Ground Resonance History of the H-34 Helicopter (Sikorsky Model S-58)

Originally designed in 1952 as an anti-submarine helicopter for the U. S. Navy, more than 2000 Sikorsky S-58’s (H-34’s, HSS-1’s) were built and delivered in the time period from 1953 through 1969. They have served on every continent of the world in both civilian roles and with the armed services of 25 countries. Whether lifting construction material or supporting police departments, this “DC-3” of helicopters is still flying well past its 50th birthday. Yet during its early introduction, the successful deployment of this marvelous machine was threatened by the specter of “ground resonance”. Early models of the H-34 could easily be excited in ground resonance. Ground resonance is a self-excited instability of the helicopter, which results from coupling between rigid body motions of the fuselage and oscillations of the rotor blades in their plane of rotation. The instability when it occurs can build up to destructive proportions in a matter of seconds. The problem was attributed to several causes: 1) insufficient roll damping characteristics; 2) inadequate oleo motion; and 3) vulnerable fuselage attachment fittings for landing gear. The H-34’s ground resonance problem was solved by the redesign of the landing gear.