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.