Avoiding Vne

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 ?

Pete Zeugma wrote:

But a glider wing breaks within milliseconds of overstressing
!

no they dont, they have to fail progressivly. no glider
would be alowed in the air by any regulating body if
a wing could break in a 1000th of a second. i dont
think you really understand the force that would be
needed to do that. the instantanious g-load to do that
would kill you out right.

Another dangerous misconception :-(((

If the actual extreme load is 6 g, the glider will loose wings as soon
as G-loading is greater than 6 g. Not 5 seconds later, not 0.5 second
later, but immediately. And not "progressively" (can you loose wings
progressively ???)

And in a modern ship near VNE, a small stick input may bring you from 1
to 6 G in less than 1 second.

Hence there is no chance at all that black-out or grey-out warn the
pilot of a too high G-load in an emergency situation.

You may have plenty of spin training on an ASK 13, it's better than
none, it will not prevent you to getting in a dangerous situation with
an open class glider. Spin training in these is prohibited, but high
speed flying is not : everybody should train to fly at higher and higher
speeds up to VNE in order to get used to the very high sensitivity of
most modern gliders at high speeds and to master the technique to use
only very small control inputs (including putting their hand firmly on
their leg to prevent unwanted or G-induced stick movements)

--
Denis

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

If the actual extreme load is 6 g, the glider will loose wings as soon
as G-loading is greater than 6 g. Not 5 seconds later, not 0.5 second
later, but immediately. And not "progressively" (can you loose wings
progressively ???)

Yes - on some gliders anyway. The Schweizer 1-34 is designed so that the
*first* failure occurs about 2/3 out from the root at about 8 Gs.

Tony V
http://home.comcast.net/~verhulst/SOARING

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)

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)

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 ?

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 ?

Started this thread (Avoiding Vne) some weeks ago with a kind invitation to
respond to the idea of pulling the airbrakes while still in the rotating
mode of a spin. The idea behind it is when rotation has been stopped with
the glider at a pitch angle of say 60° or more this will be at a lower speed
then when the airbrakes stay closed all the time. Possibly a build up of
speed to over Vne can then be avoided after that. Of course airbrakes should
be closed again in the following pull up manouvre.
Any comments?


"Denis" schreef in bericht
...
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 ?

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 ?