Ahh...the dreaded whirl mode. Unfortunately the book " The Electra Story" is long out of print and was one of the most fascinating investigations I've come across.
The problem arose when a certain combination of speed and turbulence was encountered. Keeping the speed below 400 mph ensured all stayed well.
I remember before the engine mounts were stiffened the engines used to nod up n down noticeably in turbulence .
The Lockheed test pilots were certainly made of the right stuff...when Lockheed reckoned they had fixed the problem they went out looking for turbulence trying to create the whirl mode.
When they were satisfied, as a final test they loosened the prop attach bolts on one eng and flew the a/c over the Sierra Nevadas at VNE in the severest turbulence they could find.
End of problem!
It was a magnificent a/c to fly..I am sure you will never find any pilot with a bad word to say about it.
