Avoiding Vne

Bert Willing wrote:

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.

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 ?

Bruce Greeff wrote:

First generation glass, before the finite element analysis programs
allowed the designers to design to the limit is probably much safer to
over stress than overspeed. Similarly the latest carbon designs seem to
have G limits imposed by the JAR22 deflection limits rather than
ultimate strength. Presumably these aircraft have huge strength
reserves. For interest look at the wing test on the DG1000.

I agree that *some* earlier, 15m designs may have a good safety margin
in overstress, mostly those in glassfiber (more flexible).

But not *all*, and certainly not modern open-class gliders.

I recall what I posted before, because there are facts from NTSB and
manufacturer data, which I think are more reliable than some honorable
but ill-based opinions expressed in this thread, and which nobody here
have contested yet:

the link (on Minden Nimbus 4 accident) :
http://www.ntsb.gov/NTSB/brief.asp?e...12X19310&key=1
(...)
The report quotes that the G limit for the Nimbus 4 at VNE is 3.5 g
*only* (compared to 5.3 g at Va) and the design "safety margin" is
between 1.55 to 1.75. Thus even on a plane in perfect condition, and if
the manufacturer made no mistake, it *will* break between 5.4 and 6.1 g
at VNE (even without airbrakes)

--
Denis

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

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 ?

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 ?

Earlier, Bruce Greeff wrote:

...Similarly the latest carbon designs
seem to have G limits imposed by
the JAR22 deflection limits rather
than ultimate strength...

I'll certainly agree that composite sailplane structure is bounded
more by stiffness than by strength. However, I've spent my lunch hour
searching JAR22 and I can't find anything that codifies deflection
limits. The closest thing I found seems to be:

: JAR 22.305 Strength and deformation
: (a) The structure must be able to support
: limit loads without permanent deformation. At
: any load up to limit loads, the deformation may
: not interfere with safe operation. This applies in
: particular to the control system.
: with respect to the sailplane.

Do you know of other relevant JARs that specify maximum structure
deflection in quantifiable terms? I'm not trying to nitpick or
anything, I just want to make sure I'm not missing something
important.

Thanks, and best regards to all

Bob K.
http://www.hpaircraft.com

Todd Pattist wrote:

You seem to be of the opinion that opening the brakes
guarantees not exceeding Vne. It's not true. If it was,
you'd be right to open the brakes, provided you have enough
altitude AND provided that FUD (fear uncertainty and doubt)
did not inadvertently make you pull above the max permitted
reduced G-load and break the wing tips off.

You're right that it does not guarantee not exceeding VNE, but it helps
a lot...

This accident
is one of the most common, so anyone advising opening the
brakes in a highly nose down high speed scenario had better
be sure that the net result is less fatal accidents, not
more regardless of their theoretical justification. I have
my doubts.

I'm sure that *I* would do that. And I'm sure that most flight manuals
advise that too, therefore I'm confident that it may be true.

I've seen this accident several times. I have
not seen the fatal flutter accident. I don't say they don't
happen, but it's not as common

If a glider breaks in a high speed (above Vd) recovery, I think you
cannot determine if it broke because of G-loading or flutter, except if
you have a close look to it when it breaks.

It's you that misses the fact that you can get a modern
glider into an altitude/attitude/speed where at least one
limit must be exceeded.

Yes I understand that. But I prefer recommend not to get trapped into
that situation...

--
Denis

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

Bob Kuykendall wrote:
Earlier, Bruce Greeff wrote:


...Similarly the latest carbon designs
seem to have G limits imposed by
the JAR22 deflection limits rather
than ultimate strength...


I'll certainly agree that composite sailplane structure is bounded
more by stiffness than by strength.

I've been told that is more likely true for fiberglass construction, but
not so likely to be true for carbon fiber construction, because of the
great differences in material characteristics, such as stiffness. So, it
might correct to argue that a glass fiber sailplane has a "substantial"
G loading margin, but not correct for the carbon fiber sailplane.

And the bounds might be quite different for a 15 meter glider and a 25
meter glider, or a thick wing trainer and a thin wing racer.


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

Eric Greenwell
Washington State
USA

Todd Pattist wrote:

OK, if you are going to open the brakes, you're going to
have to be sure you can stay below the reduced G-load.

Right. I admit that it is difficult to "be sure" of how many G's you
pull without an accelerometre, but it is the same with or without
airbrakes. And for my ship (ASH 26) it's not very different (0.5 G less
with airbrakes, only). How much is it in your Ventus ?

personally think we've had some accidents brakes out where
that was a contributing factor, but let's assume you're
better than I suspect I am, so you're willing to risk the
reduced G-load. Now we have to do some calculation, because
as long as your nose is below 30 degrees down, your brakes
out aren't going to keep you below Vne.

Right. But even in a 90° dive they give you almost twice more time
before reaching to VNE ; thus you may pull much less for the same speed
at the end (and the additional drag due to pulling will be by far
greater with airbrakes than without - and will stop the acceleration
*before* 30° which is the limit angle in 1 G flight)

You're also going
to lose more altitude than me,

Wrong ! I will loose more total energy, of course, but not more altitude.

If I pull the same G's, I will loose *less* altitude.

If I pull lower G's, I will loose *more or less* altitude depending of
the glider (the differences in G w and w/o airbrakes), the speed, the
initial dive angle...

Anyway the difference will be very small (either way) high enough to
avoid the ground in one case and not the other is very low (although
when somewhat low the usual reflex will be to pull harder than necessary)

I've got my brakes in, so I can develop more lift and higher
G. My higher lift and higher G turns me vertically faster

Don't forget that your turn radius increase with the square of the
speed, thus this is not true - even if your rate of turn is faster!


--
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 ?

Denis wrote:

Please quote correctly. I never experienced a spin recovery *in a Nimbus 4*

I'll take your word for it, but that is not quite what you said.
However, it was close enough to give you the benefit of the doubt.

You did say:

"And although I have some experience in Nimbus 4D
(more on ASH 25) I never experienced a spin recovery
and I hope I never will have to."

It would not be unreasonable to ASSUME you meant to say that you had
never experienced a spin recovery, "in Nimbus 4D", but your statement is
too vague for a reader to be certain.

Thanks for the clarification.



Jack