Effect of Spanwise and Chordwise Mass Distribution on Rotor Blade Cyclic Stresses
Abstract:Theory and experimental evidence indicate that rotor blade cyclic bending moments, and hence stresses, can be reduced by the proper spanwise and chordwise distribution of blade mass. The spanwise mass distribution is shown to primarily affect blade beamwise natural frequency, and hence be important for control of blade resonance effects. The primary effect of chordwise mass distribution is to produce a twisting of the blade, as a result of coupling with the blade bending motion, and thereby to produce a cyclic airload that affects the dynamic bending response of the blade. Computations based on XH‐17 flight data have shown that a third harmonic twist at the blade tip of as little as ± 0.85 degree can be enough to cancel nearly all of the normal third harmonic damping due to first mode bending. The paper reviews the theory upon which these conclusions are based, and discusses substantiating model tests and flight tests of the XH‐17 helicopter. In the case of the XH‐17 rotor blade, cyclic stresses were reduced 35 to 45 percent in all flight regimes tested. This was accomplished by adding a concentrated weight, equivalent to six percent of the blade weight, to the blade leading edge outboard of the first mode nodal point. The theory presented may be used as a basis for modifying existing rotors that experience dynamic difficulties, and it is suggested that the concepts advanced may also be employed in the design stage.
Document Type: Research Article
Affiliations: Hughes Tool Company — Aircraft Division
Publication date: April 1, 1956
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