Review of rotor loads prediction with the emergence of rotorcraft CFD

This paper reviews the state of the art in helicopter rotor loads prediction with the emergence of computational fluid dynamics (CFD) and computational structural dynamics (CSD) coupling. The focus is on steady level flight, where most of the current CFD/CSD analyses are being applied. The application of CFD to rotorcraft problems has evolved, over the period 1990-2005, as a viable means to improve the aerodynamic modeling used in rotorcraft comprehensive analyses (CA). It has the potential to meet the ultimate objective of a coupled rotor-fuselage analysis that can predict loads and vibration accurately at all critical flight conditions without semi-empirical inputs. The paper begins by identifying three critical level flight conditions. These are useful to isolate the key aerodynamic and structural dynamic mechanisms. It is followed by a review of the capabilities and limitations of current CFD analyses in representing the key aerodynamic mechanisms: three-dimensional airloads and wake, dynamic stall, unsteady transonic effects near the tip, and fuselage aerodynamic effects. The structural dynamic methods are then briefly reviewed. Finally, the recent rotorcraft CFD/CSD coupling methods are described and evaluated on the basis of their loads prediction capability. The emphasis is on the fundamental aeroelastic mechanisms which determine the critical rotor loads, and with which the accuracy and efficiency of all CFD/CSD coupled analyses should be assessed.