Numerical Identification of Laminar-Turbulent Transition Mechanisms for a Helicopter Rotor in Forward Flight

A method is presented for predicting the laminar-turbulent transition on helicopter rotors. For a robust computation of the laminar boundary layer quantities an URANS method (DLR FLOWer code) and an integral / Pohlhausen method for rotating blades are used. The approximate computation of the laminar boundary layer is tested for three experimental testcases featuring a rotating NACA 0012 airfoil, the NLF(2)-0415 swept wing and the NLF(1)-0416 laminar flow airfoil. The laminar-turbulent transition mechanisms on the 7AD helicopter rotor are investigated for a cruise and a fast forward flight condition. The investigation empirically covers different transition mechanisms and studies their influence on the rotor power and thrust. The predicted transition positions are compared to local hotfilm measurements (GOAHEAD wind tunnel test).