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National Advisory Committee for Aeronautics, Technical Notes - Effect of Sweep on Performance of Compressor Blade Sections as Indicated by Swept Blade Rotor, Unswept Blade Rotor, and Cascade Tests

An investigation has been made to determine the induced effect of
sweep on an axial-flow compressor blade. Velocities of entering and
exiting flow and blade-section pressure distributions were measured at
three radial stations on a 50° sweptéblade rotor of 0.69 hub-tip ratio
having the same blade geometric characteristics as an NACA 65-series
unsweptéblade rotor for which similar quantities were measured and the
results presented in NACA Technical Note 3806. In these tests, the blade
tip speed was l8} feet per second and the inlet Mach number relative to
the rotor ranged from 0.25 to O.h5. The blade-section pressure distri-
butions were Obtained by the use of a mercury-seal pressure-transfer
device. The data obtained were also compared with similar data for the
same blade sections obtained from a two-dimensional porous-wall cascade
tunnel.

The comparisons of blade—section pressure distributions indicated
that, in order to obtain the same effective angles of attack on the
swept-blade rotor as on the unswepteblade rotor, the swept blade would
require an additional twist of 5.80 for the entire radial span. Two-
dimensional cascade data adequately predicted the turning angle through
the swept-blade rotor if the change in axial velocity provided by the
trailing portion of the blade was taken into account.

In axial-flow compressors, the demand for higher weight flows and
fewer stages has led to higher inlet Mach numbers relative to the comp
pressor blade. When the Mach number is increased much above the critical
Mach number, the blade losses usually go up. Still further increases in
Madh number usually lead to choking of the flow, particularly for blade
sections of high solidity, high thickness, and low inlet air angle. The
application of sweepback has been suggested as a possible means of
increasing the inlet Mach number range. Sweepback is not expected to
increase appreciably the critical'Mach number for compressor blades as
it does for airdraft wings because the spanwise flow will be restrained
by the inner and outer casings. Sweepback, however, can increase the
minimum flow area in the blade passage and in that way may extend the
usable Mach number range.