Brian Gaff wrote:
> What I don't quite get is that really old planes like the Lancaster,
> Dakotas etc, seem to go on forever but a much newer craft like the vulcan is
> clapped out and mega bucks to fix much sooner. Is it the connstruction, the
> stresses or what?
>
Both. Dakotas don't have a main spar, they are a monocoque construction,
and their life is determined by cracks round the wing root, which are
easily seen from the outside. Lancasters are also easy to examine, and
nobody actually seems to have done the work that says "You can't fly it
after X hours", mainly because they were never expected to survive that
long. Piston engines are life limited only by wear of their (mostly
replaceable) parts, while jets are also limited by the effects of
temperature on the metal they're made of.
Wooden aircraft such as the Mosquito don't have a limited life, but the
condition of their structural members is regularly examined.
Newer aircraft such as the Vulcan are built to take maximum advantage of
the strength of the materials used, and the complexity of the
construction makes it harder to examine that structure, so the hours
limit laid down by the maker is used to limit their life. As the normal
limit is on the main spar, being the most highly stressed component,
that's the limit of the aircraft life. If you replaced the main spar,
the clock could, in theory, start again, but as you'd have to dismantle
and rebuild the entire aircraft to do this, it doesn't get done.
--
Tciao for Now!
John.