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 finit e element method, and the droop stop is simulated using a conditional rotat ional spring. No aerodynamic effects are modeled, Three methods of time int egrating the equations of motion were studied: 1) a direct integration of t he full finite element space equations of motion; 2) a modal space integrat ion 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-sca led articulated model rotor blade were conducted at zero rotational speed. The transient tip deflection, flap hinge angle, and strain were measured, a nd they displayed good correlation with all three analytic methods. Modal p arameter identification tests were performed on the model blade to determin e its natural frequencies and damping ratios for both hinged and cantilever ed conditions. The measured structural damping was shown to significantly i mprove correlation between the experimental and analytic results. Computati onal efficiency for the problem under consideration was not of serious conc ern. However, in a comprehensive aeroelastic analysis, it was found that a modal space integration using either hinged or cantilevered modes, dependin g on blade/droop stop contact, reduced computational time by two orders of magnitude.