Avoiding Vne

Jim wrote:

No. VNE is an indicated airspeed limit (IAS). If there is a airspeed
calibration error, VNE has been reduced to correct it. The margin is
there for instrumental errors, and *pilot* errors.


Well, this one is just a little scary. For more than one glider of my
acquaintance Vne is given as a TAS in the manual. This can be
converted to IAS of course, but the IAS would, of course, decrease
with altitude.

I did not want to raise the question of VNE at altitude (a more
difficult subject ;-) and the relation between IAS and TAS.

I replied to someone who mentioned the errors of static ports : The
calibrated airspeed (CAS) may be higher than the indicated airspeed
(IAS) depending og static ports location ; in that case, it is measured
during the tests and all limits are corrected to be given in *indicated*
airspeed.

--
Denis

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

That is not the issue Jean. I am talking about pulling the airbrakes before
the rotation of the glider has stopped. This not in the manual of course.
The idea is to avoid a high speed with the glider at a pitch angle of
something like 60° directly after it has stopped rotation.
Testing what happens when I do the whole thing with my new rather heavy low
drag Ventus-2cxT is an invitation to others to call me an "idiot". Probably
I will do that myself too.
For me it is more like an "if then" case. While instructing I have done tens
of spins with students with a ASK-13, but that's easy of course and does
hardly apply to what can happen to modern gliders.

Thanks

Karel, NL

"Jean" schreef in bericht
...
Why don't you check your glider's flight manual ?
Jean
"K.P. Termaat" a écrit dans le message de
...
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.

Karel, NL

I agree with Ed in most respects, and I think you need
to get a few facts right Denis.

Have you any reference for this affirmation ? THis
is a know issue on earlier delta wings, also in earlier
near-transsonic aircraft, but I don't know any glider
with such a problem.

Yes, as has already been mentioned the ASW20 has a
problem, where exceeding Vd can result in wing twist
that results in a terminal dive even with the stick
fully back. Not to mention what flutter can do anyway.



In most modern gliders you should be able to pull
6g+ without breaking any bits
off them.

Not at VNE !!!

Yes, I think Ed is probably right. Although the glider
will probably be a write-off after you land, because
the material will yield in highly stressed areas. Glider
wings are bloody strong, and are unlikely to snap,
but the reason you dont want to exceed the G-limits
every day is because bits will start to creep, and
your glider's structure will gradually deteriorate
and eventually fail, but as a One-off it is unlikely
to result in failure, particularly as the stalling
angle of the wing will limit the maximum g you can
pull below VNE. It will be pretty high, but is unlikely
to be more than 10g, and you shouldnt need to pull
this hard anyway. To be honest if it was a choice between
flutter, where bits like ailerons and elevator might
come off, and exceeding the G-limits where the wing
might be a write off but the plane will remain flyable
and be safe to land (or at least bail out of), I know
which I would choose.


The drag rise is not enough to avoid overspeeds. The
rise in drag
provided by *airbrakes* (not pulling too hard) is the
*only* way to
avoid either flutter (above VNE) or breaking the wings
(by overloading).

I believe that with the airbrakes open your safe positive
G-limit reduces to +2.5G. This is because you are forcing
most of the lift to be produced near the tip and thereby
increasing the wing bending moment at the root, and
there is also a hell of a shear force produced. God
knows what happens if you open them suddenly above
Va while pulling 3.5g but i suspect it would not be
pretty. (I also suspect that it may be this that caused
several big gliders to have wings come off in spin
recovery)

Ask an aerobatic pilot if you don't want to take my
word for it, but I believe their rule of thumb is that
it is always better to exceed the g-limits than to
exceed VNE, and that you should NEVER open the airbrakes
in an attempt to limit speed, because it has such a
small effect on drag and such a huge effect on the
g-limits.

Hope this in some way helps, though I think I am just
re-iterating what a lot of other people have already
said.

J.

