Avoiding Vne

K.P. Termaat wrote:

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?

well... after 114 answers, I think you have good specimens of the very
diverse opinions that have been expressed so far ;-)

in short, mine is : apply full airbrakes just after applying the initial
spin recovery control inputs, and keep them out during dive (gentle)
pull out...

--
Denis

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

Hi Denis,

If I understand you well you will wait with pulling the airbrakes until the
glider has stopped its rotation and then carefully put some back pressure on
the stick. I was considering the idea of pulling the brakes with the glider
still in its rotation mode in order to keep forward speed as low as possible
at any time. However this may frustrate the spin recovery action; I just
don't know. What's your idea about this. Of course handbooks do not say
anything about this.
B.t.w. my provisional handbook for the Ventus-2cxT forbids spin exercises.
My idea is to avoid spins with this glider any time anyway; however I will
try to get some feeling about the glider's behaviour close to entering this
"acrobatic" flying mode.

Karel, NL

"Denis" schreef in bericht
...
K.P. Termaat wrote:

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?

well... after 114 answers, I think you have good specimens of the very
diverse opinions that have been expressed so far ;-)

in short, mine is : apply full airbrakes just after applying the initial
spin recovery control inputs, and keep them out during dive (gentle)
pull out...

--
Denis

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

At the risk of flogging this subject to death it should
be remembered that this thread was started with the
question on how to avoid reaching excessive speeds.
While extending airbrakes and/or dropping the wheel
may help the only really effective way is to use gravity
to slow down the glider as opposed to accelerating
it. The priority is to prevent reaching excessive speeds,
not necessarily what to do if they are reached, when
options may be very limited.
Those who have said that pulling excessive G beyond
the design limits can cause failure are right.
Those who have said that exceeding VNE and getting
into the flutter envelope can cause failure are right.
The title of this thread is 'Avoiding VNE' and what
I have said has always been directed at this.
It is clear that at very high speeds in excess of VNE
you are in danger of flutter and catostrophic overstress
and the Nimbus accident shows that. The accident report
only gives a probable cause, did the wing fail because
of the excess G or did flutter cause the break up,
I suspect no one really knows.
What we do know is that the higher the airspeed the
more G can be applied. Bearing in mind that we are
trying to avoid VNE, because of flutter and the danger
of excessive G forces the sooner the speed can be reduced
the better. The lower the airspeed when full control
is applied the less the load which can be applied.
(The report of the Nimbus accident seems to indicate
that the airspeed was very high indeed) I advocated
using the elevator to prevent further acceleration
even if this applied a G load in excess of the placarded
limit to reduce airspeed or at least prevent further
acceleration.
What we do know is that flutter can and has caused
the breakup of aircraft (and as I have said before,
bridges). Flutter is the result of high speed.
We also know that the higher the airspeed the greater
the load that can be applied and I accept that at speeds
in excess of VNE these loads can exceed the ultimate
design load. Conversely at lower airspeeds it may not
be possible to apply a loading above the placarded
limit. Ergo, pulling hard before the speed gets excessive
is better than allowing the speed to build and then
trying to do something about it, especially when the
situation could be complicated by the onset of flutter
with total loss of control (see comments made earlier
by manufacturer). It may be possible to exceed the
design G limit of the airframe at VNE but it will be
almost impossible at 60kts. I re-stress my suggestion
was directed at avoiding high speeds, given the choice
of having to apply excessive load or exceeding VNE
I would choose the former. The risks involved in allowing
the airspeed to increase are far greater than the risks
involved in controlling the airspeed by a smaller excessive
loading at the lower speed.

It really is a case of the lesser of two evils, best
to avoid the situation in the first place.


At 07:12 06 April 2004, Denis wrote:
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 ?

K.P. Termaat wrote:

Hi Denis,

If I understand you well you will wait with pulling the airbrakes until the
glider has stopped its rotation and then carefully put some back pressure on
the stick. I was considering the idea of pulling the brakes with the glider
still in its rotation mode in order to keep forward speed as low as possible
at any time. However this may frustrate the spin recovery action; I just
don't know. What's your idea about this. Of course handbooks do not say
anything about this.

the ASH 26 handbook does say "spinning is not noticeably affected by
extending the airbrakes paddles, but it will increase the height loss
when pulling out, and is therefore less advisable"

I suppose the last sentence refers to loss of total energy (i.e. after
recovery you will re-gain more height if you made it without airbrakes
than with). It is not true of height loss down to lowest point (you will
loose less height with airbrakes because the diving speed is diminished
and the curving radius is reduced by the square of the speed -- even
with 3.5 G allowed w/ airbrakes instead of 4 G w/o the height loss
should be lesser with airbrakes out --

--
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

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

ADP wrote:

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?

If the ailerons follow the flaps, this would cause the G loading to
increase (higher lift configuration), while shifting the load towards
the tips (not instantaneously, but very soon thereafter) because the
spoiler effect.

If the ailerons don't follow the flaps, the G loading would increase but
not as much, and the load may or may not be shifted to the tips, because
the inner section would have the higher lift configuration.

I don't think the strength of the structure would be changed by the flap
deflection.
--
-----
change "netto" to "net" to email me directly

Eric Greenwell
Washington State
USA

Todd Pattist wrote:

I don't think you can, but I admit I haven't gotten out my
Matlab and done the equations. Anyone else want to do it?

Depends on what you bet ;-)

--
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 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