Abstract: To reduce the noise levels of open rotor devices rotating at supersonic tip speeds, a combination of blade sweep and aerofoil profile is employed. The sweep of the blade extends through the sonic radius, and in the region of the sonic radius the section comprises a leading edge profile with a rounded nose conforming essentially to the formulaT.about.x.sub.L.sup.a Lwhere T is the blade thicknessx.sub.L is the distance from the front of the leading edgea.sub.L is an algebraic power greater than 3/4 and less than 1.The leading edge profile extends 2-15% of the aerofoil chord and has a nose radius not greater than 2% of the chord. It can be shown that, by these measures, it is possible to moderate the perceived noise levels from a device such as an aircraft propeller rotating at supersonic tip speeds.