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Avoiding Vne



 
 
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  #1  
Old March 30th 04, 12:50 PM
Don Johnstone
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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.



  #2  
Old March 30th 04, 02:01 PM
W.J. \(Bill\) Dean \(U.K.\).
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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.




  #3  
Old March 30th 04, 04:34 PM
Bert Willing
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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



  #4  
Old March 30th 04, 07:10 PM
Eric Greenwell
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Default

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.

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

Eric Greenwell
Washington State
USA

  #5  
Old March 31st 04, 07:34 PM
Denis
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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 ?
  #6  
Old April 1st 04, 08:25 AM
Bert Willing
external usenet poster
 
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Default

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 ?



  #7  
Old March 30th 04, 08:00 PM
Bruce Greeff
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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.
  #8  
Old March 31st 04, 07:48 PM
Denis
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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 ?
  #9  
Old March 31st 04, 09:18 PM
Bob Kuykendall
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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
  #10  
Old March 31st 04, 10:59 PM
Eric Greenwell
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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.


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

Eric Greenwell
Washington State
USA

 




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