I agree with Eric and Bert - and the guys who taught
materials, structures and aerodynamics in school 20+years
ago.

Couple of points to clarify:

Some have been talking about the G-load in the manual,
others (like me) have talked about the ultimate loading
to which the airframe is tested (a bigger number).
In a panic I'd probably pull past the first, but wouldn't
get near the second. I don't think Don was recommending
anything much different - Don?

References to what aerobatic pilots do ('pull as much
as necessary') are not applicable to gliders for one
simple reason: aerobatic aircraft are generally good
for Gs past G-LOC (G-induced loss of consciousness)
- not so for gliders. For GRP or CRP structures pull
to the ultimate load at your peril. You'll probably
get away with going over the placarded limit. The main
point for me remains that I can't tell how many Gs
I'm pulling from my butt alone - at least not with
a whole lot of accuracy.

Flutter is a dynamic effect and can happen to the wing,
or any of the control surfaces - I think the horizontal
and vertical stabs are generally too stiff to go first.
Each flutter mode has a different natural frequency,
damping (positive or negative) and corresponding airspeeds
at which they can start.

I've heard of several cases of control surface flutter
in sailplanes (often older ones with looser control
circuits - and Grobs with poor mass balancing). I've
not heard of sailplanes fluttered apart in flight (though
this isn't to say it has never happened). Maybe it's
because everyone who has been forced to make a choice
pulls the wings off first.

Something to think about...


At 18:18 30 March 2004, Eric Greenwell wrote:
Bert Willing wrote:

Non-catastrophic may happen if you have a structure
which has a plastic
behavious prior to rupture.
Ironically, you don't have that with 'plastic' gliders.
You might well
enconter that you can pull more g's because the designer
has put lots of
margins, and nothing will happen
But if *something* happens, you're wings are simply
gone on a GRP/CRP ship.
The idea that you'll get away with some sort of damage
and land the ship is,
hm, fairly naive.

But to the initial question: If you are going to exceed
Vne in a dive, you
can chose between putting your joker on a good spacing
between Vne and
flutter speed, or put your joker on a pessimistic
design margin and a well
crafted serial number. There is actually no way to
tell the answer
beforehand.

I agree with Bert. To imagine Don's advice to be suitable
for all
gliders is too ignore the huge differences in design
and materials. For
example, the flexible, fiberglass wing of ASW 20 probably
means it has a
greater strength reserve because of the extra material
needed to control
flutter, while the stiffer carbon wing in the ASW 27
might give it the
reverse margins. Consider the Standard Cirrus with
it's relatively thick
fiberglass wing: where are it's margins the greatest?
And, it appears
the 25 m gliders may have special problems.

Until you have discussed the design of your _particular_
glider with
it's designer, you are simply speculating about the
dangers of
overspeeding versus overloading. Even the designer
may not know, if the
glider hasn't been tested to flutter! And if you damage
the structure
during a high G pull-up, what do you suppose will happen
to the speed at
which flutter occurs? You may now have damaged glider
experiencing flutter!

Fortunately, this situation seems to rare. Personally,
I have never
encountered it in 4500 hours of soaring, not even an
incipient spin.
Here is more speculation: I think the reality is most
pilots that have
the problem will use Don's method out of reflex, not
training or
conscious choice.

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Eric Greenwell
Washington State
USA