Devices for avoiding VNE?

Allan,

I think we're generalizing too much here.

The "distribution of forces on the wings change" as you posted, that is the
key when you reach high speeds in sub-sonic aircraft.
Supersonic aircraft have supercritical wing profiles and are a different
story altogether.

On sub-sonic, sweptback wings, the shock waves form first at the wing root
and destroy the lift there (boundary layer separates aft of it), close to
the fuselage.
Since the wing roots are "forward" of the rest of the wing in a swept back
design, the rest of the wing that continues to work being further aft,
generates the pitch down tendency.

The stories about early high-speed flights ending up flying into the ground
are related to the elevator not being the one-piece stabilators that we see
in all supersonic aircraft today.
The "regular" horizontal stabilizer-and-elevator that we see in most
sub-sonic aircraft suffers from control reversal close to Mach 1, and that's
why the one-piece stabilator was developed, it does not suffer from
control-reversal.

Mach Trim is required on SuperSonic aircraft (with supercritical airfoils),
in which the Center of Lift moves back significantly as the aircraft
accelerates through Mach 1. This phenomenon doesn't affect sub-sonic
aircraft in a very significant way because when the shock wave forms on the
wing roots, it destroys the lift there, as opposed to moving the AC back.

Either way, on our gliders, none of these things really matter.

"ADP" wrote in message
...
A for effort, D for accuracy. The DC-8 Could not be flown at altitude
with
out it's PTC (Pitch Trim Compensator) being operative.
The function of the PTC was to protect against mach tuck.

Since the DC-8 was undoubtedly certified, the argument is invalid.

Had you read my post you would have noticed the reference to supersonic
airflow which presumably does not apply to gliders.
On the other hand, a P-38 with a critical mach number of .69 is hardly a
jet
and has straight wings.
Several were lost in early testing because the phenomenon of mach tuck was
not well known.

In fact, sweepback is a design factor that helps delay critical mach to
higher numbers.

Not to make too fine a point but any aircraft, if flown fast enough
without
breaking up, can be subject to mach tuck.

Allan

"Arnold Pieper" wrote in message
m...
A+ for research initiative, D- for applicability.

Mach tuck affects certain high speed aircraft (high mach numbers, jets
only)
with swept back wings, when they exceed their Mmo.
When flying within their normal certified speed ranges, they do not
present
this abnormality.

As someone already posted, no aircraft would be certified with
instability
being a part of its normal flight envelope.

"ADP" wrote in message
...
Well,

Although not directly related to gliders (except really fast ones),
look
up
"mach tuck" and you will find several certified aircraft that tend to
nose
down as speed increases.

Critical mach is a aeronautics term that refers to the speed at which
some
of the airflow on a wing becomes supersonic. When this occurs the
distribution of forces on the wing changes suddenly and dramatically,
typically leading to a strong nose-down force on the aircraft. This
effect
led to a number of accidents in the 1930s and 1940s, when aircraft in a
dive
would hit critical mach and continue to push over into a steeper and
steeper
dive. This problem is often lumped in with the catch-all phrase
compressibility.

Wings generate much of their lift due to the Bernoulli effect; by
speeding
up the airflow over the top of the wing, the air has less density on
top
than on the bottom, leading to a net upward force. The relative
difference
in speed is due largely to the wing's shape, so the difference in speed
remains a fairly constant ratio over a wide range of speeds.

But if the air speed on the top of the wing is faster than on the
bottom,
there will be some speed where the air on top reaches the speed of
sound.
This is the critical mach. When this happens shock waves form on the
upper
wing at the point where the flow becomes supersonic, typically behind
the
midline of the chord. Shock waves generate lift of their own, so the
lift
of
the wing suddenly moves rearward, twisting it down. This effect is
known
as
mach tuck.

