In article ,
says...

On 5/7/2013 11:07 PM, J. Clarke wrote:
In article ,
says...

On Sat, 16 Mar 2013 02:27:15 +0100, Wolfgang Weisselberg
wrote:
: RichA wrote:
:
: Weakness, focus problems (very likely due to plastic's inherent
: incapability to be machined as accurately as metal,
:
: Please provide proof
:
: it's tendency to
: change shape/size radically with temperature changes.
:
: Quite unlike metal, which *never* expands or contracts.
:
: Please look up the de Havilland DH 106 Comet 1.
: Fun when the planes disintegrate in the air after 20k hours
: due to *metal* fatigue.

Likewise the Lockheed Electra. So much vibration was transmitted into the body
of the plane that at least one airline provided soft cushions for the
passengers' feet. IIRC, they still used them, even after they were required to
reduce the plane's maximum speed by about 100 knots. But after the speed
reduction the planes at least stopped falling apart in the air.

The speed reduction was temporary until the cause of the problem could
be determined. It was determined, it was fixed, and the Electra
airframe continued in production for another 50 years. The vibration
problem was also fixed--the two were not related.

I believe that Lockheed was well aware of the risk, some safety
engineers had warned management, but the warnings were ignored in the
interest of timely delivery.

You can believe anything you want to. I was an engineer at Hamilton
Standard and I worked on that propeller. These events happened before
my time but I had access to everything that was known about that
propeller and there was a lot in the files about figuring out the
vibration modes in support of Lockheed's effort to resolve the problem.
The people who worked the problem were still there and this is one of
the projects that they liked to talk about--from an engineering
viewpoint it was "interesting".

You know what a resonance is? Where you keep feeding energy into
something at the same frequency at which it vibrates when struck? Well
this was the problem. It wasn't that Lockheed was careless--they had
designed to cope with every vibration frequency that the engine and
propeller were known to produce--this wasn't horribly difficult as the
propeller and engine turned at a constant speed. The trouble is that
the propeller in this case started vibrating in a way that had not been
anticipated (by anybody, not just Lockheed) that was hitting a resonant
frequency of the engine mount.

This is not something that a "safety engineer" could predict. This is
not something that the propeller manufacturer could predict. This is
not something that, in 1959, the entire scientific community working
together could have predicted because the computing power to predict it
simply did not exist in the world. It had to be determined by extensive
testing, which was a significant engineering project in itself, and by
analysis of the wreckage, which showed unexpected damage to the bearings
on the blade shanks. Hamilton ultimately did figure it out and at that
point, knowing the nature of the problem, Lockheed was able to work up a
reliable fix.

Note that the whirl mode vibration that was breaking the airplane was
different from the vibration that causing the comfort problems and the
two fixes were unrelated--the comfort problem was fixed by changing the
angle of incidence of the nacelles, the structural problem required very
extensive modifications to the engine mount and the outer wing
structure.