Avoiding Vne

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.

At 22:18 28 March 2004, Andy Blackburn wrote:
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).



I think your definetly right Andy. My Ventus is in
the shop now replacing worn bushings in the aileron
circuits. Last year when getting very close to VNE
on final glide (you know me always coming in high
I had to hold a little right aileron into it so the
aileron would not 'start vibrating' which I interept
as incipent to flutter. Side note is I thought it was
only the bushings at the connect point on the aileron,
turns out I had wear on both ends and the 'slide' that
the tube rides on had worn half way through the tube'
Glad I had Bill check them out.

Cliff

At 22:18 28 March 2004, Andy Blackburn wrote:
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?


I think your definetly right Andy. My Ventus is in
the shop now replacing worn bushings in the aileron
circuits. Last year when getting very close to VNE
on final glide (you know me always coming in high
I had to hold a little right aileron into it so the
aileron would not 'start vibrating' which I interept
as incipent to flutter. Side note is I thought it was
only the bushings at the connect point on the aileron,
turns out I had wear on both ends and the 'slide' that
the tube rides on had worn half way through the tube'
Glad I had Bill check them out.

Cliff

At 18:54 29 March 2004, Denis wrote:
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...

probably why we use k13's and puch's then and if you
are going to examine spin characteristics of big floppy
wings, you do it for spin entry and not any further.
but then again, you shouldnt be flying at a speed where
you are going to end up in a spin in the first place,
let alone suddenly realising that you've let your speed
build up to Vne! which is the whole point of spin training........



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).


er.. didnt i say that? 5 secs....

And it is very
dependant of the condition and trainig of the pilot.

and seating position, and how much height exposure
he's had during the days flying........... (oxy saturation)

but then a well trained pilot wouldnt have got himself
into the position of entering a spin, let alone contemplating
what to do because hes suddenly at all the airframe
limits!


But a glider wing breaks within milliseconds of overstressing
!

no they dont, they have to fail progressivly. no glider
would be alowed in the air by any regulating body if
a wing could break in a 1000th of a second. i dont
think you really understand the force that would be
needed to do that. the instantanious g-load to do that
would kill you out right.



--
Denis

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

Pete Zeugma wrote:

But a glider wing breaks within milliseconds of overstressing
!

no they dont, they have to fail progressivly. no glider
would be alowed in the air by any regulating body if
a wing could break in a 1000th of a second. i dont
think you really understand the force that would be
needed to do that. the instantanious g-load to do that
would kill you out right.

Another dangerous misconception :-(((

If the actual extreme load is 6 g, the glider will loose wings as soon
as G-loading is greater than 6 g. Not 5 seconds later, not 0.5 second
later, but immediately. And not "progressively" (can you loose wings
progressively ???)

And in a modern ship near VNE, a small stick input may bring you from 1
to 6 G in less than 1 second.

Hence there is no chance at all that black-out or grey-out warn the
pilot of a too high G-load in an emergency situation.

You may have plenty of spin training on an ASK 13, it's better than
none, it will not prevent you to getting in a dangerous situation with
an open class glider. Spin training in these is prohibited, but high
speed flying is not : everybody should train to fly at higher and higher
speeds up to VNE in order to get used to the very high sensitivity of
most modern gliders at high speeds and to master the technique to use
only very small control inputs (including putting their hand firmly on
their leg to prevent unwanted or G-induced stick movements)

--
Denis

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

If the actual extreme load is 6 g, the glider will loose wings as soon
as G-loading is greater than 6 g. Not 5 seconds later, not 0.5 second
later, but immediately. And not "progressively" (can you loose wings
progressively ???)

Yes - on some gliders anyway. The Schweizer 1-34 is designed so that the
*first* failure occurs about 2/3 out from the root at about 8 Gs.

Tony V
http://home.comcast.net/~verhulst/SOARING

That's correct. If you kill the lift over part of the
wing by opening the spoilers, the rest of the wing
needs to produce a higher coefficient of lift to keep
the glider from accelerating downward. This can only
happen through an increase in angle of attack. This
will cause the wing to flex more because the center
of lift has moved outboard (imagine lifting your glider
on the ground by the wingtips versus the wing roots
to appreciate the point).

If you don't increase the angle of attack to compensate
then liftweight and the glider will accelerate downward
(accelerating sink rate). In this case the outer panels
are producing no more lift (same aoa, same airspeed)
and the inner portion of the wing is spoiled so net-net
there is less lift, less bending moment and less flex
in the wings. However, this is a transient state since
we all naturally compensate for the loss of lifting
surface by pulling back ever so slightly on the stick
and loading up the tips - hence the extra flex.

It's hard to notice the aoa shift in flight because
you need a lower pitch attitude to maintain airspeed
(due to higher drag) at the same time that aoa goes
up to produce enough lift to hold the glider up. Remember
pitch attitude and aoa are not the same.

