Avoiding Vne

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

Bert Willing wrote:

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.

But pulling the airbrakes would be fairly suicidal.

I suppose you meant "pulling the airbrakes while pulling too hard" ???

As Eric noticed it, the allowed G-loading at VNE in ASH26 (for example)
is 4 G without airbrakes, and a very close 3.5 G with airbrakes.

Thus in most cases it will be *safer* to pull airbrakes (including close
to the ground, if the dive angle is high).


--
Denis

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

Yes. However, judging g-loads with the seating position in modern gliders is
difficult - especially if you run on 100% adrenaline.

--
Bert Willing

ASW20 "TW"


"Denis" a écrit dans le message de
...
Bert Willing wrote:


But pulling the airbrakes would be fairly suicidal.

I suppose you meant "pulling the airbrakes while pulling too hard" ???

As Eric noticed it, the allowed G-loading at VNE in ASH26 (for example)
is 4 G without airbrakes, and a very close 3.5 G with airbrakes.

Thus in most cases it will be *safer* to pull airbrakes (including close
to the ground, if the dive angle is high).


--
Denis

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

Don

First - I agree entirely that you are contemplating which of two evils you
should perpetrate having got yourself into an untenable situation. But having
got there you need to at least have considered what you should do in the event.
I doubt there would be much time for deliberation.

I suggest that the correct action depends on the aircraft to some degree, but
that flutter is much more damaging to the structure than moderate overstress in
most cases since it introduces large cyclical and localised loads on the
structure in addition to whatever G load the aircraft is exposed to.

First generation glass, before the finite element analysis programs allowed the
designers to design to the limit is probably much safer to over stress than
overspeed. Similarly the latest carbon designs seem to have G limits imposed by
the JAR22 deflection limits rather than ultimate strength. Presumably these
aircraft have huge strength reserves. For interest look at the wing test on the
DG1000.

When I asked Schempp-hirth about the possibility of flutter damage in an
incident where a Std Cirrus had made a loud chattering noise on a high speed
pass, they replied that it would be unlikely to have been flutter. This because
they did not think it likely that the aircraft would remain controllable due to
control system damage in the event of flutter.

In inspection we found that the noise came from an airbrake cap that had lost
some tension in the retention springs. It was sucking slightly open and banging
against the sides of the slot as the pilot pulled up. Over one G, close to Vne
and soft springs combined to allow a millimeter or so of play. The noise was
disconcertingly loud from the ground, we thought there might be a glass-fibre
confetti shower.

I'd take a gamble on the Cirrus's wings handling more Gs than the manual said if
my life depended on it. Conversely I take great pains not to get even near that
point in a 32 year old glider.

Bruce Greeff wrote:

First generation glass, before the finite element analysis programs
allowed the designers to design to the limit is probably much safer to
over stress than overspeed. Similarly the latest carbon designs seem to
have G limits imposed by the JAR22 deflection limits rather than
ultimate strength. Presumably these aircraft have huge strength
reserves. For interest look at the wing test on the DG1000.

I agree that *some* earlier, 15m designs may have a good safety margin
in overstress, mostly those in glassfiber (more flexible).

But not *all*, and certainly not modern open-class gliders.

I recall what I posted before, because there are facts from NTSB and
manufacturer data, which I think are more reliable than some honorable
but ill-based opinions expressed in this thread, and which nobody here
have contested yet:

the link (on Minden Nimbus 4 accident) :
http://www.ntsb.gov/NTSB/brief.asp?e...12X19310&key=1
(...)
The report quotes 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)

--
Denis

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

Earlier, Bruce Greeff wrote:

...Similarly the latest carbon designs
seem to have G limits imposed by
the JAR22 deflection limits rather
than ultimate strength...

I'll certainly agree that composite sailplane structure is bounded
more by stiffness than by strength. However, I've spent my lunch hour
searching JAR22 and I can't find anything that codifies deflection
limits. The closest thing I found seems to be:

: JAR 22.305 Strength and deformation
: (a) The structure must be able to support
: limit loads without permanent deformation. At
: any load up to limit loads, the deformation may
: not interfere with safe operation. This applies in
: particular to the control system.
: with respect to the sailplane.

Do you know of other relevant JARs that specify maximum structure
deflection in quantifiable terms? I'm not trying to nitpick or
anything, I just want to make sure I'm not missing something
important.

Thanks, and best regards to all

Bob K.
http://www.hpaircraft.com

Bob Kuykendall wrote:
Earlier, Bruce Greeff wrote:


...Similarly the latest carbon designs
seem to have G limits imposed by
the JAR22 deflection limits rather
than ultimate strength...


