An Analysis of Harmonic Airloads Acting on Helicopter Rotor Blades

Rotary wing aircrafts in any flight conditions suffer from excessive vibration which makes the passengers feel uncomfortable and causes fatigue failure in the structure. The main sources of vibration are the rotor harmonic airloads which originate primarily from the rapid variation of flow around the blade due to the vortex wake. Unlike fixed wing aircrafts, helicopter wake consists of helical vortex sheets trailed behind each blade and remains under the rotor disk which induces vertical downwash velocities at chordwise and spanwise stations of the blade. In this study, a mathematical model is developed for rotor blades to compute the harmonic loads induced velocity at rotor blades for two flight conditions-vertical takeoff and landing, and forward flight. This method is useful for the performance analysis of rotor blade and selection of airfoils for the blade. The sectional lift, drag, and pitching moment are computed at a radial blade station for both flight conditions. The numerical integration of Biot-Savart relation are done for all the trailing and shed vortices to calculate the downwash through the rotor disc. The airloads are calculated using the relation between harmonic and inflow coefficients. The lift at a particular radial station is computed considering trailing and shed vortices and summing over each blade. Lifting-surface and lifting-line theories are applied to near wake and far wake, respectively, to calculate the downwash and inflow through the rotor disc. The results for lift are compared to the experimental flight-test data.