Bending stresses computation in a helicopter unmoored rotor blade blown about by the wind flow

Аuthors

Kargaev M. V.^*, Mironenko L. A.

National Helicopter Center Mil & Kamov, 26/1, Garshina str., Tomilino, Moscow region, 140070, Russia

*e-mail: kargaev_mv@mail.ru

Abstract

Ensuring the main rotor blade strength remains as before one of the main problems, the designer faces while developing a helicopter. Heretofore, at the main rotor blade design in a part of static strength ensuring, the designers confined themselves to its computing under the impact of a force of its own weight. While a helicopter operation thereat, the damages of the main and tail rotor units occur after the storm wind impact.

For the main rotor blades, the following situations are possible: the blade spar bending with residual deformation occurrence down to its destruction; corrugation occurrence at the tail sections; the blade contact with ground or the helicopter tail beam. The above said phenomena prove to be possible due to the small inherent rigidity of the rotor blades, which makes them rather sensitive to the wind loading. The designers should take measures on the wind-flow impact protection ensuring while developing rotary wing aircraft.

According to the 29.675 b item of the AC 29-2C recommendation circular, which gives procedures for determining compliance with the requirements of the AP-29 airworthiness standards, when designing the carrier system, it is necessary to avoid overloading the stops and blades in conditions of wind gusts in the parking lot, or from the rotary wing aircraft's main rotor, taxiing nearby.

The article presents a method for computing flexural stresses in an unmoored blade of a helicopter, blown over by a wind flow. It consists in determining the positions of the elastic axis points of the idealized blade model.

These positions fully determine the shape of deformations and, hence, the magnitude of flexural stresses acting in the blade. The initial equation of the blade bending by a wind loading in a linear setting by the Galerkin method is reduced to an equation relative to an unknown deformation coefficient. This coefficient is determined under the condition of neglecting the additional aerodynamic loadings stipulated by the blade elastic deformations, and with their accounting for. The load increase factor was determined from comparing the obtained relations comparison, on which basis the solution allowing avoid the direct integration of the initial equation was obtained.

The equations are presented in a form convenient for numerical determination of the elastic axis points positions of the blade, slope angles and bending moments (stresses). Computation results for the rotor blades of the Mi-8 helicopter are presented. It was shown, that accounting for elasticity introduces significant changes in the bending moments (stresses) distribution along the angle of the blade azimuthal position, which determines the direction of its blow over.

Keywords:

main rotor blade, wind loading, static strength, critical dynamic pressure

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