On linear acoustic solutions of high speed helicopter impulsive noise problems

Abstract

The nature of linear acoustic solutions for a helicopter rotor blade with a blunt leading edge operating at high transonic tip Mach number is studied. As a part of this investigation a very efficient computation procedure for helicopter rotor blade thickness noise according to linear theory is developed. Numerical and analytical results reveal that as the blade tip Mach number approaches unity, the solution develops singularities and a radiating discontinuity. It is shown that these characteristic features are caused by the contributions of the higher harmonics which decrease in magnitude only as n 1 2 in the limit n → ∞. These higher harmonics are generated by the blunt leading edge. The far field wave form at sonic tip Mach number for a blade with a NACA 0012 airfoil section has a singularity of the inverse root type at its front and a logarithmic singularity near its end. Thus caution must be exercised in applying linear acoustic theory to high speed helicopter impulsive noise problems.