;0)

Allan

I'm not sure how you want to relate your questions with what's being
discussed.
But aircraft need to have a predictable behaviour and a predictable response
to control inputs.
To be certified, a design sometimes needs to use devices so that it has a
predictable behaviour and response to control inputs.
Springs (on older aircraft and most gliders) and Bob weights are commonly
used to make pitch control heavier with higher G-loads, thus helping prevent
us from overstressing the airframe.
Springs are also used on gliders in place of aerodynamic Trim controls (such
as trim tabs or variable incidence horizontal stabilizers ), so that stick
forces remain light throughout the flight and CG envelopes.

Weights are also used in some designs to balance control surfaces (called
"mass balanced"), since a fully balanced control surface is less prone to
flutter.

All of these things, as well as Delta Fins on some jets, computer software
in fly-by-wire systems, stick pushers, stick shakers, yaw dampers, and yet
some other things, are all used to guarantee that the behaviours are
predictable and within certain standards on all flying things.
Basically to guarantee that we push to nose-over, pull to bring the nose up,
move the stick right to bank right, left to bank left, and so on.

I tried to answer your question in the most relevant way.

"d b" wrote in message
link.net...
Do you include rudder lock as an instability? How about dynamic
stability?
What about downsprings, bob weights and other stability enhancement
devices?
Stability is a really broad subject.

"Arnold Pieper" writes:

::bzzzzzt::thank you for playing::

Mach tuck affects certain high speed aircraft (high mach numbers,
jets only) with swept back wings, when they exceed their Mmo. When
flying within their normal certified speed ranges, they do not
present this abnormality.

As someone already posted, no aircraft would be certified with
instability being a part of its normal flight envelope.

Concorde, when it was acelaring through transonic speeds had to do a
large fuel xfer to the aft tanks to conpensate for the strong nose
down trim shift.

It was rumoured to be certified

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West Australia 6076
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Raw, Cooked or Well-done, it's all half baked.
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You got it. Stability isn't really defined as the aerodynamics of the plane.
It is defined as what the pilot sees. You didn't mention that
dynamic instability is quite common and is usually not a serious issue.
Rudder lock is another story. This should not happen in the speed
range of the aircraft. In the one that I flew, it happened at very low
speeds, in a highly yawed condition, on purpose, and was easy to
overcome. I was looking at this on purpose because it was evident that
the rudder forces were getting lighter with increased deflection in one
direction and different in the other direction. I thought it deserved a closer
look before any unexpected surprises might happen. Something to keep
in mind for the pilots who like spins. I don't.


In article , "Arnold Pieper"
wrote:
I'm not sure how you want to relate your questions with what's being
discussed.
But aircraft need to have a predictable behaviour and a predictable response
to control inputs.
To be certified, a design sometimes needs to use devices so that it has a
predictable behaviour and response to control inputs.
Springs (on older aircraft and most gliders) and Bob weights are commonly
used to make pitch control heavier with higher G-loads, thus helping prevent
us from overstressing the airframe.
Springs are also used on gliders in place of aerodynamic Trim controls (such
as trim tabs or variable incidence horizontal stabilizers ), so that stick
forces remain light throughout the flight and CG envelopes.

Weights are also used in some designs to balance control surfaces (called
"mass balanced"), since a fully balanced control surface is less prone to
flutter.

All of these things, as well as Delta Fins on some jets, computer software
in fly-by-wire systems, stick pushers, stick shakers, yaw dampers, and yet
some other things, are all used to guarantee that the behaviours are
predictable and within certain standards on all flying things.
Basically to guarantee that we push to nose-over, pull to bring the nose up,
move the stick right to bank right, left to bank left, and so on.

I tried to answer your question in the most relevant way.

"d b" wrote in message
hlink.net...
Do you include rudder lock as an instability? How about dynamic
stability?
What about downsprings, bob weights and other stability enhancement
devices?
Stability is a really broad subject.

d b wrote:
Do you include rudder lock as an instability? How about dynamic stability?
What about downsprings, bob weights and other stability enhancement devices?
Stability is a really broad subject.