Try going from dive brakes closed and locked to fully
open on landing rollout - AFTER you're sure you have
the tailwheel down. In this case aoa should be held
pretty constant. See what the wings do then.

9B


At 07:54 30 March 2004, Bert Willing wrote:
You're both partially right. The moment you deploy
the airbrakes and
maintain a constant angle of attack, you loose lift
which means the glider
accelerates it's sink rate. G load decreases, bending
decreases. Once the
sink rate is stabilized, the initial lift must have
been restored but it's
different distribution along the wing span increases
the bending.

BTW, Denis is fairly well know in France (an Morocco...)

--
Bert Willing

ASW20 'TW'


'W.J. (Bill) Dean (U.K.).' a écrit dans le message
de ...
You are just plain wrong. The immediate effect of
opening Schempp-Hirth
type airbrakes, if nothing else is done, is to make
the wings bend more.

Have you ever tried opening the airbrakes with a bendy
wing and watched
what
actually happens?

When I flew an ASW20L I always used some landing flap
when approaching to
land. Sometimes I would keep the brakes closed until
near the ground.
I
always opened them as I rounded out, because as soon
as I did this the
wings
bent up, to give me better ground clearance!

I remember flying a Skylark 3 at about 75 knots (fast
for the type), at
this
speed the tips bent down a bit, because of the washout.
If I then opened
the airbrakes, the wings bent up.

Your theory is wrong, it does not work! Don't try
to argue that I did
not
see what I know I did see, get in something with bendy
wings such as a
Pegasus, and try it.

I also remember seeing an article in 'Technical Soaring'
with a photo of a
Jantar 1 at Vne, and at 1 g., with the brakes fully
out. The wing bend,
at
1 g. remember, was horrendous. Don't try and give
us some theoretical
reason why this cannot happen, it does!

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?

This whole discussion has been around the point, if
you look as if you are
going to exceed Vne, what should you do?

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.

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.

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. An essential part of stall/spin recovery
training is to be
able
to distinguish at once the difference between a spin
and a spiral dive.
If
you treat a spiral dive as if it is still a spin,
this is very likely to
lead to excessive speed, as well as using more height
for the recovery.

I still think that the advice I gave in my first posting
to this thread is
correct:
'If you exceed Vne you are taking a risk, if you pull
too hard above
manoeuvring speed you are taking a risk, and if you
pull hard and roll at
the same time you are taking a risk. If you pull
the brakes you are
increasing the bending load on the wings.

'If you get it wrong and have to take one of the risks,
I am told that you
should centralise the ailerons, then pull however
hard is necessary not to
exceed Vne, and make sure the brakes stay shut.'

Denis (Denis who and from where?), if you still feel
like answering,
please
answer what I have actually written.

W.J. (Bill) Dean (U.K.).
Remove 'ic' to reply.


'Denis' wrote in message
...


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

There were postings to Rec. Aviation Soaring when
the report was
published, from pilots with experience of the Nimbus
4 and similar
models who had experience of inadvertent deployment
of the airbrakes.
If the brakes deployed inadvertently while the pilots
were recovering
from the dive, this surely may have been the reason
for the amount of
bending seen; and for the overload which led to
failure. Presumably
those investigating the accident were not aware
of these incidents
when
writing the report.


If airbrakes deploy inadvertently, the first effect
(along with the very
high drag) will be a *decrease* in G-loading *and*
bending moment), both
due to the loss of lift near the airbrakes. The increase
of bending
would happen only after the angle of attack has been
further increased
(voluntarily or not) to restore the initial G-loading
with more lift on
the outer panels (instead of the airbrakes section),
hence the higher
bending.


Denis, you are very scathing.


That is not my intention... 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'.


What do you think went wrong?

What would you have done?

Do you have any experience in the Nimbus 3 & 4 series?
I don't.

Are you more experienced or better than the pilots
who did not make
it?


I don't know them and I would not pretend to be better
(there are no
good pilots, only old pilots...). And although I
have some experience in
Nimbus 4D (more on ASH 25) I never experienced a
spin recovery and I
hope I never will have to. Therefore I don't know
what I would do in
such a situation. All I can say is what I think (sitting
comfortably in
my chair) is the better thing to do, as I said in
a previous post :

'If your speed is going to exceed VNE within this
manoeuvre [pulling
up], you should stop or reduce pulling and apply
full airbrakes. At any
dive angle up to 45° this prevents the glider to
exceeding VNE, and you
have time to recover pulling gently (under 2 g's).
This of course
supposes that there is sufficient ground clearance...
'

Denis

When I made my original post on this subject my contention
was quite simple, if faced with the choice of exceeding
VNE or pulling to avoid exceeding VNE and overstressing
the glider I would choose the latter. I would hope
that I would never get to the situation where I had
to do either and if I keep my wits about me I never
will. I stand by what I said but this was not an original
thought, it was the advice of someone who knows a great
deal more than me.

