Eta crashed

"Howard Franks" wrote in message
...
I have always assumed the recent (last 20 years) composite
gliders to be very robust, i.e. no issues flying right
up against placarded limitations at anytime (maybe
once or twice nipping over?).

Perhaps the Eta break-up is a sign that we are reaching
the structural/design limitations of the current materials
and the designers ability to optimise gliders for performance.
Are we at the point of diminishing returns where the
small increase in performance only comes at a far greater
risk of structural failure (similar to the Americas
Cup yachts).


This coupled with the recent Schempp-Hirth issues (agreed
manufacturing not design defects), and the AD restricting
GROB 103s is leading me to rethink just how tough these
things are.


There seems to be a prevalent attitude that all gliders are much stronger
than the POH says. At least, as a CFI-G, I have a hard time convincing
pilots that those placards mean just what they say. This is particularly a
problem when it comes to overloading 2-seaters. Exceed a placarded limit
and you become a test pilot - maybe a dead one.

I think gliders are just as strong as the manufacturer says, but no
stronger - and only then if the design and manufacturing rules are followed
to the letter. The problem with the Schempp-Hirth gliders seems to be that
the manufacturer's own procedures were not followed. The issue with the
G103's MAY be that errors in design occurred. None of this says that
composite construction is the problem, just that the process needs to be
carefully controlled and monitored. (It also says that reputable
manufacturers sometimes make mistakes that aren't found until after the
gliders are in the field.)

As for the Eta, this may turn out to be an intriguing case. At 31 meters it
is certainly pushing the limits. Bob K. perceptively speculated about
bending and twisting loads on the tailboom. It could also be that the spin
recovery has to be performed exactly right of the airspeeds get way, way out
of hand. I would expect that the Eta will be placarded against intentional
spins for this reason.

Bill Daniels

Bill Daniels wrote:

I think gliders are just as strong as the manufacturer says, but no
stronger

Most Gliders are built in JAR country and are certified by JAR rules.
For the utility category, JAR requires an allowed load of 5.3g at Vne
and 4.0g at Vm. JAR further requires that the break load be no lower
than 1.5 times the allowed load.

Since every extra strengh comes at a price in weight and money, the
break load of most gliders acually *is* 1.5 times the allowed load. This
is when the glider is new. Take some turbulence, and the safety cusion
is rather small.

As for the Eta, this may turn out to be an intriguing case. At 31 meters it
is certainly pushing the limits.

Actually this is the whole idea of the Eta project: pushing the limits.
Remember that the first prototype was too heavy and extra work was
required to bring the take off mass down to 850 kg! (The 850 kg limit is
again required by JAR as well as by contest rules.)

Stefan

snip

.. JAR further requires that the break load be no lower
than 1.5 times the allowed load.


Yes, 1.5 times is the ulimiate design factor for the aircraft. However
1.0 times the allowed load is the limit facter. If you exceed the
limit factor you may (will?) damage the aircraft structure but it will
not fail until 1.5 times the limit. This is especially true of metal
structures where the material itself will fail (i.e. Break) at about
1.5 times the yeild strength(i.e. bend permanently).

Brian Case
CFIIG/ASEL




snip