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National Advisory Committee for Aeronautics, Technical Notes - Compressibility Correction for Turning Angles of Axial Flow Inlet Guide Vanes

A simplified extension of the Prandtl-flauert compressibility cor-
rection for isolated airfoils was obtained for airfoils in cascade with
axial air inlet (accelerating flow) by considering the total lift
coefficient of a cascade to be composed of the sum of two components.
One lift coefficient is associated with a velocity of constant magni-
tude across the cascade and the other component is associated with a
change in velocity across the cascade. From the hypothesis, an ana-
lytical variation of lift coefficient and air turning angle with inlet
Mach number at fixed angle of attack was obtained for two-dimensional
flow.

The two-dimensionally derived turning angles were made applicable
for predicting the turning angles in annular- flow by means of the con-
cept of correcting the calculated turning angles to constant axial
velocity across the cascade. The axial-velocity correction was based
on the inlet velocity and the assumption of no change in circulation
with changes in outlet axial velocity at a fixed inlet Mach number.

Agreement was found between the compressibility variation of the
analytical corrected turning angles and limited experimental turning-
angle data obtained from annular inlet-guide —vane cascades. The ana-
lytical turning—angle variations were presented as a compressibility
correction for use in the design and the analysis of axial-flow-
compressor inlet guide vanes of moderate camber and solidity.

One of the important steps in the development of an'axial-flow
compressor is the successful design of the inlet guide vanes. The
proper design of inlet guide vanes is essential for high compressor
performance because of the dependence of the performance of the suc-
ceeding stages upon the attainment of the required- flow distributions
entering the first rotor row. In a commonly used method of inlet-
guide-vane design for axial-flow compressors, the vane camber and the
setting angles are determined from design data that provide a relation
between the angle of attack of the-given vane sections and the corre-
sponding air turning angles. Design-turning-angle data for inlet -guide-
vane application have been obtained from cascade investigations (refer-
ences l and 2) and theoretical calculations (reference 5) for essen-
tially incompressible flow (inlet Mach number 5 0.30).