The Blancolirio video is good but may be misleading. He is correct on what he says about the operating envelope, but wrong about how it might apply to this accident. The wing will only create a specific amount of lift for any specific airspeed. Lift is a function of angle of attack and the square of the airspeed, and you can only go to the critical angle of attack before the wing stalls (loses lift). G force doesn't matter when it comes to breaking a wing, only total lift force.

Yes, as the water dumps out the G forces would increase (if nothing else changes) because the total weight of the aircraft is reducing. But the total lift the wing produces stays the same. e.g. if the wing produces 8000 lbs of lift and the aircraft weighs 4000 lbs then the pilot (and the aircraft) feels 2G. When 2000 lbs of water is released the wing is still only producing 8000 lbs of lift but now the aircraft only weighs 2000 lbs, so the pilot and aircraft "feel" 4G.

As Juan says in the video, other things break at high G forces. Engine mounts, battery mounts, seat structure, etc. The wing is still only under 8000 lbs of stress regardless of the G forces.

So, that wing breaking off is not due to dumping the water but due to pulling up on the stick too hard at too fast of a speed. The way the wing twists off at the root makes me suspect corrosion or a crack from a long life of abuse and chemicals.

Not really a physicist and certainly not a pilot, but I did read Fly-sig's post and watch the Juan Browne video. Is the following any kind of valid explanation?

My understanding is that when an aircraft suddenly drops a large load, it undergoes a rapid and aggressive pitch-up motion. Would this be due to the wings applying the same lift on a much lower weight, creating a rising moment arm that tilts the nose up?

If at the same time the pilot pulls back on the yoke, rather than pushing forward, the angle-of-attack is further and catastrophically increased. The wings no longer lift, and in this case the air pressure easily overcomes the airplane's stress parameters, i.e. a wing snaps off.

quote:

My understanding is that when an aircraft suddenly drops a large load, it undergoes a rapid and aggressive pitch-up motion. Would this be due to the wings applying the same lift on a much lower weight, creating a rising moment arm that tilts the nose up?

I would think it depends on how the center of pressure and center of gravity change as the load is dumped.

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I would think it depends on how the center of pressure and center of gravity change as the load is dumped.

Yes, agree. Looking at the video, the release port seems to be between the wings, suggesting the tank is also located there, in the wing box or in the wings themself.

The "Hopper" or chemical tank, is located between the instrument panel and the engine. Designed to be the center of gravity of the plane so that the gradually reducing weight doesn't require constant trim changes.
The lesson here is don't do stupid shit, especially in old worn out
airplanes in front of crowds.

OZ

I'm seeing several articles that when a cropduster (sorry, "AG Plane") or air tanker releases its load, there is a sudden pitch up motion that must be compensated for with a firm push on the yoke. Not sure of the physics involved, but you have to push the nose down pretty aggressively, NOT pull back, the worst thing you can do in this case.

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At its limit, the entire 6,700-pound load is released in a second and a half. Pilots must anticipate the severe shift in weight and balance by pushing full forward stick simultaneously to the drop, or the aircraft will pitch up aggressively and could stall.

AOPA

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Originally posted by Oscar Zulu:
The "Hopper" or chemical tank, is located between the instrument panel and the engine. Designed to be the center of gravity of the plane so that the gradually reducing weight doesn't require constant trim changes.
OZ

Hmm, the hopper is located at the center-of-gravity, but not at the center-of-lift, which is further back. Reduce weight at the CG = rising moment arm = pitch up.

This is a very interesting and informative thread.
Certainly makes me appreciate how much information a pilot must understand, evaluate and execute before taking off and flying a plane!

I think we can all agree a lot of bad decisions contributed to this accident.

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Originally posted by Rightwire:
I think we can all agree a lot of bad decisions contributed to this accident.

And that damned gravity. It always seems to win.

The descriptions of this model of aircraft is it pitches up when the load is released, which means the hopper is forward of cg. In some aircraft the load is carried at the center of gravity, so there is no pitching up or down when released (and we'll ignore other complicating details for this discussion).

The wing, or more precisely the center of lift, is like the pivot on a playground teeter-totter. The cg is forward and tries to tip the nose down. The tail pushes down at the back, balancing the aircraft to level. Moving the stick forward or back changes the tail downforce, which tips the nose down or up. Dumping the load in this aircraft has the same effect as pulling back on the stick, which is the nose pitching up.

Things get complex because there are both dynamic and static forces at play. In this scenario it is primarily a dynamic, or changing, aerodynamic situation.

As the nose comes up it increases the angle of attack, which increases the lift the wing is generating. In a slightly simplified equation, Lift = AOA x airspeed squared. So as the speed increases, the amount of lift the wing can create increases exponentially.

For the non-aviators, AOA is the angle made by the wing to the oncoming air.

In the moment the nose is rising, the airplane is still moving forward (momentum keeps it going forward, and the climb is not yet begun), but the wing is angled up, and creates much more lift. You feel this in your seat as a bit of G force. Then in normal circumstances a climb is established over the next few seconds and the airplane is again moving in the same direction the wing is pointed, so the lift reduces to 1G.

Juan overly complicates it. His graph showing lift, airspeed, the red and yellow arc is perfect except ignore everything he says about reduced weight moving the curves to the left. That upper curve is the maximum lift the wing can produce in pounds. It happens just at the maximum AOA. Go faster, get more lift at that maximum AOA. Go too fast and the wing will produce too much lift at that maximum AOA and will break the airplane. So at high speeds you have to be gentle on the controls so as to not generate excessive forces.

Regarding this accident, the pilot released the load, and may have pulled back on the stick. Regardless, the nose pitched up which increased AOA in that moment. The wing produced a bunch more lift, which exceeded the strength it had. I expect the wing had corrosion or overstress damage already, and this was the flight it happened to fail on.

He was awfully close, those people were probably drenched in gender fluid.

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Originally posted by Rightwire:
I think we can all agree a lot of bad decisions contributed to this accident.

Totally agree. First of all risking human life with a crazy stunt.

Then I must wonder about the 32 year old pilot's backround. I get the impression that the average pilot may not understand the special dynamics of a load-dropping airplane. Was he actually trained for and/or had experience with this kind of flying? Maybe we'll get more information on his backround.

Don't get me started on that bernulie guy

^It ain't the guy, it's the principle.

If this was stateside then the NTSB would be involved. The maneuver, capabilities of the aircraft, the pilot, and the setting are likely not the complete answer. It's the maintenance or lack of.

Think words like AD, STC, NDI, repetitive inspections, and annual inspection. If there is a proper inspection of the wreckage I would expect signs of corrosion, and lack of documentation, or even signs of required repairs.

^That's an excellent point and underscores the fact that most aircraft accidents result from a perfect storm of issues and errors (or the swiss cheeze slices model if you prefer).

In this case, I would not be surprised if it turns out to be the co-incidence of bad piloting, structural defects from lack of proper maintenance, and reckless imagining of the entire stunt in the first place.

If this had happened the day before while crop dusting, it wouldn’t have made the news.