Avoiding Vne

Eric Greenwell wrote:

I believe that with the airbrakes open your safe positive
G-limit reduces to +2.5G.

For my ASH 26 E at Vne: 4 G load factor (down from 5.3 at Va); 3.5 G
load factor airbrakes extended. It looks like the G limits at Vne are
fairly close together.

Airbrakes out at Va it's 3.5 g instead of 5.3, thus the difference is
significant. I don't know why there is almost no difference at VNE, butI
suspect that in that case the margin is higher without airbrakes than with.

I am curious about why they decrease.

There is a loss of lift at the airbrakes, thus for the same G the outer
wing is more loaded, and the bending momentat wing root increases.

Perhaps the loss of lift is lesser at high speeds (lower angle of attack) ?

--
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 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.

W.J. (Bill) Dean (U.K.). wrote:
glider would be unrecoverable at some speed below 200 knots, because the
pitch down force from wing twist would overcome the pitch up force from full
up elevator, came from Schleichers probably from Waible.

This accident is a perfect illustration of how the pilot is unlikely to know
what is critical if limits are exceeded. The ASW20C did not flutter, and
did not break up in flight; the first failure was loss of control.

Was there any discussion at all about the possibility to attempt
controlling it with the flaps ?

Not that it is likely to help, possibly just the opposite, since
you'd be even more in exess of the (lower) Vne for flap positions
other than fully negative.

But if it is really out of control one would imagine trying anything,
if only as a last-ditch attempt to stabilise things a little and be
able to bail out.

Bailing out at 200 kts can't be exactly easy either.
CV

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

With some gliders it is both important and difficult to get the recovery
exactly right.

Wasn't there a thread in r.a.s not long ago in which most American
pilots considered spin training superfluous and dangerous? Hmmm...

Stefan

(Yes, I've realilzed you're British.)

"Stefan" wrote in message
...
W.J. (Bill) Dean (U.K.). wrote:

With some gliders it is both important and difficult to get the recovery
exactly right.

Wasn't there a thread in r.a.s not long ago in which most American
pilots considered spin training superfluous and dangerous? Hmmm...

Stefan

(Yes, I've realilzed you're British.)


I don't think that "most American pilots" have ever bothered to express
their opinions with respect to spin training, or any other subject for that
matter, in r.a.s. Perhaps you should have said that some pilots, some of
whom were American, expressed a concern that spin training in designs with
questionable spin related accident records should be reconsidered.
Hmmmm......

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

With some gliders it is both important and difficult to get the recovery
exactly right.


Wasn't there a thread in r.a.s not long ago in which most American
pilots considered spin training superfluous and dangerous? Hmmm...

No. Pehaps you are thinking of some other newsgroup.

--
-----
change "netto" to "net" to email me directly

Eric Greenwell
Washington State
USA

Denis wrote:

For my ASH 26 E at Vne: 4 G load factor (down from 5.3 at Va); 3.5 G
load factor airbrakes extended. It looks like the G limits at Vne are
fairly close together.


Airbrakes out at Va it's 3.5 g instead of 5.3, thus the difference is
significant. I don't know why there is almost no difference at VNE, butI
suspect that in that case the margin is higher without airbrakes than with.

I am curious about why they decrease.

There is a loss of lift at the airbrakes, thus for the same G the outer
wing is more loaded, and the bending momentat wing root increases.

I should have said "I'm curious about why the G loading (airbrakes
closed) is lower at Vne than at Va". Perhaps it is to absorb gusts, or
to allow greater control deflections.

--
-----
change "netto" to "net" to email me directly

Eric Greenwell
Washington State
USA

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............

On Mon, 29 Mar 2004 00:26:29 +0200, CV wrote:

Was there any discussion at all about the possibility to attempt
controlling it with the flaps ?

Not that it is likely to help, possibly just the opposite, since
you'd be even more in exess of the (lower) Vne for flap positions
other than fully negative.

Bailing out at 200 kts can't be exactly easy either.

Could anyone please tell me how one can get an ASW-20 to 200 kts
without noticing it? I've got a couple of hours in the 20, but I have
not found any situation where this could have been possible.



Bye
Andreas

Sorry,

when I made my comments earlier I was assuming that
people were discussing exceeding the placarded g-limits,
not the certified ultimate limits. I believe 6-7g would
result in damage but perhaps not failure (depending
on the margins), but 10g, 15g!!!, how the hell do you
get yourself in a situation where you have to pull
that hard?
Personally I think it should be unnecessary to exceed
5g in even the worst spin recovery......unless you
enter a spiral dive and do not stop the rotation, in
which case all the arguments are irrelevant. If you
are in a spiral dive and do not stop the rotation then
you will exceed both the ultimate g-limits AND vne.
Assuming you are a reasonable pilot who can recognise
a spiral dive and recover promptly, then opening the
airbrakes while pulling the 5g or so which may be necessary
to recover below vne will simply result in damage to
the wings, where not opening them would not.
I think most of the structural failures resulting from
poor spin recovery must have been spiral dives.
Again, proper pilot training should ensure that this
never happens.