Yes, it is. Interestingly, The one of the effects we've been discussing
- the stick force required at increasingly higher speeds - isn't really
an aircraft stability issue. It's a pilot control issue: the certifying
authorities believe an aircraft is easier to control if you have to push
on the stick to make it go faster, especially as you approach Vne. A
glider might still be pitch stable at high speeds, even though (as some
have reported) the stick force has decreased to zero. A lot of gliders
have an elevator with a concave (on the bottom) airfoil to provide this
increasing stick force.
--
-----
change "netto" to "net" to email me directly

Eric Greenwell
Washington State
USA

I agree, I was only pointing out that there are certified aircraft that are
dynamically unstable in pitch (and roll and yaw) - for whatever reason.
The relevance had to do with the fact that the FAA or any certifying
authority, for that matter, can not necessarily be trusted
to keep one from getting into trouble should one operate outside design
parameters.
Sometimes such authorities restrict the CG range to keep aircraft within
design parameters, sometimes the restriction is with VNE and sometimes
the restrictions specify stick shakers, PICs and/or yaw dampers.

All of this leads up to the original purpose of the thread, how to avoid VNE
and do we really have to.

Cheers,

Allan

"Arnold Pieper" wrote in message
om...
Allan,

I think we're generalizing too much here.

The "distribution of forces on the wings change" as you posted, that is
the
key when you reach high speeds in sub-sonic aircraft.
Supersonic aircraft have supercritical wing profiles and are a different
story altogether.

......Snip.....

"Eric Greenwell" wrote in message
...
d b wrote:
Do you include rudder lock as an instability? How about dynamic
stability?
What about downsprings, bob weights and other stability enhancement
devices?
Stability is a really broad subject.

Yes, it is. Interestingly, The one of the effects we've been discussing
- the stick force required at increasingly higher speeds - isn't really
an aircraft stability issue. It's a pilot control issue: the certifying
authorities believe an aircraft is easier to control if you have to push
on the stick to make it go faster, especially as you approach Vne. A
glider might still be pitch stable at high speeds, even though (as some
have reported) the stick force has decreased to zero. A lot of gliders
have an elevator with a concave (on the bottom) airfoil to provide this
increasing stick force.
--
-----
change "netto" to "net" to email me directly

Eric Greenwell
Washington State
USA


I've been following this interesting thread.

On Thursday I had a nice flight out of Boulder, CO (USA). After cruising
the Continental Divide at 18,000 feet for several hours, I decided to test
the pitch stability of the Nimbus 2C near Vne after I descended below 10,000
feet.

The Nimbus 2C was flying with the CG at 80% aft and at a wing loading of 6.1
PSF confirmed by a recent weighing. This particular Nimbus has a separate
stabilizer and elevator and not the all-moving stab. The structure and
rigging has been checked by a well respected shop within the last year.

With the flaps in full negative, the big old glider easily accelerated to
Vne. As it accelerated, the elevator forces diminished as expected. At
Vne, the stick force per G was essentially zero signifying neutral or
slightly negative static stability. While controllable, and trim-able it
would diverge nose up or down with the slightest nudge.

This behavior was almost certainly unrelated to any wing twisting since the
carbon wings are extremely stiff. I suspect the airfoil is the root cause
since it has a particularly negative pitching moment. With the flaps in
full negative, the wings pitching moment is probably moved slightly toward
neutral stability where an unflapped glider would most likely exhibit more
negative pitching moment.

One easily forms the impression that flying this glider above Vne would be
most unwise. It's also very easy to see how a nervous pilot could get into
trouble at Vne since it requires a cool hand to fly it there. I am certain
that one unintended tug on the stick would send the G loading way above the
ultimate load factor in the blink of an eye. It makes me think that some of
the in-flight break-ups were overcontrol followed by G-LOC and then airframe
breakup

Seeing a pair of gliders circling about two miles ahead, I started the nose
up with a tiny bit of backpressure. The glider responded instantly and, to
prevent unintended G buildup, I needed to push slightly to control the
pitch-up until the IAS dropped below 110 knots where the control forces
became more normal.

I don't think this is particularly unusual behavior since it confirms what I
have seen on other high performance gliders. If you are going to fly near
Vne, do so with a cool hand and steady eye. It can get pretty unforgiving
up there.

