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National Advisory Committee for Aeronautics, Report - Charts for Estimating Performance of High Performance Helicopters

Theoretically derived charts showing the profile-drag—thmst
ratio are presented for helicopter rotors operating in forward
flight and having hinged rectangular blades with a linear twist
of 0°, —8°, and ——J6°. The charts, showing the profile-drag
characteristics of the rotor for various combinations of pitch
angle, ratio of thrust coeficient to solidity, and a parameter
representing shaft power input, are presented for tip-speed
ratios ranging from 0.05 to 0.50. Also presented in chart form
are the ratio of thrust coeficient to solidity as a function of
inflow ratio and blade pitch angle and the retreating-blade
angles of attack as a function of infimv ratio and collective
pitch and as a function of power and thrust coeflicients.

The charts of this report differ from the rotor performance '

papers previously published by the National Advisory Com-
mittee for Aeronautics in that the theory on which the charts
are based includes an approximate allowance for stall in the
reversed-flow region and contains no small-angle assumptions
regarding blade-section inflow angles and velocities. The charts
of this report are therefore considered more accurate than pre-
vious ones for flight conditions involving high inflow velocities
and large regions of reversed velocity that may be encountered
by highrperformance helicopters. [the assumption is made,
however, that outside of the reversed-velocity region, the section
angles of attach: are small; thus the angles can be replaced by
their sine. In addition, other than including an approximate
allowance for stall in the reversed-velocity region, the charts do
not include stall and compressibility ejects.

The charts may be used to study the ejects of design changes
on rotor performance and to indicate optimum performance
conditions, as well as to estimate quickly rotor performance in
forward flight. They are also useful in obtaining inflow-ratio
and pitchrangle values for use in calculating flapping coefli—
cients and spanwise loadings. The method of applying the
charts to performance estimation is illustrated through sample
calculation of a typical rotor-performance problem.

INTRODUCTION

Equations were presented in reference 1 from which the
thrust, the accelerating and decelerating torque, and the
profile~drag power of a hinged rotor operating at high tip-
specd ratios and inflow angles could be calculated. Because
the equations do not place any limitation on the magnitude
of the inflow angle'or on the rotor angle of attack, they are
considered more accurate than previous analyses when
applied to high-speed helicopters and to certain types of con-

vertible aircraft. This report is an extension of reference 1
in that the equations of that reference are used as the basis of
a method for calculating the performance of lifting rotors over
a wide range of operating conditions.