An earlier posting said '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'. Of course it is possible but
I accept it is purely a matter of luck. I never made
the above statement, I did say that overstress may
cause serious damage, but flutter is much more likely
to be catastrophic.

With some gliders there is such a large margin between
placarded limits and the forces the airframe will withstand
that overstressing is definitely the lesser of 2 evils.
The Grob Acro is a perfect example of this. One of
the Acros delivered to the RAF in the UK in the 80's
was given to Slingsbys to test on a rig. After the
'normal' testing (The wing spigot problem was discovered
in this test and I was told that had this failed in
flight the airframe would probably have stayed in one
piece). Following this attempts were then made to break
the glider but despite every effort the only thing
that broke was the test rig, the glider never did.
There is no such margin for the onset of flutter. Not
all gliders have the strength of the Grob I would agreed
but there is still a margin of some sort.

The way the whole thing was explained to me was that
pulling excess G may break the glider, in particular
it may cause damage to the wing/fuselage fixing but
this damage is not necessarily total (See above re
wing spigot problem). The damage caused by flutter
is much more likely to cause total failure not only
of the wing but other aerodynamic surfaces as well
(the tailplane and fin) and a glider without a tailplane/fin
is not where I would want to be. I am told that the
weakest point on any glider is the fuselage just in
front of the fin. Again I am told that when an aircraft
breaks up in flight the cause is almost invariably
flutter, the result of overspeed, whether this is preceeded
by overstress causing loss of control or not.

There has been a lot of use of words such as rubbish
and other derogatory terms. You do not have to agree
with me but I would suggest that perhaps you might
be better researching and then posting a cogent argument
why I am wrong instead of just gainsaying. (Remember
the Python :-) UK joke)

I stand by my original post, faced with the choice
of exceeding VNE or pulling too much G I would chose
the latter as the lesser of two very great evils.

Don, I agree with you completely. You have made the points which I have
been trying to put.

W.J. (Bill) Dean (U.K.).
Remove "ic" to reply.


"Don Johnstone" wrote in
message ...

When I made my original post on this subject my contention
was quite simple, if faced with the choice of exceeding
VNE or pulling to avoid exceeding VNE and overstressing
the glider I would choose the latter. I would hope
that I would never get to the situation where I had
to do either and if I keep my wits about me I never
will. I stand by what I said but this was not an original
thought, it was the advice of someone who knows a great
deal more than me.

An earlier posting said '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'. Of course it is possible but
I accept it is purely a matter of luck. I never made
the above statement, I did say that overstress may
cause serious damage, but flutter is much more likely
to be catastrophic.

With some gliders there is such a large margin between
placarded limits and the forces the airframe will withstand
that overstressing is definitely the lesser of 2 evils.
The Grob Acro is a perfect example of this. One of
the Acros delivered to the RAF in the UK in the 80's
was given to Slingsbys to test on a rig. After the
'normal' testing (The wing spigot problem was discovered
in this test and I was told that had this failed in
flight the airframe would probably have stayed in one
piece). Following this attempts were then made to break
the glider but despite every effort the only thing
that broke was the test rig, the glider never did.
There is no such margin for the onset of flutter. Not
all gliders have the strength of the Grob I would agreed
but there is still a margin of some sort.

The way the whole thing was explained to me was that
pulling excess G may break the glider, in particular
it may cause damage to the wing/fuselage fixing but
this damage is not necessarily total (See above re
wing spigot problem). The damage caused by flutter
is much more likely to cause total failure not only
of the wing but other aerodynamic surfaces as well
(the tailplane and fin) and a glider without a tailplane/fin
is not where I would want to be. I am told that the
weakest point on any glider is the fuselage just in
front of the fin. Again I am told that when an aircraft
breaks up in flight the cause is almost invariably
flutter, the result of overspeed, whether this is preceded
by overstress causing loss of control or not.

There has been a lot of use of words such as rubbish
and other derogatory terms. You do not have to agree
with me but I would suggest that perhaps you might
be better researching and then posting a cogent argument
why I am wrong instead of just gainsaying. (Remember
the Python :-) UK joke).

I stand by my original post, faced with the choice
of exceeding VNE or pulling too much G I would chose
the latter as the lesser of two very great evils.

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.

But pulling the airbrakes would be fairly suicidal.

--
Bert Willing

ASW20 "TW"


"W.J. (Bill) Dean (U.K.)." a écrit dans le message
de ...
Don, I agree with you completely. You have made the points which I have
been trying to put.

W.J. (Bill) Dean (U.K.).
Remove "ic" to reply.


"Don Johnstone" wrote in
message ...

snip
The way the whole thing was explained to me was that
pulling excess G may break the glider, in particular
it may cause damage to the wing/fuselage fixing but
this damage is not necessarily total