Avoiding Vne

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.

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.

On Sun, 28 Mar 2004 20:30:57 +0200, Denis
wrote:

Edward Downham wrote:

It is important to note that this margin is there to cope with things such as
ASI position and calibration errors. When you fly a glider at an _indicated_
speed of Vne, you might actually be nearer Vd and the realm of the test pilot.

No. VNE is an indicated airspeed limit (IAS). If there is a airspeed
calibration error, VNE has been reduced to correct it. The margin is
there for instrumental errors, and *pilot* errors.

Well, this one is just a little scary. For more than one glider of my
acquaintance Vne is given as a TAS in the manual. This can be
converted to IAS of course, but the IAS would, of course, decrease
with altitude.

Vstall, on the other hand, seems just about always to be given as an
IAS, and as an IAS the stall speed will remain about the same as
altitude increases. This is all pretty basic stuff I know, so I
probably misinterpreted your statement about Vne.

One way to look at the "coffin corner" situation where some very
high-flying aircraft, such as the U-2, I suppose, can end up at an
altitude where the stall speed, in TAS, has come very close to the
Vne, as a TAS.

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

Jim wrote in message

One way to look at the "coffin corner" situation where some very
high-flying aircraft, such as the U-2, I suppose, can end up at an
altitude where the stall speed, in TAS, has come very close to the
Vne, as a TAS.

More precisely, I think it's when the stall speed (IAS in the cockpit)
but really a TAS issue) approaches the limiting Mach number. Since
Mach vs TAS decreases with altitude, and TAS vs IAS increases with
altitude, the problem is inevitable for the right (or wrong, depending
on your point of view!) kind of plane.

Kirk

Well. I have no experience fortunately from such situations. Statements that I have made are almost
directly taken from the book "Glider aerobatics".

About lowering the wheel: I don't know how much does it help, but if you try to fly around with the
wheel out, the performance loss is there and one can feel it. Afaik in fitting the fixed wheel
glider with retractable-one gives some 1-2 L/D points better performance.
I agree with you regarding the problem of switching hands. However, I think that in most gliders at
high g, just disarresting the wheel will lower it. One don't have to fix it.



"Eric Greenwell" wrote in message
...
iPilot wrote:
You may be correct. However, I have heard of some cases where there
have been structural failures resulting from excessive g-forces, but
the glider has remained landable after that. Aerobatics books also
recommend to "pull as hard as necessary", but to keep an eye on your
g-meter and ASI.

Aerobatic aircraft are built to much higher G and Vne limits, and the
pilots flying them are also accustomed to using high Gs. How many
sailplane pilots have a G meter and will be looking at it in a spin
recovery gone wrong?

High AoA eats energy rather fast. Flutter from
overspeeding will definately disintegrate your wing.

It is this claim that we are discussing. I know of many people that have
experienced flutter and not lost a wing. I also know flutter can have
many different modes, including those that involve the rudder, elevator,
ailerons, wing, and tail boom. It seems most dogmatic to claim "Flutter
from overspeeding will definitely disintegrate your wing". Since you are
so certain, perhaps you could tell us the reasons for your certainty?

Excessive-g may
not.

Has flutter caused any of the in-flight breakups discussed here? Perhaps
I missed them. I thought all were due to exceeding G limits, some likely
due to the reduced G loadings with the spoilers extended.

The trick is to stay within the limits as long as possible.
Therefore it's necessary to know the limits and their use. In
abovementioned Nimbus 4 incident it seems that pilot exchausted
g-limits before the Vne arrived. But no-one knows what really
happened.

BTW, there is one energy-burning device on every glider that may save
you the very necessary second or two - the wheel. Lower it as soon as
you feel the threat of overspeeding. You may lose the wheel doors
because of the speed and g-forces, but this is not nearly as
catastrophical as losing wing because of pulling the airbrakes at Vne
and high-g.

Does anyone have an idea of how much effect this would have? If it is
small, perhaps a pilot should not even think of attempting it as he
struggles with a recovery near Vne. In many gliders, it requires the
pilot to switch hands on the stick to lower the gear, and would be a
distraction at critical time in all of them.