Jon Meyer wrote:
Ask an aerobatic pilot if you don't want to take my
word for it, but I believe their rule of thumb is that
it is always better to exceed the g-limits than to
exceed VNE, and that you should NEVER open the airbrakes
in an attempt to limit speed, because it has such a
small effect on drag and such a huge effect on the
g-limits.

Didn't you mean to say "it is always better to exceed VNE
than to exceed the g-limits"?

Marc

Jon Meyer wrote:

Yes, as has already been mentioned the ASW20 has a
problem, where exceeding Vd can result in wing twist
that results in a terminal dive even with the stick
fully back. Not to mention what flutter can do anyway.

According to Bill Dean "I gather that the investigators
contacted Schleichers about the accident, and the information that the
glider would be unrecoverable at some speed below 200 knots,"

NO mention of Vd (about 160 knots), so it is speculation to say
exceeding Vd can result wing twist. It might be 170, 180, 190,... we
don't know.

In most modern gliders you should be able to pull
6g+ without breaking any bits
off them.

Not at VNE !!!


Yes, I think Ed is probably right. Although the glider
will probably be a write-off after you land, because
the material will yield in highly stressed areas. Glider
wings are bloody strong, and are unlikely to snap,
but the reason you dont want to exceed the G-limits
every day is because bits will start to creep, and
your glider's structure will gradually deteriorate
and eventually fail, but as a One-off it is unlikely
to result in failure, particularly as the stalling
angle of the wing will limit the maximum g you can
pull below VNE. It will be pretty high, but is unlikely
to be more than 10g, and you shouldnt need to pull
this hard anyway.

At Vne, my ASH 26 can pull 2.2 times the flight limits of 5.3 G. And if
you hit a gust at the same time, or have some aileron or rudder input...

To be honest if it was a choice between
flutter, where bits like ailerons and elevator might
come off, and exceeding the G-limits where the wing
might be a write off but the plane will remain flyable
and be safe to land (or at least bail out of), I know
which I would choose.

Who wouldn't choose a plane that was safe to land instead of one where
bits have fallen off? What we are discussing is if the choice is "and
exceeding the G-limits where the wings might break off".


The drag rise is not enough to avoid overspeeds. The
rise in drag
provided by *airbrakes* (not pulling too hard) is the
*only* way to
avoid either flutter (above VNE) or breaking the wings
(by overloading).


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. I am curious about why they decrease.

snip

Ask an aerobatic pilot if you don't want to take my
word for it, but I believe their rule of thumb is that
it is always better to exceed the g-limits than to
exceed VNE, and that you should NEVER open the airbrakes
in an attempt to limit speed, because it has such a
small effect on drag and such a huge effect on the
g-limits.

Are they talking specifically about gliders designed for aerobatic use?
Quite a different bird than what most of us fly, with very high G and
Vne ratings. Perhaps their rule of thumb applies just to them? And, I am
very skeptical of the "small effect on drag" remark, since at Vne, the
air brake drag will be greatest drag of all the sources. Or maybe not,
if you mean gliders designed for aerobatic use.


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

Eric Greenwell
Washington State
USA

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 ?

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

Jon Meyer wrote:

Yes, as has already been mentioned the ASW20 has a
problem, where exceeding Vd can result in wing twist
that results in a terminal dive even with the stick
fully back.

It was not at Vd but at 200 kts ! (if I remember well VNE is 145 kts in
an ASW 20)

Not to mention what flutter can do anyway.

Flutter is the likeliest cause of failure at 200 kt (40% above VNE !!!).

Don't seek any other cause even if someone reported you that he
remembered someone has told him that ;-)

In most modern gliders you should be able to pull
6g+ without breaking any bits
off them.

Not at VNE !!!


Yes, I think Ed is probably right.

See my previous post about the NSTB report that demonstrate that the
Nimbus 4 will break between 5 and 6 g (like I suppose any other
non-aerobatic glider)

angle of the wing will limit the maximum g you can
pull below VNE. It will be pretty high, but is unlikely
to be more than 10g,

again, 15 g at VNE, 20 g at design speed (three time more than the wing
can support without breaking !).

