"DSK" wrote in message
...
wrote:
I fly glassfibre aircraft, sailplanes actually - structure
lifespan is
a topic of interest when you are at 20 000' above ground level
flying
at 200 kph plus and the outside air temperature is below -15
degrees
Celcius.
Hmm. And some airliners fly at 500kts at 40 000' in -40C
the test is how well an airplane survives going through a Cumulo
Nimbus Cloud -
and development around sailplanes ( OSTIV ) recently published
a rather
reassuring article giving an estimated service lifespan of a
glassfibre
sailplane of around half a million flying hours. and that was
with a
safety factor of at least 1 order of magnitude.
the requirement for a civil aircraft is not to cause more than 1
death in every 1,000,000 flying hours. And, on average, that's
achieved.
Although yacht hulls are stressed by rigging and water loads I
suspect
that the stresses involved ( in a normal cruising boat ) are
somewhat
less than experienced by an aircraft structure.
I disagree. In fact I'd suggest that since yachts experience
structural failure more often, they are more highly loaded. Also
they experience greater load transients . (AFAIK... it may be
that aircraft experience more structural failures that I don't
hear about).
Think about how your airplane would react if you picked it up &
dropped it 10 or 20 feet (3 ~ 6 meters) three times a minute, at
varying angles, for several days.
Yup. Just like flying through a Cu Nim. Only for an hour or so
though. Hundreds of feet. On lots of flights in its lifetime.
..... The wings on my 25m
span Nimbus regularly deflect by a metre or so in normal
flight. Even
using the same figure of half a million hours one gets 47 odd
years of
'use', not just sitting in a berth.
Watch the wings of your 747 next time you lift off. They flex
through nearly 4 metres! More in sever turbulence.
I'd be willing to bet that this is well within the range of
unrelieved strain (ie does not accumulate fatigue) and the wing
is designed for this much deflection. Sailboats have to be a
good bit more rigid... I'm guessing that parts of the plane are
too, like for example how much does the fuselage flex & deflect
when you put on some rudder?
Exactly. Airplanes are designed to absorb energy by being
incredibly flexible. Yachts are designed rigid. Bad news for
yachts, so they have to be overdesigned in strength to reduce
failures. But yacht failure isn't so critical anyway; yacht
manufacturers get away with the occasional lost keel . . . only
affects a few people.
Of course if you insist on building a hull only millimetres
thick out
of Nomex, Kevlar, Carbon and other exotics, sail it in across
Biscay in
a gale with only one hull of three in the water and driving
through 5m
high swells then 'all bets are off' and you may well run into
mechanical limits of the structure.
Some production yachts have failed in lighter circumstances - ask
Bavaria about Croatia . . .
Could happen in planes too. And parachutes are cheaper than
ocean rescues 
Doubt it, there's a big industry looking after flight safety . . .
JimB