Avoiding Vne

I agree with Eric and Bert - and the guys who taught
materials, structures and aerodynamics in school 20+years
ago.

Couple of points to clarify:

Some have been talking about the G-load in the manual,
others (like me) have talked about the ultimate loading
to which the airframe is tested (a bigger number).
In a panic I'd probably pull past the first, but wouldn't
get near the second. I don't think Don was recommending
anything much different - Don?

References to what aerobatic pilots do ('pull as much
as necessary') are not applicable to gliders for one
simple reason: aerobatic aircraft are generally good
for Gs past G-LOC (G-induced loss of consciousness)
- not so for gliders. For GRP or CRP structures pull
to the ultimate load at your peril. You'll probably
get away with going over the placarded limit. The main
point for me remains that I can't tell how many Gs
I'm pulling from my butt alone - at least not with
a whole lot of accuracy.

Flutter is a dynamic effect and can happen to the wing,
or any of the control surfaces - I think the horizontal
and vertical stabs are generally too stiff to go first.
Each flutter mode has a different natural frequency,
damping (positive or negative) and corresponding airspeeds
at which they can start.

I've heard of several cases of control surface flutter
in sailplanes (often older ones with looser control
circuits - and Grobs with poor mass balancing). I've
not heard of sailplanes fluttered apart in flight (though
this isn't to say it has never happened). Maybe it's
because everyone who has been forced to make a choice
pulls the wings off first.

Something to think about...


At 18:18 30 March 2004, Eric Greenwell wrote:
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

"Andy Blackburn" wrote in message
...
snip
I've heard of several cases of control surface flutter
in sailplanes (often older ones with looser control
circuits - and Grobs with poor mass balancing). I've
not heard of sailplanes fluttered apart in flight (though
this isn't to say it has never happened). Maybe it's
because everyone who has been forced to make a choice
pulls the wings off first.

Something to think about...

IIRC, Yugo built Open Cirrus at Inkpen UK. Pilot bailed successfully low
while ascending (believe it was Irvin EB80 parachute) following a beatup as
the glider fluttered to pieces (horizontal stab?). Think this was the
incident that resulted in AD for the lowering the Vne on all the Yugo built
Open Cirruses to something like 95 knots. Don't recall it ever being
modified. There was also an AD to fit the original Open Cirruses with a
rudder damper to prevent flutter. Wings were once tested to 11g's I've
heard.

Frank Whiteley

This has yielded some good food for thought and further
investigation as the season gets going:

1) Look through your flight manual with an eye towards
operating limits, particularly with respect to G-limits
and recommended/allowed use of airbrakes in spins/dives.
I have to admit I've forgotten mine.

2) If you don't have a G-meter in your sailplane get
some stick time in a plane with one pulling 2, 3, 4
Gs to get a good sense for what it feels like by the
seat of your pants.

3) At a safe altitude, pull the spoilers and try some
steep nose down attitudes to get a sense for speed
buildup under different attitudes/configurations (don't
overdo it!). If allowed by the flight manual (and within
your comfort zone/experience), try some spin recoveries
with and without speed brakes deployed. I for one would
love to hear an actual pilot report on maximum speed
achieved, maximum Gs pulled and altitude lost under
each scenario (yes I know there are multiple possible
combinations).

4) Be aware of the likely chain of events that lead
to being sharply nose-down at high speed. A couple
of scenarios come to mind: Open-class ships where it's
just hard to stop the rotation and you end up in a
spiral dive, or late recognition of stall recovery,
resulting in rapid speed buildup. Not much to do about
the first one beyond precise flying technique. The
second one it seems can be prevented with practice
and an eye on the airspeed indicator.

Lastly, I would love to hear factory advice on potential
implications of popping speed brakes near and above
Vne. Assuming you don't exceed the G-limit are there
other issues? It stikes me as a potentially violent
change in configuration, but maybe pilot and plane
can handle the sudden deceleration onset. It seems
like a relatively important decision in a pinch, but
there has been no real resolution of the matter here.

Safe flying,

9B


At 19:12 31 March 2004, Denis wrote:
Todd Pattist wrote:

With flutter, you don't know when it will start, and
you
don't know what will happen if it does. In my experience,
fatal flutter-caused accidents are relatively rare.
G-caused breakage seems to be both more common and
more
predictable. I'll leave my brakes closed, pull to
somewhat
over my max positive G-limit (but nowhere near as
hard as I
can) and let the speed do what it has to do as I bring
the
nose up.

I agree, except for 'I'll leave my brakes closed'...

I think opening the airbrakes would allow you to do
the same without
exceeding placarded airbrakes-out G-limit and with
a lower speed at the
bottom of the recovery...

