Structural failure due to harmonic vibration

Ron Wanttaja wrote in
:

On Thu, 08 Nov 2007 09:46:52 -0500, Dudley Henriques
wrote:

Jay Honeck wrote:
I've just finished reading the tale of the first round trip
coast-to- coast flight (which was accomplished by mid-air
refueling, occasionally from milk cans) from Spokane, WA to the
east coast and back, way back in 1929.

Hmmmm....interesting definition of "coast to coast." Spokane is "on
the coast" the say way Pittsburgh is (e.g., hundreds of miles inland).

One of the pilots, Nick Mamer, went on to a career with Northwest
Air Lines. The author of the article states that he was killed in
1938 flying a Lockheed 14 Super Electra over Montana when the plane
crashed after suffering structural failure due to harmonic
vibration. All passengers and crew were killed.

Do a search for Lockheed Electra, Tell City Crash, 1960 I believe.
Reference that with propeller whirl mode, and you should come up with
all you'll ever need to know about resonant frequency as relates to
destructive force.

Wrong Electra, Dudley. Namer died in 1938 in the twin recip, twenty
years before the four-engine turboprop.

Wikipedia says, "Later, an investigation revealed that the tail
structure had failed on the new design from what is known as "natural
resonance, or period of vibration." Sounds like the natural frequency
was too low....



Yes, but the Later Electra was a classic lesson in resonant freq
failure, though quite a different thing to the failure that the earlier
Electra had. IIRC two L188s were lost when a precession induced whirl
set up a torsional action in the nacelles which in turn overstresed the
wing. in short, a bit of turbulence would get one prop wobbling whihc
would start the wing wobbling which would get the second prop on the
same side wobbling and the whole thing would increase in amplitude until
the wing failed.
A redisgend engine mount and reskinning the wings with the next gauge
aluminum cured the problem.


Bertie