Vne, Va and lift?

On Sat, 12 Sep 2009 09:25:22 -0400, "Morgans"
wrote:


"Stealth Pilot" wrote

this diagnosis didnt make sense to me. how could an aircraft that has
just shed it's load fail? with the shedding of the load the airframe
gets relatively stronger.
the locheed reports indicate that the aircraft had an undetected crack
in the root of the mainspar that had grown to such an extent that the
structure was compromised. the events that you see on the video are
coincidental and not the cause of the crash.
the crack grew to the point that it broke up in flight. that was the
cause.

It shed its load of water, by dropping it on a fire. If you keep your
control surfaces in the same position, you will suddenly pull more G's when
the plane is much, much lighter. Those G's were more than a plane with an
already compromised wing could stand, so it broke up.

Does that make more sense?

yes but I'm still not convinced.

In article ,
"Morgans" wrote:

"Stealth Pilot" wrote

this diagnosis didnt make sense to me. how could an aircraft that has
just shed it's load fail? with the shedding of the load the airframe
gets relatively stronger.
the locheed reports indicate that the aircraft had an undetected crack
in the root of the mainspar that had grown to such an extent that the
structure was compromised. the events that you see on the video are
coincidental and not the cause of the crash.
the crack grew to the point that it broke up in flight. that was the
cause.

It shed its load of water, by dropping it on a fire. If you keep your
control surfaces in the same position, you will suddenly pull more G's when
the plane is much, much lighter. Those G's were more than a plane with an
already compromised wing could stand, so it broke up.

Does that make more sense?

Not really. You're pulling more gees because you're lighter. The wings
are exerting the same force as before, thus the spar is under the same
load as before.

The reason you have G limits as well as loading limits is because of
fixed-weight components in the structure. For example, it's my
understanding that the engine attachments in light singles are a major
factor in having G limits instead of just loading limits. Your wings
don't care if you're pulling 3 Gs at max gross or 6 Gs at half max
gross, but in the 6 G case your engine mounts have to bear twice the
load.

In a case like this, where it's the wings that failed, it can't be due
to attachments holding fixed-weight items. My totally uninformed guess,
since there was a crack, is that suddenly shedding this load caused the
wings to flex DOWN, and this flexing was the final straw that caused the
crack to fail catastrophically.

--
Mike Ash
Radio Free Earth
Broadcasting from our climate-controlled studios deep inside the Moon

On Sep 12, 11:58*am, Mike Ash wrote:
In article ,





*"Morgans" wrote:
"Stealth Pilot" wrote

this diagnosis didnt make sense to me. how could an aircraft that has
just shed it's load fail? with the shedding of the load the airframe
gets relatively stronger.
the locheed reports indicate that the aircraft had an undetected crack
in the root of the mainspar that had grown to such an extent that the
structure was compromised. the events that you see on the video are
coincidental and not the cause of the crash.
the crack grew to the point that it broke up in flight. that was the
cause.

It shed its load of water, by dropping it on a fire. *If you keep your
control surfaces in the same position, you will suddenly pull more G's when
the plane is much, much lighter. *Those G's were more than a plane with an
already compromised wing could stand, so it broke up.

Does that make more sense?

Not really. You're pulling more gees because you're lighter. The wings
are exerting the same force as before, thus the spar is under the same
load as before.

The reason you have G limits as well as loading limits is because of
fixed-weight components in the structure. For example, it's my
understanding that the engine attachments in light singles are a major
factor in having G limits instead of just loading limits. Your wings
don't care if you're pulling 3 Gs at max gross or 6 Gs at half max
gross, but in the 6 G case your engine mounts have to bear twice the
load.

In a case like this, where it's the wings that failed, it can't be due
to attachments holding fixed-weight items. My totally uninformed guess,
since there was a crack, is that suddenly shedding this load caused the
wings to flex DOWN, and this flexing was the final straw that caused the
crack to fail catastrophically.

--
Mike Ash
Radio Free Earth
Broadcasting from our climate-controlled studios deep inside the Moon- Hide quoted text -

- Show quoted text -

I am not an aeronautical engineer, but I think this 'model' makes
sense. The C130 was in coordinated flight with a heavy load of water.
that means it had to be trimmed for a lot of nose up to carry that
load. Now, drop the load. the airplane will pitch nose up because of
the trim setting, but its momentum will want it to continue straight
ahead. The wings now have a much greater angle of attack, much more
lift than was needed before. If you were flying straight and level
then yanked back on the yoke which I think is pretty much the same
thing aerodynamically, you might expect the wings to fail upward.

