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National Advisory Committee for Aeronautics, Technical Notes - Effect of Rotor Blade Twist and Plan Form Taper on Helicopter Hovering Performance

A theoretical analysis of the hovering performance of rotors having
various combinations of twist and plan—foam; taper was made in order to
estimate the effects of blade twist and taper on rotor efficiency in
hovering. The calculations were made with the aid of a strip—analysis
procedure similar to that used in propeller analysis, the comparisons
being made for rotors of equal solidity. The solidity of the tapered
blades was computed by means of an equivalent chord which was derived by
weighting the values of chord at each blade element according to the
square of its radial distance.

The results indicated that at typical Operating throust coefficients,
the addition of linear twist of the order of —8° to ~12° to rectangular
blades having 0.060 solidity may be expected to increase the rotor thrust
at fixed power by approximately 3 to ll— percent or, if the blades are
initially tapered with a ratio of root chord to tip chord of 3, by approxi—
matel;r 2 to 3 percent. An increase in thrust of about 5 percent is indi—
cated for a combination of —8° or —12° twist and a taper ratio of 3 to 1.
An additional 2—percent gain is indicated if a nonlinear optimum combi—
nation of twist and taper is used.

Examination of the effects of a moderate change in solidity and of
the use of partial rather than full taper indicated that these factors
did not significantly affect the preceding results, when the same anoints
of twist and taper were investigated over the same thrust—coefficient
range.

Methods for the improvement of rotor hovering performance are being
sought, especially in relation to the current interest in large, slow—
mcving, load-carrying helicopters. The selection of the blade-pitch
distribution and plan form that would yield the maxixmm rotor thrust for
way of achieving this end. The importance of a percentage increase in
thrust is more apparent when it is realized that, with the current ratio
of pay load to gross weight (approx. 20 percent), the percentage increase
in pay load is approximately five times the percentage increase in thrust.