Avoiding Vne

Edward Downham wrote:

It is important to note that this margin is there to cope with things such as
ASI position and calibration errors. When you fly a glider at an _indicated_
speed of Vne, you might actually be nearer Vd and the realm of the test pilot.

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.


As Bill Dean has mentioned, some gliders lose a lot of their control authority
over Vd. You may find yourself flying faster and faster and unable to do much
about it. Also, the faster you get, the more likely you are to overstress the
airframe with control inputs (and I don't just mean the elevator).

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.

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

Not at VNE !!!

The rise is drag is high, and will pretty certainly arrest any
further speed build-up and bring the nose up quite smartly. Also you won't get
so close to the ground, if this is a factor.

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).

Except when too close to the ground this is always the only thing to do.

And to come back to the original question, I think there is nothing to
fear pulling the airbrakes before getting out of the spin, since it will
help stopping the rotation (the outer airbrake will have a greater drag
due to greater 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 ?

No, it is only one point.

There have been several cases of certificated gliders overstressed in
stall/spin recoveries, some of them broke up.

I saw a Ka6E being repaired after such an incident (in this case a
deliberate stall/spin for self-training, the pilot was a current
instructor), you can read about it in "Gliding Safety" by Derek Piggott, on
page 53 in my copy (second edition), under "Stall and spin-related
accidents", "Light stick forces".

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.

With some gliders it is both important and difficult to get the recovery
exactly right. I suspect the modern generation of gliders, Standard, 15M
and Open come into this category especially when fully ballasted and flown
in really strong conditions. The critical manoeuvre is likely to be an
inadvertent departure from a steep thermal turn in rough strong lift.
Don't let anyone tell you that it won't depart when flown like this, it may,
even if you can't get it to do it deliberately.

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


"Stefan" wrote in message
...


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

It is clear that some types are marginal in recovering from an
inadvertent spin entry without exceeding limits, especially if the entry
is from an accelerated stall. An ETA broke up recovering from a
deliberate stall/spin entered for certification purposes.


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.

Stefan.

My comments on the ASW20C accident are based on my memories of what I saw at
the time, and on my memories of the subsequent investigation and report. I
never saw the actual report, but comments were published at the time.

The investigators estimated the speed when the glider hit the ground, it was
certainly of the order of 200 knots. 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, 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.

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


"Andreas Maurer" wrote in message
...


On Fri, 26 Mar 2004 14:00:08 -0000, "W.J. \(Bill\) Dean \(U.K.\)."
wrote:

I once saw the wreck of an ASW20C just after it crashed at about 200
knots. It turns out that the stability and control characteristics of
that type are such that above a certain speed it is impossible to
recover from a dive even with the stick on the back stop; this is
because the wing twists.


pretty interesting case - I had expected that the 20 (and 20C) would
lose its wings due to aileron/flap flutter at this speed.

snip

Bye
Andreas

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

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...

--
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:

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:

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.

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 ?