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The Lockheed L-188 Electra, notoriously, suffered from early problems with whirl-mode flutter, which caused two Electras to break apart in flight in 1959 and 1960, forcing an expensive redesign of the Electra's engine mounts and killing off Electra sales.
In contrast, the other similar quadturbopropliners of the era (Viscount, Britannia, Il-18, An-10, CL-44, Vanguard) - even those which entered service before the Electra ever flew - had no such problems.
Why?
Flutter is a natural frequency resonance phenomenon. Something is oscillating, and its oscillation, at some point, starts to match the natural frequency of the structure it's attached to, and a mutual input/feedback cycle starts that stores and returns the energy in such a way that it magnifies the initial input and away you go.
The outer wing was the saxophone... the engine nacelle was the reed.
The other airplanes? Well, when you get down to it, fortunately they were just out of tune you might say, so they weren't affected.
the forward structural section of the engine nacelle which supported the reduction gearbox for the propeller drive and connected the gearbox mounts to the rest of the engine supports were designed in such a way that if the plane made a hard landing, those structures would be weakened enough to allow the establishment of undamped whirl-mode flutter/resonance.
The dynamics were beyond the capabilities of the computer modeling tools available at the time, so the root-cause investigation of the crashes was based instead on the construction of precision scale models of the entire airplane which were then "flown" in a wind tunnel, with the tests captured on film with movie cameras.
In this way, the structural deficiency of the gearbox-and-engine mounts was uncovered.
