Piper crop spraying type crash in Mexico.

[megan]

You Tube

[MechEngr]

That video is a bit off. The curve as weight decreases scales rather than shifts to the left by the square root of the ratio of the actual weight over the maximum weight. This is because the dynamic pressure, and therefore the resulting lift at a particular airspeed and AoA, varies as the square of the airspeed. Cut the weight in half and, for a given AoA, the plane will need 70.7% of the original speed (sqrt(1/2)) to develop matching lift.

Not sure about this case as I don't know how much water was on board, but if it was 1000# of water and max wgt of 2900#, then the factor for reaching stall AoA for a particular load factor will be scaled left by 0.8, so the maneuvering speed, by that calculation should be 80% of the fully loaded case.

I feel like I am missing why the maximum load factor doesn't go up as the aircraft is lighter. Sure, by 8 or 9 Gs the pilot is the limiting participant, but design for fatigue life results in higher attachment capacities. It also doesn't seem clear why where the payload is stored makes no difference to the maximum load factor. Putting that container on a wingtip would certainly change the characteristics in a way the simple calc fails to capture.

[IFMU]

Originally Posted by MechEngr

I feel like I am missing why the maximum load factor doesn't go up as the aircraft is lighter.

Pretty hard to calculate allowed G loading on the fly. Plus if the actual loads are lower from being under gross, that is adding margin to the life of the aircraft.

My aircraft is rated 4.4G at gross, and 6G at aerobatic gross. What if I'm 20 lbs under aerobatic gross? Can I pull 6.2G? What aircraft manufacturer wants to give a pilot that leeway?

[Pilot DAR]

I feel like I am missing why the maximum load factor doesn't go up as the aircraft is lighter.

Mathematically, it does. But, there will not be any approved data to support operations into that corner of the envelope. The limiting G factors are prescribed by the design standards and regulation, not convenience to the pilot for pushing it.

If a pilot chooses to pull really close to the G limit on a decades old airplane, I hope that they know its airworthiness intimately!

[Uplinker]

Out of interest, do small aircraft like this and others have C and D checks, as airliners do?

[EXDAC]

Originally Posted by Uplinker

Out of interest, do small aircraft like this and others have C and D checks, as airliners do?

No, they don't, at least not in USA. Typically, for non commercial use, an annual inspection is required. In commercial use a 100 hour inspection may be required if 100 hours reached before the next annual inspection is due.

Airworthiness Directives may impose addition inspection requirements.

[megan]

I feel like I am missing why the maximum load factor doesn't go up as the aircraft is lighter

Because such things as the battery container, engine mounts etc are designed to support the unchanging weight of the respective items to the certified "g" limit, going beyond the certified limit will over stress those respective mounts. Don't think there are many pilots who would appreciate their engine falling off..

[markkal]

Originally Posted by megan

Because such things as the battery container, engine mounts etc are designed to support the unchanging weight of the respective items to the certified "g" limit, going beyond the certified limit will over stress those respective mounts. Don't think there are many pilots who would appreciate their engine falling off..

The G'load neither up or down when the A/C is lighter or heavier, It's the stall speed that is affected. A heavier aircraft will stall at a higher velocity than a lighter one for the same amount of G's applied.

[ZeBedie]

Is there a CofG change when the hopper is dumped?

[IFMU]

Originally Posted by ZeBedie

Is there a CofG change when the hopper is dumped?

The hopper on a Pawnee is on the CG.

[FlexibleResponse]

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.

The answer to the accident has been revealed in some of the posts but it is still possibly not clear to all of us.

@blancolirio in his video (above posted by @megan) referred to and explained the phenomena quite well using the terms of change of maneuvering speed with weight change.

Expanding on his line of reasoning, Lift required for level flight is equal to Weight, sometimes expressed as L = Nz x W (where N = g and z = vertical plane and W = weight).

So a Pawnee flying at say 2900 lbs at 124 mph Max weight man speed) in level flight is creating 2900 lbs of lift at the current AOA (L = 1g x 2900).

If the weight of the Pawnee is reduced by 1200 lbs of hopper emergency jettison and the AOA remains constant, the aircraft wings are still generating 2900 lbs Lift, but the Weight has reduced to 1700 lbs.

ie, 2900 = Nz x 1700

That translates to Nz = 2900/1700 which gives Nz = 1.7g

However, the pilot pulls back the stick to almost the stall AOA to achieve the max g of 4.5g at 124mph (the max man speed for 2900 lbs) which is expressed as L = 4.5g x 2900, which gives L = 13.050.

But, he reduces the weight of the Pawnee during the pullup with the emergency jettison to 1700 lbs (13,050 = Nz x 1700).

That translates as Nz = 13,050/1700 which gives Nz = 7.67g

Which clearly is an overstress far in excess of the aircraft ultimate g limits

Now I am not that expert with math so I would kindly ask the Flight Test Engineers on the forum to cross-check my reasoning and figures for accuracy.

