which can be associated with the periodic reactions on the medium
arising from the distribution of pressure rotating along with.the blades.

This pressure distribution consists in part of a distribution due to
thickness of the blades, whose resultant force in subsonic potential flow
is zero, and in part a distribution due to angle of attack and camber
of the blades whose integrated effect includes the induced drag and corre—
sponds almost wholly to the thrust and torque distribution.over the blade.
Another source of propeller noise may be associated with flow separation
and with friction or shear due to the boundary layer; both effects lead
to vorticity shed into the wake and hence the designation vortex noise.
The vortex noise and the noise due to thickness (where wave drag is not
a large factor) are, however, for actual propellers normally of a consid-
erably smaller magnitude than the rotational sound due to torque and
thrust; hence only the latter effect will be considered in the present
work.