Bill Daniels

Bill Daniels wrote:

I've been following this interesting thread.

On Thursday I had a nice flight out of Boulder, CO (USA). After cruising
the Continental Divide at 18,000 feet for several hours, I decided to test
the pitch stability of the Nimbus 2C near Vne after I descended below 10,000
feet.

The Nimbus 2C was flying with the CG at 80% aft and at a wing loading of 6.1
PSF confirmed by a recent weighing. This particular Nimbus has a separate
stabilizer and elevator and not the all-moving stab. The structure and
rigging has been checked by a well respected shop within the last year.

Is the elevator concave on the bottom side? If not, is it supposed to
be? Some people have filled in the concavity on their gliders (in
general, not the Nimbus 2 in particular) to decrease drag a bit.

With the flaps in full negative, the big old glider easily accelerated to
Vne. As it accelerated, the elevator forces diminished as expected. At
Vne, the stick force per G was essentially zero signifying neutral or
slightly negative static stability.

I'm not a real aerodyanicisit, but I don't think stick force is an
indicator of fixed-stick (while you are holding it) static stability.
This is determined by other factors, such as hinge placement, control
surface shape, trim springs, and probably other stuff.

While controllable, and trim-able it
would diverge nose up or down with the slightest nudge.

This is an indicator of static instability, if you mean "moving the
stick slightly forward leads to continually increasing airspeed". If the
airspeed increases some but then stabilizes, the glider is still
statically stable. "Stabilizes" has to be interpreted loosely, since
most gliders do have a long time constant (10-15 seconds) oscillation
that can be divergent.

This behavior was almost certainly unrelated to any wing twisting since the
carbon wings are extremely stiff. I suspect the airfoil is the root cause
since it has a particularly negative pitching moment. With the flaps in
full negative, the wings pitching moment is probably moved slightly toward
neutral stability where an unflapped glider would most likely exhibit more
negative pitching moment.

One easily forms the impression that flying this glider above Vne would be
most unwise. It's also very easy to see how a nervous pilot could get into
trouble at Vne since it requires a cool hand to fly it there. I am certain
that one unintended tug on the stick would send the G loading way above the
ultimate load factor in the blink of an eye.

A cogent description of why I become nervous when people suggest
"pulling hard" over Vne.

It makes me think that some of
the in-flight break-ups were overcontrol followed by G-LOC and then airframe
breakup

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

Eric Greenwell
Washington State
USA

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

I've been following this interesting thread.

On Thursday I had a nice flight out of Boulder, CO (USA). After
cruising
the Continental Divide at 18,000 feet for several hours, I decided to
test
the pitch stability of the Nimbus 2C near Vne after I descended below
10,000
feet.

The Nimbus 2C was flying with the CG at 80% aft and at a wing loading of
6.1
PSF confirmed by a recent weighing. This particular Nimbus has a
separate
stabilizer and elevator and not the all-moving stab. The structure and
rigging has been checked by a well respected shop within the last year.

Is the elevator concave on the bottom side? If not, is it supposed to
be? Some people have filled in the concavity on their gliders (in
general, not the Nimbus 2 in particular) to decrease drag a bit.

Very slight concavity. The apearance of the elevator as well as the
logbooks suggest that it has not been modified.


With the flaps in full negative, the big old glider easily accelerated
to
Vne. As it accelerated, the elevator forces diminished as expected. At
Vne, the stick force per G was essentially zero signifying neutral or
slightly negative static stability.

I'm not a real aerodyanicisit, but I don't think stick force is an
indicator of fixed-stick (while you are holding it) static stability.
This is determined by other factors, such as hinge placement, control
surface shape, trim springs, and probably other stuff.

While controllable, and trim-able it
would diverge nose up or down with the slightest nudge.

This is an indicator of static instability, if you mean "moving the
stick slightly forward leads to continually increasing airspeed". If the
airspeed increases some but then stabilizes, the glider is still
statically stable. "Stabilizes" has to be interpreted loosely, since
most gliders do have a long time constant (10-15 seconds) oscillation
that can be divergent.

