Experimental and theoretical correlation of helicopter rotor blade-droop stop impacts

The transient response of a nonrotating articulated rotor blade undergoing a droop stop impact is examined. The rotor blade is modeled using the finite element method, and the droop stop is simulated using a conditional rotational spring. No aerodynamic effects are modeled. Three methods of time integrating the equations of motion were studied: 1) a direct integration of the full finite element space equations of motion; 2) a modal space integration using only hinged modes; and 3) a modal space integration using either hinged or cantilevered modes, depending on blade/droop stop contact. Given a range of initial flap hinge angles, drop tests of a one-eighth Froude-scaled articulated model rotor blade were conducted at zero rotational speed. The transient tip deflection, flap hinge angle, and strain were measured, and they displayed good correlation with all three analytic methods. Modal parameter identification tests were performed on the model blade to determine its natural frequencies and damping ratios for both hinged and cantilevered conditions. The measured structural damping was shown to significantly improve correlation between the experimental and analytic results. Computational efficiency for the problem under consideration was not of serious concern. However, in a comprehensive aeroelastic analysis, it was found that a modal space integration using either hinged or cantilevered modes, depending on blade/droop stop contact, reduced computational time by two orders of magnitude.