Boundary-layer transition measurements on Mach-scaled helicopter rotor blades in climb

Abstract

In this work, laminar-turbulent boundary-layer transition is investigated on the suction side of Mach-scaled helicopter rotor blades in climb. The phenomenon is assessed by means of temperature-sensitive paint (TSP). Results are compared to a data sample acquired by infrared (IR) thermography and accompanied by integral thrust- and local surface pressure measurements at two radial blade sections. Spatially, high-resolved data allow for precise detection of boundary-layer transition along the outer 60% of the blade span. Results obtained via TSP and IR show remarkable agreement with minor deviations due to different surface qualities of the respective blades tested. TSP data are obtained at various collective pitch angles and three different rotating speeds corresponding to chord Reynolds and Mach numbers based on blade tip speed of $$Re_{\rm tip} = 4.8 - 9.3\times 10^5$$ and $$M_{\rm tip} = 0.29 - 0.57$$ , respectively. The transition position is detected with an accuracy of better than 1% chord and the findings show overall coherence as blade loading and tip chord Reynolds number are varied. Experimental findings are shown to be consistent with two-dimensional simulations using the $$e^N$$ -envelope method for transition prediction. Based on quantitative agreement between measured and calculated surface pressures, a comparison of the corresponding transition results suggests a critical amplification factor of $$N_{\rm cr.} = 5.5$$ best suited for transition prediction in the rotating test facility of the DLR Gottingen.