A nonlinear hub spring design and results of the subsequent flight testing as a concept to provide increased mast bumping safety margin for the UH-1H helicopter is presented. Although there is a need for additional testing, the hub spring is shown to provide an increased margin of safety by reducing main rotor flapping in all conditions tested. As a part of this effort, a hybrid computer program was verified as able to predict mast loads due to flapping stop contact. Using this program a parametric study of mast loads as a function of rotor flapping was performed in order to develop a design criteria to ensure that mast loads can be sustained during in-flight flapping stop contact. In addition, a comparison of the main rotor flapping predicted by the hybrid computer and the digital computer C81 program using elastic blades is shown, Also, an evaluation of U. S. Army helicopter tactics to determine which NOE maneuvers are susceptible to high main rotor flapping is presented. (Author)


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