That's my take on explaining what I've seen in the video. Give me
enough speed and enough elevator authority and I might be able to fail
the wings of any airplane.

In article
,
a wrote:

I am not an aeronautical engineer, but I think this 'model' makes
sense. The C130 was in coordinated flight with a heavy load of water.
that means it had to be trimmed for a lot of nose up to carry that
load. Now, drop the load. the airplane will pitch nose up because of
the trim setting, but its momentum will want it to continue straight
ahead. The wings now have a much greater angle of attack, much more
lift than was needed before. If you were flying straight and level
then yanked back on the yoke which I think is pretty much the same
thing aerodynamically, you might expect the wings to fail upward.

That's my take on explaining what I've seen in the video. Give me
enough speed and enough elevator authority and I might be able to fail
the wings of any airplane.

Makes sense to me. Seems like there are several potential explanations:
sudden flexing of the wings like I said, sudden pitch up like you said,
or simply CG changes causing increased load on the wing. Lots of ways
for this failure to occur given a weakened wing, but the idea of the
wings failing under constant load with more Gs due to less weight
doesn't seem to make sense.

--
Mike Ash
Radio Free Earth
Broadcasting from our climate-controlled studios deep inside the Moon

On Sep 12, 2:42*pm, Mike Ash wrote:
In article
,

*a wrote:
I am not an aeronautical engineer, but I think this 'model' makes
sense. The C130 was in coordinated flight with a heavy load of water.
that means it had to be trimmed for a lot of nose up to carry that
load. *Now, drop the load. the airplane will pitch nose up because of
the trim setting, but its momentum will want it to continue straight
ahead. The wings now have a much greater angle of attack, much more
lift than was needed before. *If you were flying straight and level
then yanked back on the yoke which I think is pretty much the same
thing aerodynamically, you might expect the wings to fail upward.

That's my take on explaining what I've seen in the video. Give me
enough speed and enough elevator authority and I might be able to fail
the wings of any airplane.

Makes sense to me. Seems like there are several potential explanations:
sudden flexing of the wings like I said, sudden pitch up like you said,
or simply CG changes causing increased load on the wing. Lots of ways
for this failure to occur given a weakened wing, but the idea of the
wings failing under constant load with more Gs due to less weight
doesn't seem to make sense.

--
Mike Ash
Radio Free Earth
Broadcasting from our climate-controlled studios deep inside the Moon

Mike, if you look closely at the video I think you'll see the change
of pitch occur when the water is dropped. Of course if the airplane
stayed straight and level the reduced weight would reduce the wing
loading, but my theory is related to the dynamics, not the steady
state. We have all been taught to be gentle with the contols, this is
an argument that we have to be gentle with pseudo control changes too.
It would be like flying into a sharp edged updraft -- that can take
one's wings off too.

There are lots of theories here, but my bias is showing!

"Mike Ash" wrote

Makes sense to me. Seems like there are several potential explanations:
sudden flexing of the wings like I said, sudden pitch up like you said,
or simply CG changes causing increased load on the wing. Lots of ways
for this failure to occur given a weakened wing, but the idea of the
wings failing under constant load with more Gs due to less weight
doesn't seem to make sense.

It does seem counter-intuitive. I had problems with the concept when it
came to explaining max maneuvering speed.

I had it explained to me, something like this: You are cruising along at
low weight, and hit a strong upward air column, suddenly. With a light wing
loading, the strength of the updraft will make the machine move upward
rapidly, which will cause a G to register on your G meter.

Now, you take the same plane, loaded to max weight and going the same speed
as before. You hit the same updraft, but the plane has a higher wing
loading, and higher mass, but the same wing area, so it will accelerate
upwards more slowly. That will register a lower G on your meter. Same
force applied to the higher mass is equal to less acceleration, as shown in
F=MA.

In thinking about max maneuvering speed, the more gradually you move into an
updraft, the less force will suddenly be applied, and I think another factor
comes into play in this. The same wing with a higher wing loading will not
be as efficient at creating more lift. It will slip, or "mush" through the
air more at higher wing loading.

I believe the same factor took place in the fire fighting airplane that
pulled the wing off. With the lighter load, the wing slipped less, and
created more lift at the lighter weight. It changed direction much more
quickly, which converts to higher G's, which broke it's wing.