[Jhieminga]

I'm no FTE, but your calculations appear to be correct. Even if he didn't pull the full 4.5g he was still adding a significant amount of g to the 1.7 incurred by the load drop leading to the disastrous outcome. It also illustrates, in a sad way, how Va (manouvring speed) is not a fixed number but changes due to a change in aircraft mass.

[RichardJones]

Once again, we are reminded of this. If you bend a piece of metal, long enough and oftern enough it is going to fail. The trick is to find the impending fail point, before it gets you

Also jettisoning a load, at that rate, puts a lot of strain on the airfram. Sadly it is able to bring any fail points to the fore

[Cedrik]

Normally dumping even when pulling up does not overstress the aircraft. Do all the maths you like, doesn't make it so. Talk to an Ag pilot, they actually know. Take the standard waterbombing aircraft now, an 802 is stressed to lower G than a Pawnee, the job is to dump water on fires. The Pawnee referred to was deficient with maintenance which caused catastrophic airframe failure. It is a silly thing to do gender reveals in any way let alone an aircraft.

[RichardJones]

Originally Posted by Cedrik

Normally dumping even when pulling up does not overstress the aircraft. Do all the maths you like, doesn't make it so. Talk to an Ag pilot, they actually know. Take the standard waterbombing aircraft now, an 802 is stressed to lower G than a Pawnee, the job is to dump water on fires. The Pawnee referred to was deficient with maintenance which caused catastrophic airframe failure. It is a silly thing to do gender reveals in any way let alone an aircraft.

Well I did 10 years ag flying, on 6 different types. Started 50 + years ago, when just turned 21 years old.
We did practice jettions, using water, every so oftern for possible fire fighting use. We applied solids also, straight out the bottom of the hopper. There is a noticable "rumbling" type vibration if you like. You could feel it through the airframe and stick.
Let me tell you, there is quite a lot of aerodynamic bufferitng, not to mention trim changes. My gut told me it was hard on an aircraft. Disturbed airflow from beneath the elevator/tailplane.
Also chemical is highly corrosive of course. That may have had a part to play. The pullup at the at the point of jettison, whether commanded or uncommanded would have put a lot of stress on that spar, in this accident. The result is there for all to see.
I was asked to do the odd demo. I always refused. The reason being, if I screwed up, I didn't want it to be witnessed by a lot of people. There is no dignity in that.
Air show pilots, have a lot of guts IMHO.

[Peter Fanelli]

Originally Posted by Bksmithca

What's a little pesticide amongst friends

Good point, and silence from the chemtrail people.

[FlexibleResponse]

Originally Posted by RichardJones

We did practice jettions, using water, every so oftern for possible fire fighting use. We applied solids also, straight out the bottom of the hopper. There is a noticable "rumbling" if you like. You could feel it through the airframe and stick.
Let me tell you, there is quite a lot of aerodynamic bufferitng, not to mention trim changes. My gut told me it was hard on an aircraft. Disturbed airflow from beneath the elevator.
.... The pullup at the at the point of jettison, whether commanded or commanded would have put a lot of stress on that spar, in this accident. The result is there for all to see.

These comments from Richard Jones are very interesting, especially his gut telling him that the drop was hard on the aircraft.
Is there something going on that we have not factored in to load dumping?
I wonder if the high density of the water flowing from the hopper under the wing would increase the forces applied around the wing root
attachment structure and strut attachments as a consequence just from the high density water flow and/or buffeting around the wing and tailplane in itself?




Image lifted from:
https://aviation.stackexchange.com/q...g-payload-drop

[RichardJones]

Originally Posted by FlexibleResponse

These comments from Richard Jones are very interesting, especially his gut telling him that the drop was hard on the aircraft.
Is there something going on that we have not factored in to load dumping?
I wonder if the high density of the water flowing from the hopper under the wing would increase the forces applied around the wing root
attachment structure and strut attachments as a consequence just from the high density water flow and/or buffeting around the wing and tailplane in itself?




Image lifted from:
https://aviation.stackexchange.com/q...g-payload-drop

Thankyou for taking the time to read and consider my observations etc.
That's quite an accolade, coming from an academic, for a dumb farmboy, who left school at 15 with nothing.
None of us know all the answers. No one.

[RichardJones]

You Tube

[Cedrik]

Yes it does increase the load on an aircraft but.......... maintenance or lack of it was the factor in all three of those wing off accidents. There are over 1000 firebomber 802's flying. As I said they are not stressed for as much G as a normal ag aircraft, they drop 3 ton a time. There have been zero incidents of 802's loosing wings. Every ag pilot dumps loads for training and practice in every aircraft from Pawnee through to the 802 from solids to liquids. There are no supplements any any of the ag aircraft manuals about not dumping loads, it's part of the brief.
I have been flying ag since 1980 and flown most machinery including the 8 on firebombing. Design faults have been cause in the early 60's of a few accidents, ones since have been a lack of maintenance or negligence by the operator and is what ag pilots have to put up with to be employed. I still think gender reveals are a stupid act of any sort.