ME 163

RESONANCE AND THE LOCKHEED ELECTRA

The Lockheed L-188 Electra has an important place in the history of commercial airliners. It is the only large American airliner with turboprop engines. It first flew in 1957, and its first commercial flight was on January 12, 1959. Although widely praised by pilots and passengers, its first year in service was marred by a series of five crashes. After investigation, three of the crashes were ruled to be the result of natural disasters that could have happened to any aircraft. But in two crashes, the plane seemed to disintegrate in mid-air. An extensive investigation revealed that there was a resonance problem. The engine/propeller combination produced a forcing vibration that was close to one of the natural frequencies of the wing, and this resonance destroyed the wing. Why did the engineers produce a design with such a resonance? Actually they didn't. A normal Electra did not have the problem. However, Electras with engine mounts that had received apparently minor damage (from a heavy landing for example) had radically different characteristics. The damaged mounts allowed an engine vibration at a different frequency than the normal mounts, and this frequency was unfortunately close to the wing frequency. Once this was understood, it was possible to modify the design to prevent the resonance, but the plane never really regained popularity.

The attached brief description of the Electra is from The Illustrated Encyclopedia of Propeller Airliners, edited by Bill Gunston, Exeter Books, p. 208- 209, 1980, New York. For a contemporary pilot's view of the Electra, see Chapter 11 of L-1011 TriStar and the Lockheed Story, by Douglas J. Ingells, Aero Publishers Inc., 1973, Fallbrook, CA.