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Piper crop spraying type crash in Mexico.

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Piper crop spraying type crash in Mexico.

Old 6th Sep 2023, 09:26
  #41 (permalink)  
 
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Blancolirio nailed it: The problem is the manoeuvring speed lowers when you shed mass. Or weight as he says.

He apparently used to be involved with air tanker operations and states that they're training to unload the wing slightly during the drop, and to never pull high g/bank immediately after the drop. Because your manouevring speed is now lower...

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Old 6th Sep 2023, 10:56
  #42 (permalink)  
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In my opinion, using aircraft as "attention getters" during social events is a poor idea, and to be avoided. For obvious reasons, a failing during such "displays" ruins the event, and probably lives. Pilots should consider declining such requests, and just stand there with everyone else and enjoy a finger food.

As silly as this pilot was, I have to hope what whatever he was dispensing over a crowd had more colour than mass. Though I agree with the theme that a sudden load release changes the physics of the flight, I would be surprised to hear that this load, in this situation, was a major factor in the structural failure.

The video shows the left wingtip twisting upward (increasing angle of incidence relative to the root), then the entire left wing folding up from the fuselage. It happened so fast, that the additional momentary lift of the left wing could not induce any brief roll to the right. So I doubt that the wing spar itself failed, but more a failure of the wing structure in torsion, so that a sudden increase in lift resulting from the torsional failure, caused the left upper wing strut attach at the fuselage to fail. As those wing struts act in compression, the failure would be instantly total. I opine that overly exuberant pilot pushed a less than ideally airworthy airplane just a little too far.
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Old 6th Sep 2023, 11:40
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I've noticed that our pilots in the AT-802s feed in increasing forward stick and down elevator during the unload of a fire drop as seen in this photo during a training exercise a few years ago. The elevator deflection is very obvious here; note also that SOP involves a small degree of flap application during the actual dump.


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Old 6th Sep 2023, 12:41
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In aircraft with strut braced wings the spar attachment to the fuselage is usually a simple single bolt (or pin) hinge. Without the struts in place the wing would be free to hinge up and down and could not support any lifting load. There is no spar bending load at the wing root.
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Old 6th Sep 2023, 14:58
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Originally Posted by physicus
@IFMU Just do a simple thought experiment on the load factor formula: n = lift / weight.

Assume 1 for lift as it remains the same throughout the exercise. Further assume weight = 1: n = 1/1= 1. If you shed half the weight, your new weight now is 0.5. so n becomes 1/0.5 = 2. So you just doubled the load factor by sheeding half your weight.
You have doubled the load factor, but you have not doubled the load (i.e., lift). After the drop it remains at 1, same as before.

That explains why the load factor momentarily and appreciably increases when you suddenly dump a largish amount of liquid from an airplane. that may well increase it beyond its structural limits.
But the structure fails at a certain load, not load factor.
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Old 6th Sep 2023, 21:56
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It makes zero sense for the structural loads to go up when the load is dropped. The angle of attack will momentarily be the same, lift will be the same, and the aircraft will rise. As you get more vertical speed the relative wind effectively reduces angle of attack and also you have less weight.
Newton's second law of motion applies

Force = mass x acceleration

Force exerted by elevator is the same throughout manoeuver

Mass reduces on drop

Therefore acceleration (i.e. g-load) must increase for same force

I never realised the implication for aerial application: I have learned from this poor chap's accident.
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Old 6th Sep 2023, 22:02
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That is a pretty unfortunate what to end the day.

From the start of the pitch up, the aircraft achieves an attitude change of around 20+ degrees, (needs measurement to determine the aspect change) but that happens in... ~11 frames of a 30-FPS image but he looks of it, and that is a pretty impressive pitch attitude change angle, the rate is much more impressive. The load on the aircraft is related to the resultant normal force which comes from the angle of attack that is applied. The angle of attack is analogous to the difference between the instantaneous pitch and the flight path vector. Normally we consider steady state conditions and these are not, this is a dynamic pitching case, and the usual assumption that the wing will stall at a certain angle of attack and that will limit the normal force that may be derives is not quite correct.