--
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 have a much better idea, practice your spin recovery
so that you dont end up going through Vne, or having
to pull excessive G to prevent it! It's quite easy
really. There really is no excuse for allowing a spin
to develop beyond the wing drop stage in any other
situation other than forced spinning exercises.

I dont particularly want to read an accident report
for a pilot practicing what has been 'recommended'
as the correct way to recover from a spin in these
cercumstances.......... Do it right from the start
is the only solution!

At 20:06 31 March 2004, Andy Blackburn wrote:
This has yielded some good food for thought and further
investigation as the season gets going:

1) Look through your flight manual with an eye towards
operating limits, particularly with respect to G-limits
and recommended/allowed use of airbrakes in spins/dives.
I have to admit I've forgotten mine.

2) If you don't have a G-meter in your sailplane get
some stick time in a plane with one pulling 2, 3, 4
Gs to get a good sense for what it feels like by the
seat of your pants.

3) At a safe altitude, pull the spoilers and try some
steep nose down attitudes to get a sense for speed
buildup under different attitudes/configurations (don't
overdo it!). If allowed by the flight manual (and within
your comfort zone/experience), try some spin recoveries
with and without speed brakes deployed. I for one would
love to hear an actual pilot report on maximum speed
achieved, maximum Gs pulled and altitude lost under
each scenario (yes I know there are multiple possible
combinations).

4) Be aware of the likely chain of events that lead
to being sharply nose-down at high speed. A couple
of scenarios come to mind: Open-class ships where it's
just hard to stop the rotation and you end up in a
spiral dive, or late recognition of stall recovery,
resulting in rapid speed buildup. Not much to do about
the first one beyond precise flying technique. The
second one it seems can be prevented with practice
and an eye on the airspeed indicator.

Lastly, I would love to hear factory advice on potential
implications of popping speed brakes near and above
Vne. Assuming you don't exceed the G-limit are there
other issues? It stikes me as a potentially violent
change in configuration, but maybe pilot and plane
can handle the sudden deceleration onset. It seems
like a relatively important decision in a pinch, but
there has been no real resolution of the matter here.

Safe flying,

9B


At 19:12 31 March 2004, Denis wrote:
Todd Pattist wrote:

With flutter, you don't know when it will start, and
you
don't know what will happen if it does. In my experience,
fatal flutter-caused accidents are relatively rare.
G-caused breakage seems to be both more common and
more
predictable. I'll leave my brakes closed, pull to
somewhat
over my max positive G-limit (but nowhere near as
hard as I
can) and let the speed do what it has to do as I bring
the
nose up.

I agree, except for 'I'll leave my brakes closed'...

I think opening the airbrakes would allow you to do
the same without
exceeding placarded airbrakes-out G-limit and with
a lower speed at the
bottom of the recovery...

--
Denis

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

Finally, someone bothered to get the regs out.

I still believe that the G-limit is better understood
in most designs than the Vne limit, just due to the
difference in testing approach. G-loads are tested
to destruction, Vne is not.

In either case it's good to know the demonstrated margins
in excess of certified limits - just in case.


At 13:12 04 April 2004, Bruce Greeff wrote:
HI Bob

That is what I was referring to.

The deformation limit for carbon designs with thin
wings appears to be the point
at which it becomes impossible to maintain control
movement.

As an example, there are various apocryphal tales of
uncommanded airbrake
openings on open class aircraft with thin flexible
wings. The Nimbus 4 appears
to be the most common suspect here.

So the deflection limit is not a 'x degrees from rest',
or a plastic deformation
(although there is a requirement for this in the regulations)
but a deflection
beyond which the control actuators do not work correctly
or have unacceptably
high resistance.

My point came from published discussions on the construction
of the Eta, and the
DG1000 where both constructors commented that the ultimate
strength of the
structure was well in excess of the limit load, and
that the limit load was
imposed by the deflection of the wing.

There is an interesting test story at:

http://www.dg-flugzeugbau.de/bruchversuch-e.html

The destructive test requirement is that the wing must
withstand 1.725* the
limit load for three seconds at a temperature of 54Celsius.
The DG1000 wing
withstood this - and eventually failed at 1.95 times
the design load limit. This
is one reason why I believe you would probably be able
to get away with a brief
overstress load. I am not sure of the limits on older
designs, but would expect
there to be less margin of strength.

As I understand it the modern thin section wings are
flexible enough that the
load limit is imposed by control freedom limitation,
and the wing must withstand
1.725 times this load in test. Flutter is the subject
of speed limitation which
give speeds and margins that the designer/manufacturer
must demonstrate flying
to. The regulations imply that the glider must be demonstrated
safe at a minimum
of 23% margin above the placarded Vne. So your margins
for flutter, versus
ultimate strength are 1.23 vs 1.725 in JAR22 (unless
I got the math wrong)

Andy Blackburn wrote:

Finally, someone bothered to get the regs out.

