Avoiding Vne

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 ?

On 3/26/04 1:04 PM, in article ,
"Denis" wrote:


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 !

You write like a guy who got all his experience from a book, or a seminar.
But that could just be a language problem, I suppose. How much test pilot
work have you done?



Jack

Jack wrote:

You write like a guy who got all his experience from a book, or a seminar.
But that could just be a language problem, I suppose. How much test pilot
work have you done?

None (and I never pretended to have, did I ?). Although I worked for
years as a flight test engineer and a few thousand flight hours as a
pilot. Also some hours reading books, but I did not log them ;-)

--
Denis

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

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

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.

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?


--
-----
change "netto" to "net" to email me directly

Eric Greenwell
Washington State
USA

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

As Eric has indicated, it is a given that once you exceed Va by a
significant margin, whether or not you exceed Vne, you can pull hard
enough to cause a structural failure.

I think there is a point here that some are missing, and I'd like to
hear some discussion around this. With long wing gliders, it is easier
to unstall the wing, than it is to stop the rotation. If you are
unstalled and rotating, you are in a spiral. If you pull in a spiral,
your speed will increase, not decrease. The most important thing to
remember, as far as I'm concerned, is never pull before the rotation has
stopped...

Marc

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 ?

On Sun, 28 Mar 2004 20:30:57 +0200, Denis
wrote:

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.

Well, this one is just a little scary. For more than one glider of my
acquaintance Vne is given as a TAS in the manual. This can be
converted to IAS of course, but the IAS would, of course, decrease
with altitude.

Vstall, on the other hand, seems just about always to be given as an
IAS, and as an IAS the stall speed will remain about the same as
altitude increases. This is all pretty basic stuff I know, so I
probably misinterpreted your statement about Vne.

One way to look at the "coffin corner" situation where some very
high-flying aircraft, such as the U-2, I suppose, can end up at an
altitude where the stall speed, in TAS, has come very close to the
Vne, as a TAS.

Jim wrote in message

One way to look at the "coffin corner" situation where some very
high-flying aircraft, such as the U-2, I suppose, can end up at an
altitude where the stall speed, in TAS, has come very close to the
Vne, as a TAS.

More precisely, I think it's when the stall speed (IAS in the cockpit)
but really a TAS issue) approaches the limiting Mach number. Since
Mach vs TAS decreases with altitude, and TAS vs IAS increases with
altitude, the problem is inevitable for the right (or wrong, depending
on your point of view!) kind of plane.

Kirk