Unsteady effects are generally assumed to start to feature when a change in state occurs within the time that the flow takes to travel the chord of the airfoil. That has been the considered case, however, research on dynamic pitching of rotor blades shows that the effects occur outside of that assumption, and they alter the characteristic curves of CL, CD and CM vs AOA significantly. Where a wing may stall at say, 15 AOA normally, dynamic pitching will delay stall and values well above 20 AOA will still result in increasing CL, and therefore over short periods normal force from the wing. The video shows that the flight path is changing, and that rate of change comes off the pitch change to give eventually the steady state AOA. In FBW systems from the F16 through A320 and B777, there is a pitch rate ramp up that is achieved. For a fly by cable design, the elevator deflection is essentially instantaneous to the control input by the pilot, the rate is purely dependent on the drivers physiological arm motion limits. Dynamic CL, CD and CM are spectacularly different to the assumed vanilla steady state charts of characteristic CL, CD, CM to AOA, and are subject to biggly hysteresis, CL ends up looking like an italic P that over lays and extends beyond the inverted U [sine] of the CL curve; CD ends up looking like a drunk oblate spheroid, a flattened O with the case of the leans, and instead of mainly being a slightly increasing negative line until a fairly abrupt shift to positive, CM looks like a scribbled letter T, inked by someone with a bad dose of MND)
Pulling abruptly might be good for lawn mower starters, but is not great on planes. Actually, it isn't good for lawnmowers either.

AA 587 resulted from an sequence of inputs that just had to be about as unfortunate a career move as is conceivable. While nasty, in isolation each input would not have resulted in a bad day, but the sequence resulted in a rapid increase in bending and torsion loads, alternating direction and being exacerbated by the resultant yaw which lags. Even then, one of the fundamental badnesses was the structure of the tail had a primary and secondary load path as is needed by failsafe design, but the failure of one results in an effective lever load applied to the other that exceeds its isolated load limit at the same time. The force limiting of the A300-605 in common with the OEMs practice is a throw limiter which results in an increase in sensitivity compared to a limiter that reduces the hydraulic pressure applied, but allows the full movement of the rudder pedals.

The LE translates upwards while the TE initially stays near the normal geometry, so the outboard fitting of the forward compression strut would be interesting to look at. Once it fails, the torsion as well as the rolling moment resulting from the geometry change is going to lead to a rapid bad day.

Be gently on the onset of control inputs, someone else might have been as rough as we are, most materials have a really long memory, and planet earth doesn't give way for aircraft.

Did the wing fail prematurely? I suspect... not. The PA-25 by recollection has a 4412 equivalent airfoil, which has a fairly soft stall break, dynamic pitching will lead to much higher CL than expected before the section stalls, so it is possible to get higher instantaneous loads. The forward fitting will show a compressive/buckling failure, but, there could be prior damage, a close look at the components will tell the story. That the strut gave way in the sequence is on the video, the question is why did it start to buckle. If you really need to hit hard pull ups, go do it in a Pitts or Extra, not in a normal or utility category aircraft I would suggest.

Last edited by fdr; 7th Sep 2023 at 00:17. Reason: rewriting the derivatives.... to be comprehensible
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Old 6th Sep 2023, 22:49
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The dynamic load must have been dramatic. He dropped water, within a second reducing wing load, wing relaxing / flexing down. Would not be surprised if the resonant frequency of that wing structure is about 1 / s. Just after half that period or so he increased the wing load abruptly, at the worst possible instant. AA587 comes to mind indeed.
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Old 6th Sep 2023, 23:09
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Originally Posted by Capn Bug Smasher
Newton's second law of motion applies

Force = mass x acceleration

Force exerted by elevator is the same throughout manoeuver

Mass reduces on drop

Therefore acceleration (i.e. g-load) must increase for same force
The question is, which of these correlates to wing stress?

Last edited by Vessbot; 6th Sep 2023 at 23:52.
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Old 7th Sep 2023, 02:23
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Originally Posted by Vessbot
The question is, which of these correlates to wing stress?
All of them
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Old 7th Sep 2023, 04:33
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Originally Posted by DuncanDoenitz
The result is, of course, tragic and traumatic.

But if I've got this right, and I don't care how well they've cleaned it, they are using pesticide-delivery equipment to spray vapour onto the participants at a family event which includes a pregnant woman.
The risk of any actual contamination is extremely low in that instance. The likelihood that the aircraft was well decontaminated prior to this event is very high. I would very much doubt if the unrealistic fear of agricultural chemical contamination is going to form a part of the investigation.