I'll certainly agree that composite sailplane structure is bounded
more by stiffness than by strength.

I've been told that is more likely true for fiberglass construction, but
not so likely to be true for carbon fiber construction, because of the
great differences in material characteristics, such as stiffness. So, it
might correct to argue that a glass fiber sailplane has a "substantial"
G loading margin, but not correct for the carbon fiber sailplane.

And the bounds might be quite different for a 15 meter glider and a 25
meter glider, or a thick wing trainer and a thin wing racer.


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Eric Greenwell
Washington State
USA

HI Bob

That is what I was referring to.

The deformation limit for carbon designs with thin wings appears to be the point
at which it becomes impossible to maintain control movement.

As an example, there are various apocryphal tales of uncommanded airbrake
openings on open class aircraft with thin flexible wings. The Nimbus 4 appears
to be the most common suspect here.

So the deflection limit is not a "x degrees from rest", or a plastic deformation
(although there is a requirement for this in the regulations) but a deflection
beyond which the control actuators do not work correctly or have unacceptably
high resistance.

My point came from published discussions on the construction of the Eta, and the
DG1000 where both constructors commented that the ultimate strength of the
structure was well in excess of the limit load, and that the limit load was
imposed by the deflection of the wing.

There is an interesting test story at:

http://www.dg-flugzeugbau.de/bruchversuch-e.html

The destructive test requirement is that the wing must withstand 1.725* the
limit load for three seconds at a temperature of 54Celsius. The DG1000 wing
withstood this - and eventually failed at 1.95 times the design load limit. This
is one reason why I believe you would probably be able to get away with a brief
overstress load. I am not sure of the limits on older designs, but would expect
there to be less margin of strength.

As I understand it the modern thin section wings are flexible enough that the
load limit is imposed by control freedom limitation, and the wing must withstand
1.725 times this load in test. Flutter is the subject of speed limitation which
give speeds and margins that the designer/manufacturer must demonstrate flying
to. The regulations imply that the glider must be demonstrated safe at a minimum
of 23% margin above the placarded Vne. So your margins for flutter, versus
ultimate strength are 1.23 vs 1.725 in JAR22 (unless I got the math wrong)

Bruce Greeff wrote:

As I understand it the modern thin section wings are flexible enough
that the load limit is imposed by control freedom limitation, and the
wing must withstand 1.725 times this load in test. Flutter is the
subject of speed limitation which give speeds and margins that the
designer/manufacturer must demonstrate flying to. The regulations imply
that the glider must be demonstrated safe at a minimum of 23% margin
above the placarded Vne. So your margins for flutter, versus ultimate
strength are 1.23 vs 1.725 in JAR22 (unless I got the math wrong)

It's perhaps mathematically true but your argument is wrong (if your
conclusion is to say that there is more risk of flutter than
overloading). You cannot compare pourcentages of load and speed !

It takes less tenth of second at any moment to take the 2 or 3 g's that
will exceed your (supposed) 72.5% load margin, whereas it will take
several seconds to take the 60 or 65 km/h of margin in speed (supposing
23% margin), or depending of the dive angle you might never get over the
speed margin...

And although it may be true that some parts of the wing (e.w. center
section) has more stress margin due to deflection limit, it does *not*
guarantee you that all the parts of the wing has the same extra margin:
in the Nimbus 4 accident the central wing did not break, but the outer
wing did, with fatal consequences :-(


--
Denis

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

At 19:18 30 March 2004, Bruce Greeff wrote:


When I asked Schempp-hirth about the possibility of
flutter damage in an
incident where a Std Cirrus had made a loud chattering
noise on a high speed
pass, they replied that it would be unlikely to have
been flutter. This because
they did not think it likely that the aircraft would
remain controllable due to
control system damage in the event of flutter.

and this is really the crux of the whole thing. Once
flutter starts there may be a complete loss of control;
end ex.
Probably one of the best demonstrations of flutter
I have ever seen was the video of a suspension bridge
breaking up in high wind. Once started complete destruction
is guaranteed unless the cause can be removed (speed
reduced), In the case of the bridge the wind speed
did not reduce, it not being controlled. If loss of
control of the glider occurs, same result, lots of
little bits.

The other point I neglected to mention earlier was
that any margin on the VNE is established on an airframe
where the control hinges are a good fit and all the
control rods have no slack.

Having looked at and lifted the bits of my ASW17 I
think I am happy that there is a reasonable margin
on the loading placard.