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INFLUENCE ON THE SIDE MEMBER FORM natural frequency BLADE

INFLUENCE ON THE SIDE MEMBER FORM natural frequency BLADE

If you look at the resonance curves constructed for the various designs of the blades to swivel mount, we find that they differ slightly. Moreover, unlike most often determined by the difference in the rigidities for bending of the blades, to a lesser extent - in the mass deviations blade characteristics. This is because the designer during operation guided by a number of different requirements that limit the possibilities of varying the parameters of the blade spar.

For a given total mass of the structure, the maximum rigid blade is obtained if the longitudinal member of the spar is positioned along the contour of the profile, i.e. If you inscribe the spar in the profile of the blade. In this case, a large percentage of the mass of the blade can be embedded in its power element - spar. Such blades are usually most advantageous in terms of the magnitude of the acting stresses. The blades with the free cross-sectional shape of the spar (for example, in the form of a pipe) that are not inscribed in its profile were simpler in production. Such blades possess low resistance to bending and give the least successful resonance diagram for oscillations in the plane of the stroke.

According to the dynamic characteristics of the following types of blades can be identified in the sweep of the plane.

The blade is based on a tubular steel spar with the idle frame in bending, it has a low rigidity in the flapping plane. In this type of blade is especially sharp resonances occur at low speed flight modes, so their life is usually limited to a stay at low speed mode.

With increasing frequency of natural stiffness of the blade vibrations are moving away from resonance. Structurally, this is done in the form of blades with a contour (or close to this form) inscribed in the profile spar - steel, dyuralyuminovym, titanium, or made from the CM.

When designing the blade, it is also necessary to provide detuning from the resonances and in the plane of greatest stiffness of the blade, which usually coincides with the plane of the chords. Therefore, the stiffness characteristics of the blade in this plane can vary over a wider range than in the plane of the stroke. Starting with a round tube, the cross member section can be enlarged to dimensions that occupy almost the entire profile from the front to the trailing edge. However, certain restrictions come into force here. Thus, increasing the width of the spar along the chord necessarily leads to a shift of the blade centering to the trailing edge, which is usually unacceptable from the point of view of the requirements for excluding the flutter.
By reducing the stiffness of the spar by reducing its width while falls and torsional stiffness of the blade. This circumstance is one of the obstacles to the creation of the blades with very low stiffness in the plane of rotation.

The blades with the tubular spar and idle frame has a flexural natural frequency in the rotational plane about the same as in the thrust plane.

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