Curious,
how do giant tankers like the DC10s and 747s handle losing tons of payload in a matter of seconds?



I'm operating under the presumption that the flight handling characteristics would notably change; but not being a pilot, I don't know if that's an issue at all.

If however that is indeed the case, then how is it mitigated?
Software? Flight training? Both? Something other?

Also, does flying at such generally low altitudes add to any difficulty incurred by such immediate weight shifts?

It's not that different from giant WW2 bombers dropping a load out of bombs when you really think about it. Besides getting a sudden bump in performance, if everything stays in CG limits why should it handle differently?

Think of it this way, the Evergreen tanker carried around 74,000 kg of water. A commercial flight crew fly the airplane no differently on a ferry flight versus a flight at MTOW, save except for flying different speeds etc.

Branzino wrote:

It's not that different from giant WW2 bombers dropping a load out of bombs when you really think about it. Besides getting a sudden bump in performance, if everything stays in CG limits why should it handle differently?

Think of it this way, the Evergreen tanker carried around 74,000 kg of water. A commercial flight crew fly the airplane no differently on a ferry flight versus a flight at MTOW, save except for flying different speeds etc.

Few crews get their load teleported in to or out of the hold during low level cruise
You have to get rid of 75t of lift or accept that the plane will go up or accelerate.

Branzino wrote:

Besides getting a sudden bump in performance, if everything stays in CG limits why should it handle differently?

At a given speed the aircraft produces a certain amount of lift. This amount of lift should be equal to the weight of the aircraft, otherwise you climb or descend. When the lift is suddenly (within a few seconds) 740,000N greater than the weight of the aircraft, this is something the pilot should counteract I would guess.

LAX772LR wrote:

I'm operating under the presumption that the flight handling characteristics would notably change; but not being a pilot, I don't know if that's an issue at all.

I would suggest the handeling would not change as they would keep the load within the CG envelope at all times.

I think the handling won't change too differently (maybe there are slight differences from CG, but still in the normal envelope) but the airplane will jump quite a bit (lost of weight means excess lift)

benbeny wrote:

but the airplane will jump quite a bit (lost of weight means excess lift)

This is exactly what I had in mind.
Perhaps the term "handling" doesn't accurately encompass/convey that.

I was just wondering how hard it is (if at all) for pilots to deal with this.

LAX772LR wrote:

benbeny wrote:

but the airplane will jump quite a bit (lost of weight means excess lift)

This is exactly what I had in mind.
Perhaps the term "handling" doesn't accurately encompass/convey that.

I was just wondering how hard it is (if at all) for pilots to deal with this.

recall seeing some kind of documentary on these types of aircraft, and the pilots saying that there was a strong nose-up tendency as soon as the load was dumped. They train to combat that and keep the aircraft in level flight.

LAX772LR wrote:

benbeny wrote:

but the airplane will jump quite a bit (lost of weight means excess lift)

This is exactly what I had in mind.
Perhaps the term "handling" doesn't accurately encompass/convey that.

I was just wondering how hard it is (if at all) for pilots to deal with this.

Just ride it out (as level as possible), until you stop climbing. Over a large fire, there won't be a pattern or traffic to worry about above.

For something like a 744, they may also be pretty close to stall at the drop. If you look at how far those flaps are extended, they're well below Vref anyway.

Dropping a large payload is obviously going to make the nose lift, but I believe having the flaps extended should subdue that somewhat due to the counteractive nose-down tendency.

Like others have said, if the CG doesn't move, it's just very quickly lightening the airplane, so just lower the nose unless you want to climb. If the CG shifts, you'll get a pitch moment in addition to the reduced weight, which (depending on the direction) would increase or decrease the pitch control needed at that moment.*

The big deal with cargo drops, though, is stuff like this, where the CG is definitely moving:
http://www.youtube.com/watch?v=64cU_p4dwVc&t=0m53s

Imagine you're doing that, but the stick breaks and the forward stuff stays put and the aft half of the stick falls out the back of the airplane. Or they're going out, but something breaks and they jam with the remaining stuff all towards the rear.

On the bright side, a parachute unfurling behind the airplane on a hung drop would help in a spin.

