US Patent 7997872 Fan blade

The suction surface blade angle of a transonic fan blade, subject in use to a shock wave, progressively reduces along part of the suction surface, beginning at a position upstream of the shock wave position. The increased area variation at the location of the shock results in the shock position becoming less sensitive to small geometric imperfections. The reduced shock sensitivity reduces the variation in aerodynamic load and hence reduces the untwist variation with respect to small geometric imperfections. This has the effect of stabilising the untwist deflections of the fan.