Avoiding Vne

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

Bert Willing wrote:
Nonsense. Exceed the structural g-load limit and you fly without wings.

Not so much fly as.... plummet.

Tony V. :-)

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
loose its wings due to aileron/flap flutter at this speed.


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.

The eta case is particularly interesting:

Afaik it did not break up because it exceeded Vne (in fact it lost its
tail at "only" about 200 kp/h), but because it exceeded the yellow
line with fully deflected rudder (from the spin recovery) and the
resulting torque broke the tail boom that was not designed for this
case of load (fully deflected rudder at this speed).

Bye
Andreas

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

Unless you are the designer, you don't know what was the limiting factor
which sets the limit speed.

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.

are YOU the designer of the ASW20C ???

--
Denis

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

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

If you get it wrong and have to take one of the risks, I am told that you
should centralise the ailerons, then pull however hard is necessary not to
exceed VNE, and make sure the brakes stay shut.

No.

You should "pull however is necessary but not exceeding the maximum g
permitted near VNE (usually it is reduced to 4 g's at that speed, even
without airbrakes). And certainly not "pull hard" because a even very
small force backwards on the stick is likely to cause higher than
expected g's !

If your speed is going to exceed VNE within this manoeuvre, you should
stop or reduce pulling and apply full airbrakes. At any dive angle up to
45° this prevents the glider to exceeding VNE, and you have time to
recover pulling gently (under 2 g's). This of course supposes that there
is sufficient ground clearance...

--
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:
Don Johnstone wrote:

I am not sure that enough G could
be pulled at speeds below VNE to cause serious catastrophic
failure as there is a margin between the placarded
limit and the design limit of most gliders.


IIRC, JAR requires a safety margin of 1.5

Va (maneuvering speed) is generally accepted as the highest speed you
can make full control deflections without exceeding the flight limits.
If there is a 1.5 safety margin, a speed only 22% higher would allow you
to exceed the design limits. On my ASH 26 Va is 99 knots, so this speed
would be 121 knots, 25 knots _below_ Vne. I am sure enough G can be
pulled at speeds below Vne to cause serious catastrophic failure.

Further, At Vne (47% higher than Va), you could pull 2.2 times the
flight limits!

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Eric Greenwell
Washington State
USA

I would tend to side with Don here.

Given a choice between exceeding Vne or the placard 'g' loading, I would go for
pulling hard every time.

From what I can remember, the glider manufacturer comes up with a 'Design Dive
Speed', Vd, which is then reduced (c.10%) to give a 'Never Exceed Speed', Vne.

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.

Flutter is a big problem and I am told that it is usually the determining
factor in the calculations used to produce Vd. Remember this is for a new
glider, with factory mass-balancing and pristine seals on the flying surfaces.

On the other hand, the airframe 'g' limits have: (a) a regulatory margin of
1.5, often much greater by the time the glider is made (it musn't be below 1.5.
so ends up being 1.6+) and (b) been promulgated for max. flight-mass (leading
to an even bigger margin if you are below this).

To use an example, the K-21 has a Vne of 280Kph/151Kts and declared flight
manual 'g' limits of +5.3/-4.0. That makes Vd around 166Kts. The design 'g'
limits work out as 8/-6 and it is unlikely to fail at that point.

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).

I agree with other posters about the use of airbrakes; either pull the brakes
OR pull hard but NOT BOTH. The Nimbus-4 which broke up near Minden was handled
in this way.

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

Don Johnstone wrote:

2. Pulling excess G can damage the airframe, however
the damage is likely to be far less than the damage
caused by flutter.

Both may cause the same damage : loosing the wings (and both have)

I am not sure that enough G could
be pulled at speeds below VNE to cause serious catastrophic
failure

Below manouevreing speed, no. Above it (and below VNE) yes !

I stand to be corrected but I suspect it is unlikely
that 8G could be attained at an airspeed less than
VNE.

If 8 g is the extreme limit this includes a safety margin and the real
limit is what is placarded (in most gliders it's about 5 or 5.5 g). And
this limit is only valid at manouevering speed, at VNE it is much lower.

yes 8 g may be attained well below VNE (if stalling speed is 70 km/h,
you may exceed 8 g at 198 km/h, as lift depends on square of speed.) At
270 km/h you'd reach 14.9 g !!!

3. Airframe flutter can occur at less than VNE. The
likelyhood of flutter increases dramatically above
VNE and the severity increases with the speed.

Nonsense. Flutter cannot appear under Vc, a design speed that is just
above VNE.

Not all modes of flutter are catastrophic (it depends of the damping)
but most are explosive. At any speed above flutter speed.

The N really does stand for never.

Yes, but the G limits are *never* to be exceeded too, even if nobody has
thought to call it GNE. It is exactly the same.

Will airbrakes effect the
recovery from a spin, I don't know yet,

That was the question. Thus if you don't know, please don't reply !

--
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

Edward Downham wrote:

I would tend to side with Don here.

Given a choice between exceeding Vne or the placard 'g' loading, I would go for
pulling hard every time.

Does "pulling hard" mean: a) pulling hard on the stick, or b) pulling
"high Gs"? There is a big difference!

To use an example, the K-21 has a Vne of 280Kph/151Kts and declared flight
manual 'g' limits of +5.3/-4.0.

My manual, as do many, show a _reduced_ allowable G load at increasing
speeds, going from 5.3 G (Va) to only 4 G at Vne.

That makes Vd around 166Kts. The design 'g'
limits work out as 8/-6 and it is unlikely to fail at that point.

"Unlikely"? How about gusts or simultaneous control deflections (say, a
bit of rudder or aileron)? Have you talked to a Schleicher designer (or
any designer) about this, or are you speculating?


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Eric Greenwell
Washington State
USA