Devices for avoiding VNE?

From: "Arnold Pieper"
Date: 3/31/2004 1:16 PM Pacific Standard Time
Message-id: m

Maybe true if "near vertical", however, staying at a near vertical dive is
something that requires a conscient effort.
The glider won't just stay there on its own.
To remain in a 90 degree vertical dive requires a significant amount of
forward stick force and concentration.
As speed increases, the nose will come up (away from vertical) even if you
don't want it to, and even with full forward trim, it would still require an
honest push on the stick to maintain that attitude.

Not true with all gliders. Several modern racing ships will, when trimmed
within CG limits for best climb performance, "tuck" or pitch down with
increasing airspeed, resulting in a large outside loop or exceeding VNE without
pilot input. The crossover speed for this in my own ship is about 115 kts,
above which slight back pressure is needed to maintain speed.

-
Mark Navarre
2/5 black ace
LoCal, USA
-

Mark Navarre wrote:

As speed increases, the nose will come up (away from vertical) even if you
don't want it to, and even with full forward trim, it would still require an
honest push on the stick to maintain that attitude.


Not true with all gliders. Several modern racing ships will, when trimmed
within CG limits for best climb performance, "tuck" or pitch down with
increasing airspeed, resulting in a large outside loop or exceeding VNE without
pilot input. The crossover speed for this in my own ship is about 115 kts,
above which slight back pressure is needed to maintain speed.

Which glider is that? I thought a pitch-up tendency with increasing
speed was a certification requirement? And what is the CG position for
this to happen? Is this "feature" mentioned in the flight manaul? Sounds
kind of scary.

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Eric Greenwell
Washington State
USA

Nimbus 3 and 4 and Duo Discus brakes do not meet the
'speed limiting in a 45 degree dive' requirement, but
do achieve that in a 30 degree dive. That's why they
are non-aerobatic.

At 15:18 31 March 2004, W.J. \bill\ Dean \u.K.\. wrote:
NO. This is thoroughly misleading.

HISTORICAL.

When the first gliders with good (for the day) performance
were built,
it was found that the good performance made them difficult
to land.

So they were fitted with spoilers as a landing aid.

Then pilots started to cloud fly, and some lost control
in cloud
and overspeeded and overstressed their gliders, which
broke up.

This was countered by developing and fitting speed-limiting
airbrakes
(DFS, e.g. Weihe and Slingsby Sky, and Schempp-Hirth).
These were intended
to be speed limiting in a true vertical dive.

In the U.K. it was a requirement that the glider was
test flown to prove
that at max. all up weight in a vertical dive Vne was
not exceeded,
I understand that the Slingsby Skylark series all passed
this test.

Note that max. manoeuvring and rough air speeds WOULD
be exceeded.

Later, it was found that with higher wing loadings,
thinner wing sections
and higher aspect ratios it became practically impossible
to fit true speed
limiting brakes (in the sense that Vne would not be
exceeded in a true
vertical dive at max. a.u.w.). The first U.K. built
gliders for which this
applied were, I believe, some at least of the Slingsby
Dart series.

Also, if the rules were relaxed life would become a
lot easier for the
designer, because it would save weight and cost.
So the rules were
relaxed, and 'Speed limiting' came to mean 'In a dive
at X degrees', usually
I understand of 45 degrees.

TODAY

Most gliders today, including I believe all those built
in Europe, are
designed to JAR 22.

See:
Joint Aviation Authorities, Europe. http://www.jaa.nl/
,
JARs – Section 1 – JAR-22 http://www.jaa.nl/section1/jars/445499.
pdf .

The relevant clause is:

'JAR 22.73 Descent, high speed

'It must be shown that the sailplane with the airbrakes
extended, will not
exceed VNE in a dive at an angle to the horizon of:

'(a) 45° when the sailplane is approved for cloud flying
and/or aerobatics
when certificated in the Aerobatic or Utility Category;

'(b) 30° in other cases.

'[Ch. 5, 28.10.95]'

Some modern gliders, including some being built today,
probably still have
true speed limiting brakes by the strict old definition
given above; my
guess is that these would all be gliders with trailing
edge brakes or
braking flaps such as the early Pik 20; but this would
not necessarily be
true for all gliders with such brakes.

Some gliders were built with tailchutes, either in
an attempt to comply with
the old strict requirement, or because it was necessary
if they were to
comply with the relaxed rule. I have always understood
that the Janus was
fitted with a tailchute to be speed limiting in a 45
degree dive at max.
a.u.w. with full water ballast.

At what dive angle would a Duo-Discus with full brakes
go through Vne?
I would be astonished if this is more than 45 degrees,
it may very well be
30 degrees.

So if in a spin recovery, or for any other reason,
you are diving at a very
steep angle your air-brakes are unlikely to save you
from exceeding Vne.
I am sure they won't in the Nimbus 3/4 series; it was
not a requirement for
certification.

W.J. (Bill) Dean (U.K.).
Remove 'ic' to reply.