To be honest if it was a choice between
flutter, where bits like ailerons and elevator might
come off, and exceeding the G-limits where the wing
might be a write off but the plane will remain flyable
and be safe to land (or at least bail out of), I know
which I would choose.

Neither choice is good. Your choice belongs to you. But don't let other
pilots think that exceeding G-limits "is not good, but I can do it from
time to time". Please don't play with /their/ lives.

Ask an aerobatic pilot if you don't want to take my
word for it,

Aerobatic gliders have a much higher allowed G-loads (the physiological
limits of most pilots are below those of the glider).

And the question was about open-class gliders. Did you see many
open-class aerobatic gliders ?

Hope this in some way helps, though I think I am just
re-iterating what a lot of other people have already
said.

Unfortunately yes, many people think like you (that the gliders are much
stronger that what is written in the manual, don't worry until 10 G,
etc.). That was perhaps true with older, low-span gliders (because the
design was less accurate than today and the margins greater). But it's
definitely *not* true.

--
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 believe that with the airbrakes open your safe positive
G-limit reduces to +2.5G. This is because you are forcing
most of the lift to be produced near the tip and thereby
increasing the wing bending moment at the root, and
there is also a hell of a shear force produced. God
knows what happens if you open them suddenly above
Va while pulling 3.5g but i suspect it would not be
pretty. (I also suspect that it may be this that caused
several big gliders to have wings come off in spin
recovery)

I once saw a discus, 'A' I beleive, due a high speed low pass and half
way through his pull up, which seemed to be agressive, deploy his
airbreak/spoiler (?). I thought for sure his wings from the airbreaks
out were going to snap off as they had a significant greater angle of
'bend' than the rest of the wing. I don't know why, but I know what I
saw.

Don

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.

I don't see anything obviously bad about deploying
speed brakes early in the recovery, particularly if
they are terminal velocity brakes and the flight manual
allows for their use. It ups the pilot workload a bit,
so I'd be wary of getting overloaded - fly the airplane
first.

I'm not as keen on the idea of pulling out the breaks
in situations where the G-loading in pullout is likely
to be near the limit. As has been observed, with the
breaks deployed the G-limit is significantly lower
because with no lift on the inner portion of the wing,
the bending moments on the wing are a lot higher for
any given G-loading.

I disagree strongly with the statement that you can
over-G a composite sailplane and encounter non-catastrophic
structural damage. This may be somewhat true for aluminum,
but the characteristics of composites are such than
they flex elastically until the break in spectacular
fashion - there is no intermediate 'plastic deformation'
mode. If you reach the ultimate limit there will be
essentially no warning before you turn into a high-speed
lawn dart. I know of several cases where this has happened.

If you find yourself steeply nose down and accelerating,
I would consider pulling the breaks only if I have
enough altitude for a relatively low-G pullout and
I am not too fast already. Under all other circumstances
I would pull smoothly back on the stick until I reach
the G-limit (question, if you don't have a G-meter,
how well calibrated is your backside?). I'd only pull
harder: a) to avoid hitting the ground, or b) if the
speed was still building at an alarming rate - of course
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.

9B

At 19:12 26 March 2004, K.P. Termaat wrote:
That is not the issue Jean. I am talking about pulling
the airbrakes before
the rotation of the glider has stopped. This not in
the manual of course.
The idea is to avoid a high speed with the glider at
a pitch angle of
something like 60° directly after it has stopped rotation.
Testing what happens when I do the whole thing with
my new rather heavy low
drag Ventus-2cxT is an invitation to others to call
me an 'idiot'. Probably
I will do that myself too.
For me it is more like an 'if then' case. While instructing
I have done tens
of spins with students with a ASK-13, but that's easy
of course and does
hardly apply to what can happen to modern gliders.

Thanks

Karel, NL

'Jean' schreef in bericht
...
Why don't you check your glider's flight manual ?
Jean
'K.P. Termaat' a écrit dans le message de
...
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.

Karel, NL