Envelope Protection System for Iced Airfoils Using Flap Hinge Moment

A stall prediction method based on unsteady hinge moment measurements was previously developed from experimental hinge moment measurements on a NACA 3415 airfoil model under a clean configuration and four iced configurations. The stall prediction algorithm was based on three separate detector functions. Additional hinge moment measurements have been obtained experimentally for a NACA 23012 airfoil model, using these same clean and iced configurations. Tests were also conducted with boundary-layer trips on the model upper and lower surfaces. The addition of simulated icing degraded the performance of the NACA 23012. Upon application of the hinge moment stall prediction method, two of the six configurations provided detector function outputs that were inconsistent with the other four configurations. Further investigation revealed differences in the hinge moment signal between contamination configurations due to the presence of different types and extents of primary stall mechanisms. Larger amounts of energy were observed in the hinge moment at low-frequency ranges for the clean and simulated horn-ice cases than the other contamination configurations. This increase in energy was attributed to greater energy content in the pressure forcing on the flap upper surface at these low frequencies.