Obviously, with the glider at Vne, I'm not going to experiment with stick
free stability if I suspect the resulting behavior will be divergent.
However, once a nose-up input started the nose up and the airspeed down, it
appeared from the stick forces that the pitch rate was divergent above 110
knots. I detected no oscillatory behavior.

Don't confuse static and dynamic stability. All low drag gliders exhibit
dynamic instability (meaning they exhibit a phugoid oscillation and are, to
some degree, susceptible to PIO's) IF they are also statically stable. If
static stability is absent, then they will not also be dynamically unstable
since no restoring force exists to sustain the oscillation. (In my view
this is a damn good reason not to make gliders too statically stable.)

A glider with zero static stability will have zero stick force per G and no
tendency to hold any particular airspeed - elevator trim will be
unneccessary. It's easy to confuse this with stability since the glider
will not change its attitude very much in response to turbulence and no
phugiod will be evident. I actually prefer this since, to me at least, it
represents the lowest pilot workload.


This behavior was almost certainly unrelated to any wing twisting since
the
carbon wings are extremely stiff. I suspect the airfoil is the root
cause
since it has a particularly negative pitching moment. With the flaps in
full negative, the wings pitching moment is probably moved slightly
toward
neutral stability where an unflapped glider would most likely exhibit
more
negative pitching moment.

One easily forms the impression that flying this glider above Vne would
be
most unwise. It's also very easy to see how a nervous pilot could get
into
trouble at Vne since it requires a cool hand to fly it there. I am
certain
that one unintended tug on the stick would send the G loading way above
the
ultimate load factor in the blink of an eye.

A cogent description of why I become nervous when people suggest
"pulling hard" over Vne.

It makes me think that some of
the in-flight break-ups were overcontrol followed by G-LOC and then
airframe
breakup


Should I ever find myself over Vne in a dive. First, if no essential parts
of the glider had departed so far, I would assume that they would not as
long as the airspeed didn't increase further and the G loads remained low.
If there were any rolling/turning I would stop that with aileron and then,
with ailerons neutral, I would use the smallest up elevator input possible
that would start the airspeed on a decreasing trend. As the airspeed
decreased, I would progressively add further tiny up inputs to ever so
slightly increase the G loading and accelerate the rate of airspeed
decrease. Once the dive angle was less than about 50 degrees, the airspeed
should be approaching Vne and I would increase the load on the wings
judiciously to recover from the dive as quickly as possible.

If the glider had conventional plug-type spoilers, I would refrain from
using them until under Va. If it had trailing edge air brakes, as the
Nimbus 2C has, I would probably use them on a situational basis.

If, in the above scenario, it appeared that the trajectory would intersect
the ground, it would be a good time to think (damn quickly) about the
parachute.

Bill Daniels

With the flaps in full negative, the big old glider easily accelerated to
Vne. As it accelerated, the elevator forces diminished as expected. At
Vne, the stick force per G was essentially zero signifying neutral or
slightly negative static stability. While controllable, and trim-able it
would diverge nose up or down with the slightest nudge.


"The elevator forces diminished as expected"...
I don't know why you expected this behaviour, since this goes against
certification requirements and against normal flight behaviour.
None of the gliders and aircraft that I've flown in the past 24 years
present this characteristic.

The certification requirements (both JAR and FAR), spell out that stick
forces have to increase with increasing G-loads, all the way to VNE.
Static stability requiremens for certification say that the airspeed has to
return to within 15% (10% in the case of FARs) of trimmed speed, for all
trimmable speeds between stall speed and VNE, and any significant change in
airspeed HAS TO cause a variation in stick force plainly percepbible to the
pilot.

JAR-22 says about Dynamic Stability that "any short period oscillations
between Stall Speed and Vdf must be heavily damped" with the primary
controls both free and fixed. Vdf is the demonstrated design speed, VNE is
90% of Vdf.