'Arnold Pieper' wrote in message
. com...

John,

The airbrakes were designed not only to be used for
approach and landing,
but also to avoid reaching VNE.

Look at your glider's POH and check what is the maximum
speed to deploy
the airbrakes, and what becomes the VNE with them
deployed.

In most modern design gliders, the airbrakes can be
deployed up to VNE,
and they will prevent the glider from reaching VNE
when fully opened.

The airbrakes are designed for this purpose.

Once the airbrakes are opened and will prevent you
from going over VNE,
there's no need to pull at anything even close to
the design limit G.

Spin training therefore, is the best way to ease this
fear and learn how
to pull without overstressing the airframe.

AP


'John Galloway' wrote in message
...

Through the contributions to the avoiding VNE thread
runs the theme of the difficulty of avoiding overspeeding
and/or overstressing some modern designs in accidental
spin recovery. This is made more difficult than
in
older composite gliders because they had a little
more
drag and a little more (fortuitous) margin in the
g
limits.

Is it not blindingly obvious that there is a need
for
an emergency drag device that does not reduce the
G
limits of gliders? Clearly if we all handled the
recovery
from inadvertent spins etc perfectly all would be
well
but equally clearly that does not always happen and
it is a shame to lose pilots in this situation.

As the Phoebus pilot pointed out a tail chute is
ideal
for this - providing that it can be made to actuate
and jettison reliably. (I found the design used
on
the Kestrel particularly good and I never once had
a failure for landing use.) On the other hand they
are expensive and inconvenient to replace and there
are several ways that they can fail.

So can anyone think of a better idea than a chute?
The best I can come up with is some sort of flush
fitted rectangular-with the-long-edge-horizontal
rear
hinged airbrakes (like old fashioned automobile suicide
doors) located on the fuselage sides somewhere in
the
region below or below/behind the wings. If they
opened
to about 45 degrees with a spring actuator (and limited
by sliding metal stays that hinge/attach to the front
of the panel and whose inner ends slid along in runners)
then they would provide a lot of drag without any
deep
internal mechanism (such as wing airbrakes have).
Once they have done their job the rear end of the
brakes
could be released by a spring loaded mechanism similar
to the front end so that the brakes would then instantly
spring to as position set out from and parallel to
the fuselage so that there would be very little drag
- only that provided by the stays at both ends and
the brake panels edge on to the wind. That configuration
would be good enough to fly home with. It would
only
be possible to reset these brakes on the ground and
they would not replace conventional wing airbrakes
for approach control - although they could have a
secondary
use for emergency approach control.

I am envisaging something the could be included in
new designs although there does not seem to be any
obvious reason why such a device could not be retrofitted
as a fairly major modification. The contours of
the
brake panels would be specific to the individual
fuselage
type but the mechanism could be generic. The assembly
would be fairly shallow and complete within itself
apart from e.g. a cable release attachment.

I am not advocating a technical solution to this
problem
in place of spin recovery practice but I do think
that
there must be something that the combined intellects
of the gliding community can come up with other than
observing that if we get into that particular overspeeding/steep

attitude condition we are stuffed.

Anyone got any simpler or better ideas? I am definitely
not an engineer.

John Galloway

I think positive static stability means that this doesn't
happen. The only certified aircraft I'm aware of that
tucks in pitch is the Learjet 20/30/50 series, but
this happens at transonic speeds as the center of lift
moves aft. The aircraft is certified with a stick puller
to ensure you never get there.

In most aircraft the phugoid mode is pretty lightly
damped and has a period on the order of tens of seconds,
so the speed can build up a bit before the nose comes
up. Are you sure that's not what you're experiencing?
The only other thing I can think of is a c.g. out
of limits, so it might be worth double checking your
weight and balance if your aircraft behaves this way.


At 06:18 01 April 2004, Eric Greenwell wrote:
Mark Navarre wrote:

As speed increases, the nose will come up (away from
vertical) even if you
don't want it to, and even with full forward trim,
it would still require an
honest push on the stick to maintain that attitude.


Not true with all gliders. Several modern racing
ships will, when trimmed
within CG limits for best climb performance, 'tuck'
or pitch down with
increasing airspeed, resulting in a large outside
loop or exceeding VNE without
pilot input. The crossover speed for this in my own
ship is about 115 kts,
above which slight back pressure is needed to maintain
speed.

Which glider is that? I thought a pitch-up tendency
with increasing
speed was a certification requirement? And what is
the CG position for
this to happen? Is this 'feature' mentioned in the
flight manaul? Sounds
kind of scary.

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Eric Greenwell
Washington State
USA

"Eric Greenwell" wrote in message
...
Mark Navarre wrote:

As speed increases, the nose will come up (away from vertical) even if
you
don't want it to, and even with full forward trim, it would still
require an
honest push on the stick to maintain that attitude.