I still believe that the G-limit is better understood
in most designs than the Vne limit, just due to the
difference in testing approach. G-loads are tested
to destruction, Vne is not.

Another difference: if *you* survived to overspeed (i.e. flutter did not
occur), your glider is still safe for you or *other pilots*

If you survived overloading (i.e. over limit G-load but the wings did
not break) your glider may be *unsafe* and next time might break well
under extreme G-load limit...


--
Denis

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

OK taking your point about the Nimbus 4. Exactly why
did the wing break, because of pilot induced overstress
or because of overstress caused by flutter? What did
the crew say in evidence?

At 17:48 05 April 2004, Denis wrote:
Bruce Greeff wrote:

As I understand it the modern thin section wings are
flexible enough
that the load limit is imposed by control freedom
limitation, and the
wing must withstand 1.725 times this load in test.
Flutter is the
subject of speed limitation which give speeds and
margins that the
designer/manufacturer must demonstrate flying to.
The regulations imply
that the glider must be demonstrated safe at a minimum
of 23% margin
above the placarded Vne. So your margins for flutter,
versus ultimate
strength are 1.23 vs 1.725 in JAR22 (unless I got
the math wrong)

It's perhaps mathematically true but your argument
is wrong (if your
conclusion is to say that there is more risk of flutter
than
overloading). You cannot compare pourcentages of load
and speed !

It takes less tenth of second at any moment to take
the 2 or 3 g's that
will exceed your (supposed) 72.5% load margin, whereas
it will take
several seconds to take the 60 or 65 km/h of margin
in speed (supposing
23% margin), or depending of the dive angle you might
never get over the
speed margin...

And although it may be true that some parts of the
wing (e.w. center
section) has more stress margin due to deflection limit,
it does *not*
guarantee you that all the parts of the wing has the
same extra margin:
in the Nimbus 4 accident the central wing did not break,
but the outer
wing did, with fatal consequences :-(


--
Denis

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

Don Johnstone wrote:
OK taking your point about the Nimbus 4. Exactly why
did the wing break, because of pilot induced overstress
or because of overstress caused by flutter? What did
the crew say in evidence?

I have no information except the link that have been provided by Bill
earlier in this thread :
http://www.ntsb.gov/NTSB/brief.asp?e...12X19310&key=1

The likliest cause of the outer wings failure seems to be pulling out of
the dive beyond extreme load, since the observed wing bending (45°)
correspond to that expected by the manufacturer for ultimate load limit

NTSB Identification: LAX99MA251. The docket is stored on NTSB microfiche number DMS.
14 CFR Part 91: General Aviation
Accident occurred Tuesday, July 13, 1999 in MINDEN, NV
Probable Cause Approval Date: 9/30/02
Aircraft: Schempp-Hirth NIMBUS 4DM, registration: N807BB
Injuries: 2 Fatal.

The glider broke up in flight during the recovery phase after a departure from controlled flight while maneuvering in thermal lift conditions. Airborne witnesses in other gliders who saw the beginning of the sequence said the glider was in a tight turn, as if climbing in a thermal, when it entered a spiral or a spin. With a 45-degree nose down attitude, the speed quickly built up as the glider completed two full rotations. The rotation then stopped, the flight stabilized on a northeasterly heading, and the nose pitched further down to a near vertical attitude (this is consistent with the spin recovery technique specified in the Flight Manual). The glider was observed to be pulling out of the dive, with the wings bending upward and the wing tips coning higher, when the outboard wing tip panels departed from the glider, the wings disintegrated, and the fuselage dove into the ground. Several witnesses estimated the wing deflection reached 45-degrees or more before the wings f
ailed. Examination of the wreckage disclosed that the left and right outboard wing sections failed symmetrically at 2 locations.

The glider is a high performance sailplane with an 87-foot wingspan and is constructed from fiber reinforced plastic (FRP) composites. The manufacturing process uses a hand lay-up of carbon and glass materials with applied epoxy resins. The glider is certificated in the normal category in Germany under the provisions of the European Joint Airworthiness Regulations.

Pilots with experience in the Nimbus 4 series gliders stated that the glider was particularly sensitive to over input of the rudder control during turns due to the 87-foot wingspan, with a resulting tendency for unwanted rolling moments. The manufacturer reported that to avoid undesired rolling moments once the bank is established the ailerons must be deflected against the bank.

