Ed Huntress wrote:

"Tim Shoppa" wrote in message
ups.com...

On Sep 30, 10:00 am, "Ed Huntress" wrote:

However, they also don't know how much fatigue becomes a problem in
all-aluminum aircraft that were designed over the last few decades. DC3's
are still flying because the engineers didn't know what a reasonable
safety
margin was. Now they know, and the life of those planes is finite.

Overdesigned aircraft can have a ridiculously long airframe life.

The B-52's airframe service life runs through ****ing 2040 by current
military planning. Can you believe that? The kids flying them will be
in an airplane made when their great-grandparents were kids or not
even born. It'll be a timespan equivalent to the military still using
Wright flyers. But not exactly the same, because even in 2040 there
still won't be another airplane that'll do everything a B-52 does.


Yeah, when you study fatigue you learn that the margins are really very slim
between limited life and extremely long life. The curves climb (or fall off,
if the curve represents time to failure) sharply; I don't know what they are
mathematically, but there's some positive exponent in there.

Aluminum is particularly nasty in that regard. Fortunately aluminum fatigue
has been studied and researched to beat hell because of the aircraft
applications, and there is little guesswork in applying the calculations
today.

--
Ed Huntress



The C-130 is another aircrft that will be around for a long time. On
the newer aircraft a lot of the parts are titanimum rather than
aluminum. On the older aircraft intergranular corrosion was a ongoing
problem. I was told it was from improper heat treating of the aluminum.
The aluminum would turn to a white powdery substance, mainly on heavier
aluminum pieces.


John