Not true with all gliders. Several modern racing ships will, when
trimmed
within CG limits for best climb performance, "tuck" or pitch down with
increasing airspeed, resulting in a large outside loop or exceeding VNE
without
pilot input. The crossover speed for this in my own ship is about 115
kts,
above which slight back pressure is needed to maintain speed.

Which glider is that? I thought a pitch-up tendency with increasing
speed was a certification requirement? And what is the CG position for
this to happen? Is this "feature" mentioned in the flight manaul? Sounds
kind of scary.

--
-----
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Eric Greenwell
Washington State
USA


Though it was certainly not a "modern racing ship" my old Lark IS28 would go
neutral pitch stability at about 90 knots and become very nose heavy above
that even with negative flaps.

Bill Daniels
Nimbus 2C

From: Andy Blackburn
Date: 4/1/2004 3:35 AM Pacific Standard Time
Message-id:

In most aircraft the phugoid mode is pretty lightly
damped and has a period on the order of tens of seconds,
so the speed can build up a bit before the nose comes
up. Are you sure that's not what you're experiencing?

Could be this is true, I have never let the glider have it's way for long
enough to find out what the trim speed will be if I let it go. Something for a
future test flight....maybe.


-
Mark Navarre
2/5 black ace
LoCal, USA
-

Bill Daniels wrote:
"Eric Greenwell" wrote in message
...

Mark Navarre wrote:


As speed increases, the nose will come up (away from vertical) even if

you

don't want it to, and even with full forward trim, it would still

require an

honest push on the stick to maintain that attitude.


Not true with all gliders. Several modern racing ships will, when

trimmed

within CG limits for best climb performance, "tuck" or pitch down with
increasing airspeed, resulting in a large outside loop or exceeding VNE

without

pilot input. The crossover speed for this in my own ship is about 115

kts,

above which slight back pressure is needed to maintain speed.

Which glider is that? I thought a pitch-up tendency with increasing
speed was a certification requirement? And what is the CG position for
this to happen? Is this "feature" mentioned in the flight manaul? Sounds
kind of scary.

Though it was certainly not a "modern racing ship" my old Lark IS28 would go
neutral pitch stability at about 90 knots and become very nose heavy above
that even with negative flaps.

Was that a common experience, or just your particular IS28? I'm
wondering if there might be a rigging error. None of the gliders I've
owned has exhibited that; in fact, all of them ran out (or required
nearly full) of forward trim nearing Vne. THese gliders are the Ka-6E,
Std Cirrus, Libelle H301, ASW 20 C, ASH 26 E, Blanik. The Cirrus had a
particularly noticeable concave under section on the elevator to induce
this behavior.

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Eric Greenwell
Washington State
USA

On Thu, 01 Apr 2004 07:58:46 -0800, Eric Greenwell
wrote:

Though it was certainly not a "modern racing ship" my old Lark IS28 would go
neutral pitch stability at about 90 knots and become very nose heavy above
that even with negative flaps.

This behaviour of a neutrally instable aircraft is typical for a CG
that is out of range.

Was that a common experience, or just your particular IS28? I'm
wondering if there might be a rigging error. None of the gliders I've
owned has exhibited that; in fact, all of them ran out (or required
nearly full) of forward trim nearing Vne.


Just like you I also have never flown an aircraft that did not hve
positive pitch stability.

Bye
Andreas

I don't know where you guys get these things from, but this behaviour means
these gliders should have their certifications cancelled.

I've never flown anything with these characteristics.

"Mark Navarre" wrote in message
...
From: "Arnold Pieper"
Date: 3/31/2004 1:16 PM Pacific Standard Time
Message-id: m

Maybe true if "near vertical", however, staying at a near vertical dive
is
something that requires a conscient effort.
The glider won't just stay there on its own.
To remain in a 90 degree vertical dive requires a significant amount of
forward stick force and concentration.
As speed increases, the nose will come up (away from vertical) even if
you
don't want it to, and even with full forward trim, it would still require
an
honest push on the stick to maintain that attitude.

Not true with all gliders. Several modern racing ships will, when trimmed
within CG limits for best climb performance, "tuck" or pitch down with
increasing airspeed, resulting in a large outside loop or exceeding VNE
without
pilot input. The crossover speed for this in my own ship is about 115
kts,
above which slight back pressure is needed to maintain speed.

-
Mark Navarre
2/5 black ace
LoCal, USA
-

You're affraid of something that

"Todd Pattist" wrote in message
...
"Arnold Pieper" wrote:

Don't be affraid to open the airbrakes at high speed, believe me, the
manufacturer is just a little smarter than that.

I'm not afraid to open them at high speed, I'm afraid to
lower the G-limit when I need G-s to recover. I've seen too
many accident reports where structural failure was
attributed to overstressing with the brakes open.

You have to be careful at high speed just because they tend to jump out
more
easily, so, have a firm hand on it.

Real aerobatic training (as opposed to some occasional loops) will
clarify a
lot of this.

Although it's been long time, my training was by a national
glider aerobatic champion.


Todd Pattist - "WH" Ventus C
(Remove DONTSPAMME from address to email reply.)