Maneuvering speed (Va) is 180 km/h (97 kts) and the AFM notes that full control surface deflections may only be applied at this speed and below. Never exceed speed (Vne) is 285 km/h (154 kts) and control deflections are limited to one third of the full range at this speed and a bold print cautionary note reads, "Avoid especially sudden elevator control movements." The manufacturer reported that design dive speed (Vd) is 324 km/h (175 kts). The manufacturer also said that, assuming a 45-degree nose down attitude with airbrakes closed, the glider would accelerate from stall speed to Vne in 8.6 seconds, with an additional 1.8 seconds to accelerate from Vne to Vd. While no specific information on stick force per 'g' was available, certification flight test data showed that the elevator control stick forces were relatively light, with only 11.9 pounds of force (nose down) required to hold a fixed attitude at Vne versus the neutral stick force trim speed of 135 km/h (72.89 kts).


Detailed examination of witness marks and other evidence in the wreckage established that the pilot extended the airbrakes at some point in an attempt to slow the glider during the descent prior to the break-up. Concerning limitations on use of the airbrakes, the AFM notes that while airbrakes may be extended up to Vne they should only be used at such high speeds in emergency or if the maximum permitted speeds are being exceeded inadvertently. The manufacturer noted that the airbrakes function like spoilers and have the effect of shifting the aerodynamic loads outboard on the wings. The control linkages for the airbrakes and flaps are interconnected so that when full airbrake deployment is achieved, the flaps are extended to their full down limit.

The maximum maneuvering load factor limits (in units of gravity or g's) change with variations in glider speed and flap/airbrake configuration. From a "flaps up" configuration at Va to the condition of airbrakes and flaps extended at Vne, the maximum maneuvering load factor limits decrease from positive 5.3 to a positive 3.5.

The pertinent certification regulations require a minimum safety margin of 1.5 above the design limit load, which is defined as ultimate load. Review of the manufacturers data on safety margins in the wing spar disclosed that in the area of the primary wing failures, the structural design safety margin ranged between 1.55 and 1.75.

The manufacturer supplied data of the wing deflections under various load and aerodynamic conditions. At the design load limit (3.5g's) with airbrakes extended and at Vd, the wings were deflected to a 31-degree angle. 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.

An extensive series of scientific investigations were undertaken to establish: 1) if the structure as built conformed with the approved production drawings; 2) that the wing design met pertinent certification standards for strength safety margins; and 3) whether or not the failures occurred in overload beyond the ultimate load limits of the structure. While production control type discrepancies were found in the structure that differed from drawing specifications, none contributed to the failures. The testing established that the structure as built exceeded the minimum safety margin requirements. All the wing failures were overload in character and occurred at loadings well above the ultimate design load limits.

The National Transportation Safety Board determines the probable cause(s) of this accident as follows:
The pilot's excessive use of the elevator control during recovery from an inadvertently entered spin and/or spiral dive during which the glider exceeded the maximum permissible speed, which resulted in the overload failure of the wings at loadings beyond the structure's ultimate design loads.

Full narrative available

Index for Jul1999 | Index of months





--
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, but we still don't know, whether there was any speed margin left. It seems very much that the
major factor in destroying the craft was fear of exceeding Vne and consequently pulling up too hard.
But nobody knows now.

One interesting thing is written there about airbrakes:

"Detailed examination of witness marks and other evidence in the wreckage established that the pilot
extended the airbrakes at some point in an attempt to slow the glider during the descent prior to
the break-up. Concerning limitations on use of the airbrakes, the AFM notes that while airbrakes may
be extended up to Vne they should only be used at such high speeds in emergency or if the maximum
permitted speeds are being exceeded inadvertently. The manufacturer noted that the airbrakes
function like spoilers and have the effect of shifting the aerodynamic loads outboard on the wings.
The control linkages for the airbrakes and flaps are interconnected so that when full airbrake
deployment is achieved, the flaps are extended to their full down limit."

Outboard is where the wing broke




"Denis" wrote in message
...
Don Johnstone wrote:
OK taking your point about the Nimbus 4. Exactly why
did the wing break, because of pilot induced overstress
or because of overstress caused by flutter? What did
the crew say in evidence?

I have no information except the link that have been provided by Bill
earlier in this thread :
http://www.ntsb.gov/NTSB/brief.asp?e...12X19310&key=1

The likliest cause of the outer wings failure seems to be pulling out of
the dive beyond extreme load, since the observed wing bending (45°)
correspond to that expected by the manufacturer for ultimate load limit

At the risk of reviving a flogged horse, does anyone find this part of the
analysis strange:

"The control linkages for the airbrakes and flaps are interconnected so that
when full airbrake
deployment is achieved, the flaps are extended to their full down limit."

What do you think the extension of full flaps at hight speed does to the
load distribution and the strength of the wing structure?

Allan