The effect of blade deformation rotorcraft acoustics

A derivation of Farassat's formulation 1A is presented that includes two additional terms related to the surface deformation. the effect of making a rigid-blade approximation in the acoustics formulation is investigated for a utility helicopter, and it is found that for this case the thickness noise can be in error by 1-2 dB in the plane of the rotor and as much as 10 dB out of plane; the loading noise can be in error by 3-4 dB out of plane and up to 10 dB in plane. An investigation of the effect of the additional terms when used in conjunction with a deforming permeable acoustic data surface is investigated. Finally, the thickness noise of a speculative advanced "smart materials" blade is examined to set an upper limit on the impact of the new formulation on rotorcraft acoustics. It is found that for present rotorcraft, the additional terms in the acoustic formulation for flexible surfaces may be neglected safely, although the aerodynamic and aeroelastic effects of blade flexibility should be included.