Unsteady Boundary-Layer Transition Measurements and Computations on a Rotating Blade under Cyclic Pitch Conditions

The presented work tackles the lack of experimental investigations of unsteady laminar-turbulent boundary-layer transition on rotor blades at cyclic pitch actuation, which are important for accurate performance predictions of helicopters in forward flight. Unsteady transition positions were measured on the blade suction side of a four-bladed subscale rotor by means of the non-intrusive Differential Infrared Thermography (DIT). Experiments were conducted at different rotation rates corresponding to Mach and Reynolds numbers at 75% rotor radius of up to M_75 = 0.21 and Re_75 = 3.3 x 10*5 and with varying cyclic blade pitch settings. The setup allowed to measure transition across the outer 54% of the rotor radius. For comparison, transition was also measured using conventional infrared thermography for steady cases with collective pitch settings only. The study is complemented by numerical simulations including boundary-layer transition modelling based on semi-empirical criteria. Promising results reveal a plausible development of measured transition positions over the pitch cycle, a reasonable comparison to experimental results obtained using the already established sigma c_p method, and noticeable agreement with numerical simulations. The result is the first systematic study of unsteady boundary-layer transition on a rotor suction side by means of DIT including a comparison to numerical computations