I don't know. I hope to always (usually?) explain things in the least
technical way possible. That is the teacher side of me trying to make
things make sense to people who are not experts in the subject that I am
attempting to explain. It makes sense to me, but maybe I'm all wet.
Something must make it true, because that is what people say who know how to
make fancy math work as related to aeroplanes.
--
Jim in NC

"Mike Ash" wrote

Makes sense to me. Seems like there are several potential explanations:
sudden flexing of the wings like I said, sudden pitch up like you said,
or simply CG changes causing increased load on the wing. Lots of ways
for this failure to occur given a weakened wing, but the idea of the
wings failing under constant load with more Gs due to less weight
doesn't seem to make sense.

What causes something to break more easily; a steady pull, or a sharp
impact (or pull)?

A steady bend is the result of more weight carried by the airplane. A
sudden G load causes the wing to flex rapidly.

At least that's my story, and I'm stickin' to it! g
--
Jim in NC

In article ,
"Morgans" wrote:

"Mike Ash" wrote

Makes sense to me. Seems like there are several potential explanations:
sudden flexing of the wings like I said, sudden pitch up like you said,
or simply CG changes causing increased load on the wing. Lots of ways
for this failure to occur given a weakened wing, but the idea of the
wings failing under constant load with more Gs due to less weight
doesn't seem to make sense.

What causes something to break more easily; a steady pull, or a sharp
impact (or pull)?

A steady bend is the result of more weight carried by the airplane. A
sudden G load causes the wing to flex rapidly.

At least that's my story, and I'm stickin' to it! g

In the case of the firefighting plane, if it was flying level and
dropped a large weight (slurry), the wings would have the same load,
either with or without the dropped weight. Other airframe components,
such as engine mounts, fixed equipment, crew, however, would experience
a sudden increase in G loading. If the plane was flying at too high
speed, sudden updrafts/gusts could overload the wings.

IIRC, the aircraft in question was an older C-130 and did not have the
proper, mandatory inspections performed on the wing spar box sections.

--
Remove _'s from email address to talk to me.

"Orval Fairbairn" wrote

In the case of the firefighting plane, if it was flying level and
dropped a large weight (slurry), the wings would have the same load,
either with or without the dropped weight. Other airframe components,
such as engine mounts, fixed equipment, crew, however, would experience
a sudden increase in G loading. If the plane was flying at too high
speed, sudden updrafts/gusts could overload the wings.

My point is that the plane had a load and probably had substantial up trim
in, and full power, so when it's load was released with the power kept the
same, it probably zoomed into a climb, mostly on its own. The pilot also
very well could have pulled back on the yolk after the load was released as
is normal practice, to gain altitude. They have to drop very close to the
ground, so gaining altitude after a drop is S. O. P.

IIRC, the aircraft in question was an older C-130 and did not have the
proper, mandatory inspections performed on the wing spar box sections.

Sadly, also true. The fact remains that the wing was overloaded (for it's
condition) soon after release, though some combination of plane's
aerodynamic characteristics and pilot actions.
--
Jim in NC

Orval Fairbairn wrote
In the case of the firefighting plane, if it was flying level and
dropped a large weight (slurry), the wings would have the same load,
either with or without the dropped weight. Other airframe components,
such as engine mounts, fixed equipment, crew, however, would
experience a sudden increase in G loading. If the plane was flying at
too high speed, sudden updrafts/gusts could overload the wings.

Say What!!

You may know what you are trying to say, but it sure didn't come out
making sense.

From Wikipedia:
The g-force experienced by an object is its acceleration relative to
free-fall. The term g-force is considered a misnomer, as g-force is not
a force but an acceleration.

You probably meant to say "wings would have the same LOAD FACTOR".
Clearly, the load supported by the wing of a loaded aircraft is more
than the wing loading of an empty aircraft even though both are
experiencing only 1g. If the pilot doesn't reduce the angle of attack
(amount of lift produced by the wing)as the load is dropped, the wing
root will experience an increase in g-force. G-force is equal to the
actual lift being produced by the wing (at that angle of attack and
airspeed) divided by the weight being lifted.

From a aerodynamic viewpoint, the smart thing to do would be to push-
over (reduce the angle of attack) just as the fire retardant is
released, thereby reducing the g-force on the wing root. This, however,
tends to prevent the retardant from exiting the aircraft. What the
pilots seem to be doing is pulling up AND turning at the point of drop
and thereby making a bad situation even worse.

Bob Moore