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Eric Greenwell
Washington State
USA

Jim wrote:

No. VNE is an indicated airspeed limit (IAS). If there is a airspeed
calibration error, VNE has been reduced to correct it. The margin is
there for instrumental errors, and *pilot* errors.


Well, this one is just a little scary. For more than one glider of my
acquaintance Vne is given as a TAS in the manual. This can be
converted to IAS of course, but the IAS would, of course, decrease
with altitude.

I did not want to raise the question of VNE at altitude (a more
difficult subject ;-) and the relation between IAS and TAS.

I replied to someone who mentioned the errors of static ports : The
calibrated airspeed (CAS) may be higher than the indicated airspeed
(IAS) depending og static ports location ; in that case, it is measured
during the tests and all limits are corrected to be given in *indicated*
airspeed.

--
Denis

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

Jack wrote:

I never experienced a spin recovery and...[t]herefore I don't know what I

would do in such a situation.

With your (claimed) thousands of hours of flight experience?

Please quote correctly. I never experienced a spin recovery *in a Nimbus 4*


--
Denis

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

W.J. (Bill) Dean (U.K.). wrote:

You are just plain wrong.

Who are you answering to ? What are you speaking about ?

Please reply after what you quote and not before.

You also say:
"all I want is to give my opinion when I think something is said here that
may lead to dangerous flying - such as sentences like "don't exceed VNE, but
no problem if you exceed permitted G-loading" ".
Who said that, which posting?

You in :

" pull however hard is necessary not to exceed VNE,"


Exceeding Vne is outside limits and dangerous, so are any of the
alternatives - the discussion is about which of the alternatives is the
least worst.

No. Pulling airbrakes at or below VNE is safe and permitted, if you
respect the G limits. The other two (exceeding VNE or exceeding
g-limits) are unsafe and prohibited. I really don't understand why you
(and not you alone, unfortunately) cannot understand that.


With the Minden accident on 13th July 1999, it is clear from the report that
the glider was pitched down to well beyond a 45 degree dive, so the
airbrakes would not have been speed limiting.

Of course not. But it would have considerably limited the speed increase
in the few seconds needed to get at or below 45° dive.

You say "I never experienced a spin recovery", presumably you mean in a
large span glider. I hope you have done plenty in training and short span
machines.

Yes. Including some with a VNE at 550 kt...

Denis (Denis who and from where?),

Does it really import my family name or where I am from ? You'd better
try to find more convincing arguments.

Anyway the answer to your questions is in my headers or any search engine.

--
Denis

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

I'd recommend reading about the asymmetric loading arguments
presented at:

http://www.avweb.com/news/columns/182086-1.html

The meat of the matter starts about halfway down
the page. Essentially it says full aileron and full elevator
when applied simultaneously create a much greater chance of wing
failure at a lower airspeed than just applying either one independently.

I'm not aware of how this may or may not apply to
gliders. Perhaps some expert in wing construction
can give most welcome educated opinion...

F.L. Whiteley wrote:

"Mark James Boyd" wrote in message
news:4064994c$1@darkstar...
K.P. Termaat wrote:
Yesterday evening I talked with a friend about avoiding excessive speed
when
recovering from a spin in a modern low drag glider with the somewhat
larger
span.

A lot has been written here about G loads. I recall that the
T-34 (an aerobatic power plane I have a little time in
which is sortof a tandem Beech 33) had some issues with wings
coming off during aerobatics. The recorded G loads and mauevers
indicated the aircraft wasn't flown outside of G limits.

How did the wings separate? Some smarty folks said it was
because the twisting G load that the wing could endure was
much less than the static tested load. If the ailerons were
deflected and the thing was in a steep spiral (as opposed
to straight dive) there were twisting loads.

Thought I heard that inspections showed T-34 wings were suffering from
fatigue cracks. Kind of shot down some of the 'fighter dude' thrill rides
(we have/had one in Colorado). We have a disassembled T-34 wrapped in
plastic in our hangar. I recall a conversation about the value dropping by
about 50% when the crack problem was discovered.
--

------------+
Mark Boyd
Avenal, California, USA