I may not be exactly right about this, but I believe
that most certified aircraft (including gliders) are
tested to, and must be able to hold together at, a
static G-load of roughly 150% of maximum. To meet this
requirement the wings are generally tested to destruction
(check out the DG website for a video of this procedure
for the DG 1000).

With respect to the flutter speed, I believe that the
manufacturer must demonstrate dives with some margin
above Vne. Question: does anyone know how much faster
over Vne a sailplane must demonstrate flight and under
what combinations of G-loading and control inputs?
Needless to say, certified sailplanes are not generally
tested to destruction by flutter - for obvious reasons.

Conclusion: There is a relatively certain G-load beyond
which you will pull the wings off your glider - just
multiply the certified G-limit by the certification
margin. With respect to flutter at speeds beyond Vne,
it's more of a roll of the dice, since flutter is a
dynamic interaction between aerodynamic forces and
structural ones. Flutter can be affected by control
inputs, control balance, and G-loading, among other
factors. The speed at which wing or tail flutter starts
is not usually an empirical number (Grob 102/103 not
withstanding - customers established these speeds after
the fact), since flight tests don't confirm absolutely
when flutter begins. All we know is the speed below
which flutter doesn't start (Vne x certifcation margin).
That's all that's demonstrated. You may in fact be
able to get away with a bit faster - of course, you
are taking your chances. In summary, it seems like
a choice between certain catastrophe versus possible
catastrophe.

Also, I find it a bit strange that some here feel that
it is possible to over-G a sailplane to damage, but
not destruction. It seems like a fine point to me and
there are several examples of unlucky souls who have
misjudged the point.

One additional thought - I suspect that for older gliders,
wearing of bearings/bushings may degrade the flutter
margin faster than normal material aging degrades the
G-limit, so you might need to think differently if
you have a high-time glider.

Thoughts?


At 21:30 28 March 2004, Denis wrote:
W.J. (Bill) Dean (U.K.). wrote:

There have been several cases of certificated gliders
overstressed in
stall/spin recoveries, some of them broke up.

(...)

You can also read about the Nimbus 4DM at Minden;
99.07.13 - LAX99MA251 -
http://www.ntsb.gov/NTSB/brief.asp?e...09X01702&key=1
Nimbus 4DM -
Minden - Two killed.

the link is actually
http://www.ntsb.gov/NTSB/brief.asp?e...12X19310&key=1

the conclusion is 'The pilot's excessive use of the
elevator control
during recovery (...) resulted in the overload failure
of the wings at
loadings beyond the structure's ultimate design loads.'

[the possibility of speed being over VNE or Vd is neither
confirmed nor
being one of the causes of the wing failure according
to the report]

This supposes that unfortunately the pilots did what
Bill told : 'pull
however hard is necessary' with the result that 'At
the ultimate load
limit, the deflection was 46.5-degrees, similar to
the witness
observations of the wing deflection just prior to the
break up.'

Do you imagine you may safely 'pull however hard you
need' with your
wings bent at 45° up ??? I don't.

The report quotes also that the G limit for the Nimbus
4 at VNE is 3.5 g
*only* (compared to 5.3 g at Va) and the design 'safety
margin' is
between 1.55 to 1.75. Thus even on a plane in perfect
condition, and if
the manufacturer made no mistake, it *will* break between
5.4 and 6.1 g
at VNE (even without airbrakes)

Remember that at that speed (285 km/h) you could pull
about 16 g ! and
at Vd (324 km/h) more than 20 g...

Yes it was certificated, but certification does *not*
guarantee you that
the glider will not break if you pull 20 g... !

--
Denis

R. Parce que ça rompt le cours normal de la conversation
!!!
Q. Pourquoi ne faut-il pas répondre au-dessus de la
question ?



'Stefan' wrote in message
...


This is exactly the point: certificated gliders can
always be recovered
from a spin without exceeding the limits, otherwise
they wouldn't have
been certificated. The ETA wasn't certificated
and broke up during a
test flight. It won't get certificated before this
issue is fixed.