Avoiding Vne

Andy Blackburn wrote:

I'm sure everyone agrees the best advice is not to
get into a situation where you have to choose between
Vne and the G-limit. Thinking ahead with respect to
attitude and configuration as you initiate recovery
is your best bet.

(...)

if you get to this point you are in a world of hurt
anyway so the amount of over-G versus over-Vne is subject
to your personal risk profile.

I agree with all Andy said. I would add that "pulling as hard as
required to avoid VNE" is easier to say that to do, because :

- it is impossible, if you are not an experimented glider aerobatics
pilot, to know how many g's you need to avoid exceeding VNE,

- depending on dive angle and speed, it may be just impossible to avoid
VNE without airbrakes, even if pulling 15 g's (supposing the wings have
not briken before)

- it is impossible without a g-meter to know if you pull 5 g (or just a
little more, comprised in the "safety margin"), or 10 g's or more.
Especially at high speeds, because a very small stick input may result
in high g's, or pilot induced oscillation, etc.

And, last but not least, if you have been above VNE and lucky enough not
to have encountered flutter, you are lucky andy the glider is still safe.

If you have pulled too many g's and the wings have not broken, you are
lucky but the *glider structure may have been damaged* and you, or
another pilot, may encounter a catastrophic failure in a subsequent
flight within the certificated flight enveloppe !

--
Denis

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

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.

Now I ain't no aerodinymakist. I took a plastic spoon and
tried to bend it in half. No luck. Took the same
spoon and twisted it and bent it in half. Bingo. What
does this mean? Don't make wings out of plastic spoons...

Anyway, I also recall the Sprint Ultralight had some airframe
separations, and before I flew it the first time, the
owner (who was also an A&P) told me he'd installed additional
bracing wires to reduce wing twisting.

Come to find out, there are more than a few aerodyne designs
which use drag and anti-drag wires (maybe the wrong technical
name but yo get the idea) inside and outside the wing to help
prevent twisting.

I tell ya, I'm personally a big fan of wings-level
dives vs. spirals. I'd go 1.2 x Vne in a dive before I'd go
1.0 x Vne in a steep spiral. Dunno if this is right,
but that's what my instinct tells me...
--

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

Now if you used carbon rods for the spars one could pull about 15 G's and
not suffer a failure . In our designs we only design to the deflection
desired.... and the resulting G loading is always well beyond the required
specs.

Just a thought! -mat

"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.

Frank Whiteley

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

That's what I recall causing the dive in values. The quick fix was to
tighten the speed envelopes I thought. But that was 1999.

This is now.

One of our CFI-G/Tow pilots starts his Navy BPT next month. He said all
T-34B's(? or civilian use?) were grounded a couple of weeks ago and there
will be spar mods required. Don't know how the A differs from the B, but
the Navy trainers are still flying for now. He's looking forward to flying
with a turbo. He thinks his winch launching experiences have him ready for
carrier ops:^)

Frank

"Mark James Boyd" wrote in message
news:406b0c5e$1@darkstar...
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

On Thu, 25 Mar 2004 08:53:00 GMT, "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.
He came up with the idea of pulling the airbrakes when still recovering from
the rotating mode. I am not sure this can be done without disturbing the
recovering action or without hurting the glider.
Any comment will appreciated.

What has not been discussed so far in this thread is the acceleration
of the glider in a steep dive:

If you extend the airbrakes (far) below Vne, you have a lot more time
to pull out of the dive until your airspeed reaches Vne because the
acceleration of the glider is a lot slower.

Typical case: You extend your airbrakes once you exceed a certain
speed (for example Va or slightly higher).

Glider airbrakes are typically designed to keep the glider under Vne
at a dive angle of more or less 30 degrees, so if your nose-down
attitude is less than 30 degrees, the glider will decelerate while you
are still pulling out of the dive - this means that once you have
reached 30 degrees nose-down attitude, you are already safe and can
take all the time in the world to pull out the last 30 degrees till
level flight (...if you have enough height, of course) without
worrying about exceeding Vne.

You are probably going to loose more height during the recovery
(because of the "soft" pullout) this way, but the g-load will be kept
realtively low.



With retracted airbrakes the glider will accelerate quickly, therefore
you are going to need to pull higher g-load to get out of the dive
before you exceed Vne - and the glider will accelerate all the time
until it is nearly in level flight. And as others have already pointed
out in this thread, extending the airbrakes close to (or over) Vne at
high g-load is probably going to ruin your day...



Bye
Andreas

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.

At 17:42 28 March 2004, Denis wrote:
Andy Blackburn wrote:

I'm sure everyone agrees the best advice is not to
get into a situation where you have to choose between
Vne and the G-limit. Thinking ahead with respect
to
attitude and configuration as you initiate recovery
is your best bet.

(...)

if you get to this point you are in a world of hurt
anyway so the amount of over-G versus over-Vne is
subject
to your personal risk profile.

cant remember ever having to pull more that 5g in the
odd incipient spin in the ls6, but then you have also
the additional problem in your spin recovery of having
to dump the flaps into full reflex.


I agree with all Andy said. I would add that 'pulling
as hard as
required to avoid VNE' is easier to say that to do,
because :

- it is impossible, if you are not an experimented
glider aerobatics
pilot, to know how many g's you need to avoid exceeding
VNE,

you pull as hard as you need, and its not impossible
to know either, thats why you should practice spin
recovery.

- depending on dive angle and speed, it may be just
impossible to avoid
VNE without airbrakes,
even if pulling 15 g's (supposing the wings have
not briken before)

and presuming you have not blacked out by then also,
i start to grey out at about 7g from more than 5 secs
exposure at that level.


- it is impossible without a g-meter to know if you
pull 5 g (or just a
little more, comprised in the 'safety margin'), or
10 g's or more.

you would probably have blacked out about 9g




as seems to be typical here, not much mention of prevention,
or the possible reason for why you got yourself into
a position where you are approaching Vne. When it says
'ease the stick forward until the glider unstalls'
that does not mean push it to the forward stop and
dive out of the spin............

Pete Zeugma wrote:

you pull as hard as you need, and its not impossible
to know either, thats why you should practice spin
recovery.

AFAIK spin training is prohibited in most, if not all, open class ships...

and presuming you have not blacked out by then also,
i start to grey out at about 7g from more than 5 secs
exposure at that level.

you would probably have blacked out about 9g

Yes, except grey out or black out happens only after a few seconds (the
time needed for the blood to fall into your lower boby). And it is very
dependant of the condition and trainig of the pilot.

But a glider wing breaks within milliseconds of overstressing !


--
Denis

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