We have many dual purpose Air Tractors across the USA and Australia spraying in the morning and firebombing in the afternoon. Even our chemical-phobic government agencies don't blink an eye at the low potential of significant contamination of off-target areas despite the fact that it has been drawn to their attention.

Of course there are a number of Air Tractors dedicated to firebombing operations only but many aren't. Most have a recent history of both spraying and firebombing operations.
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Old 7th Sep 2023, 08:22
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Lucerne; Thank you!

(Still not sure it's on my bucket list though).
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Old 7th Sep 2023, 08:30
  #53 (permalink)  
 
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Originally Posted by Cedrik
Gender reveal stupidity is American, there have been a few aircraft crash doing this.
Maybe just trail a blue or pink ribbon next time ... ?
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Old 7th Sep 2023, 09:46
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Quite low coming in with trees ahead, did the combined dump & pull-up simply turn the wing into a high drag barn door which the structure wasn't capable of supporting?
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Old 9th Sep 2023, 06:25
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An interesting analogy for those of us who have dropped bombs (and it would include other loads such as water bombers and helo external loads)…

Imagine a Lancaster bomber of a nominal weight of 45,000 lbs carrying a Tallboy bomb of 12,000 lbs for an AUW of 57,000 lbs .

The bomber is flying and trimmed in straight and level in (unaccelerated) flight at 18,000 feet and at an airspeed of 150 KIAS.

To do so, the power is set at say, 85% power and the wing AOA is generating lift force of 57,000 lbs to equal the total weight in level flight.

The crew then release the bomb over their target without changing any of the control positions or power setting. (Let’s ignore any trim changes due to changes in CG or airflow effects).

On releasing the bomb, the aircraft now weighs only 45,000 lbs…but crew maintains the power, speed and pitch attitude which are still set for a lift force of 57,000 lbs.

Will the aircraft:

A. Continue to fly straight and level at 150 KIAS;
B. Immediately descend with the bomb release;
C. Immediately climb as the lift force of 57,000 lbs far exceeds the new a/c weight of 45,000 lbs (increase in g); or
D. Enter a victory roll?

Now try the same bomb release in accelerated flight pulling up or recovering from a dive with increasing AOA and ask the same question re g force change.
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Old 9th Sep 2023, 09:07
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The Lancaster bomber will climb after bomb release, but the forces in the wing structure will be less than they were when they were carrying the bomb.

With the bomb on board the wings are 'pushing up' with a force of 57,000lbs. After the bomb is released, the wings are still pushing up with a force of 57,000lbs, but the weight they are carrying is now 12,000lbs less, so the forces in the wing structure must be less - even though the aircraft climbs, and even though the aircrew will feel positive g.
i.e. The wings do not suddenly push up more at the point of bomb release - they are 'pushing up' the same as they were.

I have watched the video explanation about manoeuvring speed, but am confused that reducing the fuselage weight the wings are carrying somehow increases the stress in their structure? How can that be ?

I think this pilot simply pulled up too aggressively, and that is what over-stressed the wing. The reason he pulled up too aggressively was that he applied a larger pitch force that would have been required with the payload on board - learned as he took off and flew the aircraft towards the venue - but he forgot to moderate his control inputs after the payload had gone.
e,g, say he would need N degrees of up elevator to pitch up when heavy; he would only need, say, a half or a third of N to pitch up when empty, but he applied N, which in the now very light plane was way too much elevator input.

If he had held the stick and not moved it in pitch at payload release, the aircraft would have climbed, since the elevator was already applying a high degree of elevator pitch-up to enable the wings to carry the payload. But then on top of that he applied even more pitch up to pull up, and this combined with the already high elevator deflection, simply rotated the (now empty) aircraft much too aggressively which rapidly took the wing AoA way beyond what it was built for, (at the speed he was flying), which overstressed the wing structure, (or the wing structure was weakened due to age or corrosion).

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Old 9th Sep 2023, 16:11
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The pilot appeared to approach the drop very low, quite fast, and possibly in a slightly decent.

