A new method of dynamic and static stall detection using infrared thermography

A new method of detecting flow separation for static and pitching airfoils is described, with application to the generation of stall maps for helicopter rotors. An airfoil is heated using a lamp, and a high-speed infrared camera monitors the surface temperature. Subtracting consecutive images and performing a spatial standard deviation over a region of interest yields a single \(\sigma \hbox {DIT}\) value which is used to detect boundary layer separation on the airfoil. The data can be analysed to identify attached flow (low values of \(\sigma \hbox {DIT}\)) and separated flow (high values of \(\sigma \hbox {DIT}\)). Although appropriate filtering can significantly improve the signal-to-noise ratio, the method is robust regarding the exact method of analysis and the unfiltered data are sufficiently clear to be analysed without additional processing. For the test airfoil used, stall was measured up to a pitching frequency of 5 Hz, and signal-to-noise ratios indicate that it should be possible to measure stall for a pitching frequency of 20 Hz for a carbon-fibre surface with the thermal properties used.