Avoiding Vne

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

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

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 ?

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 ?