Almost immediately after dropping the load, there was a dramatic pitch up, which appears to have been the cause of the failure. As others with engineering / aeronautical knowledge have said, dropping the load by itself doesn't instantaneously add any load to the wings, so it should be discounted as the direct or sole cause of the over-stressed wing. Most likely, in the adrenalin rush of the drop, the brain's messages to the hands on both the release control and on the joystick might well have become conflated to a significant extent. This might have been exacerbated by the rapidly approaching trees, and even the desire for a dramatic exit - after all, the desired effect was a dramatic display. Whether for not and pre-existing faults were present, clearly and sadly it all conspired to end this pilot's life shortly afterwards.

It is a sad fact that too many pilots, planes, helicopters and innocent bystanders have become victims of this understandable desire to give the most dramatic display at various weddings, fetes, or other gatherings of friends / family, and often to pilots who do not have the display flying skill, judgement or currency to pull of the attempted maneuvre successfully, or without putting themselves and other at considerable risk. The desire to impress is a very dangerous one, which has and will continue to cost too many lives.
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Old 9th Sep 2023, 16:35
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Originally Posted by FlexibleResponse
An interesting analogy for those of us who have dropped bombs (and it would include other loads such as water bombers and helo external loads)…

Imagine a Lancaster bomber of a nominal weight of 45,000 lbs carrying a Tallboy bomb of 12,000 lbs for an AUW of 57,000 lbs .

The bomber is flying and trimmed in straight and level in (unaccelerated) flight at 18,000 feet and at an airspeed of 150 KIAS.

To do so, the power is set at say, 85% power and the wing AOA is generating lift force of 57,000 lbs to equal the total weight in level flight.

The crew then release the bomb over their target without changing any of the control positions or power setting. (Let’s ignore any trim changes due to changes in CG or airflow effects).

On releasing the bomb, the aircraft now weighs only 45,000 lbs…but crew maintains the power, speed and pitch attitude which are still set for a lift force of 57,000 lbs.

Will the aircraft:

A. Continue to fly straight and level at 150 KIAS;
B. Immediately descend with the bomb release;
C. Immediately climb as the lift force of 57,000 lbs far exceeds the new a/c weight of 45,000 lbs (increase in g); or
D. Enter a victory roll?

Now try the same bomb release in accelerated flight pulling up or recovering from a dive with increasing AOA and ask the same question re g force change.
Originally Posted by Uplinker
The Lancaster bomber will climb after bomb release, but the forces in the wing structure will be less than they were when they were carrying the bomb.

With the bomb on board the wings are 'pushing up' with a force of 57,000lbs. After the bomb is released, the wings are still pushing up with a force of 57,000lbs, but the weight they are carrying is now 12,000lbs less, so the forces in the wing structure must be less - even though the aircraft climbs, and even though the aircrew will feel positive g.
i.e. The wings do not suddenly push up more at the point of bomb release - they are 'pushing up' the same as they were.
Agree with that.
If the pilots keep AOA constant the lift produced will remain at 57K so the aircraft will start pitching up and climbing. The pilots will feel positive G, but the wing load will remain steady (eventually the increasing pitch will lead to a decrease in speed).
If the pilots kept the altitude constant the lower pitch would lead to an increase in speed. The pilots will feel no G load, but the wings attachments will initially experience a reduction in load. (eventually the speed would stabilize at a higher point)
If the pilots keep the pitch constant it would be somewhere in-between the previous two scenarios.
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Old 9th Sep 2023, 16:40
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Originally Posted by hans brinker
Ahm no. No. NO.
Imagine a balloon filled with helium with a weight attached to it floating around. Cut the string, and the weight falls down and the balloon shoots up. That is what happens when an aircrafts releases its payload in flight. Either the aircraft goes up, or the load factor is reduced. But absolutely never will the load factor increase from a decrease in payload.

​​​​​​​
Originally Posted by Jhieminga
That analogy doesn't work. Or if you do want to stick with this: When you cut the string, the lift on the balloon is appreciably more than its weight (I know, it should be mass...) which equates to a L/w of more than 1. If you were to be inside that balloon, you would feel that as a perceived G-force.
And that is what I said. The pilot will feel an increase in G load. But the string (=wing attachment) feels a decrease in load, because the weight stopped pulling from the other side.
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Old 9th Sep 2023, 16:50
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Originally Posted by DaveReidUK
Maybe just trail a blue or pink ribbon next time ... ?
Fly the pink plane or the blue one.

I went to a gender reveal once. Misunderstood the nature of the event. No one there returns my calls.
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