*Technically, if the CG didn't move at all but the weight decreased, the nose would drop because the center of lift is behind the CG, so a no-pitch-moment weight reduction would require the CG to move forward as the weight reduced.

Florianopolis wrote:

Like others have said, if the CG doesn't move, it's just very quickly lightening the airplane, so just lower the nose unless you want to climb. If the CG shifts, you'll get a pitch moment in addition to the reduced weight, which (depending on the direction) would increase or decrease the pitch control needed at that moment.*

The big deal with cargo drops, though, is stuff like this, where the CG is definitely moving:
http://www.youtube.com/watch?v=64cU_p4dwVc&t=0m53s

Imagine you're doing that, but the stick breaks and the forward stuff stays put and the aft half of the stick falls out the back of the airplane. Or they're going out, but something breaks and they jam with the remaining stuff all towards the rear.

On the bright side, a parachute unfurling behind the airplane on a hung drop would help in a spin.

*Technically, if the CG didn't move at all but the weight decreased, the nose would drop because the center of lift is behind the CG, so a no-pitch-moment weight reduction would require the CG to move forward as the weight reduced.

What about torque forces from the wing changing as the lift changes?

Wonder if they had to do any unusual CG tests or it was all theoretical and a pretty known quantity in the 744.

Fred

flipdewaf wrote:

What about torque forces from the wing changing as the lift changes?

Wonder if they had to do any unusual CG tests or it was all theoretical and a pretty known quantity in the 744.

Fred

I don't think they need to do that. The tank itself, being heavy, must be located as close as possible to the empty plane center of gravity, or at least within limits of CG.
Being said, if the tank itself is in the center of gravity, no matter how heavy the load it will not affect the CG itself.

PS: what's interesting, though, how do they manage to design baffles that will reduce sloshing while in the same time maintains high rate of delivery?

Florianopolis wrote:

The big deal with cargo drops, though, is stuff like this, where the CG is definitely moving:
http://www.youtube.com/watch?v=64cU_p4dwVc&t=0m53s

Those parachute extractions are not as bad on CoG shifts since everything happens in a blink of an eye. You can't trim fast enough to counter it.
Obviously when you get a hang-up or cargo stuck in the door the consequences are dire.

These type of gravity extractions take a lot longer and hence the pilots feel the forces way more
https://youtu.be/qijcEN1V95U?t=50

As said before sudden weight loss is really no problem. Nothing happens that sudden that negative G's are encountered, and as long as the CoG stays within limits no issue.

benbeny wrote:

flipdewaf wrote:

What about torque forces from the wing changing as the lift changes?

Wonder if they had to do any unusual CG tests or it was all theoretical and a pretty known quantity in the 744.

Fred

I don't think they need to do that. The tank itself, being heavy, must be located as close as possible to the empty plane center of gravity, or at least within limits of CG.
Being said, if the tank itself is in the center of gravity, no matter how heavy the load it will not affect the CG itself.

PS: what's interesting, though, how do they manage to design baffles that will reduce sloshing while in the same time maintains high rate of delivery?

Solid at the top of the tank down to a level close to the bottom, then slatted at the bottom, maybe at an angle like a partially opened vertical blind. The sloshing near empty isn't going to matter much, though it will impact CG on climb.

So maybe it's multichambered?

Maybe baffled with flapper valves to mitigate a sudden change in CG?

I am more interested in how the 744 maneuvers down to the ocean to pick up more water.

Flighty wrote:

I am more interested in how the 744 maneuvers down to the ocean to pick up more water.

lands, gets filled ( 30min ) Takes off.
There site has a lot of good pics and videos. some impressive stuff here.
http://globalsupertanker.com/b747-400-supertanker/

Just plug a new ZFW figure into the FMC and keep flying?

Beyond the weight change garnering more lift than needed, what if it was coupled with a strong thermal (you are flying over fires). That could be a fun ride. Wouldn't the pilot tend to fly slightly nose down on a (bombing) run in anticipation?

I was involved with the Tanker industry as a Government employee (Fire Fighter and Airport Dispatch) and had the chance to ride on B-17, PB4Y, DC4, 6 and 7 as a passenger on bomber runs. In most cases they are wanting to climb anyway to either enter another cycle on a split drop or to return to the Airport. One of the pilots told me that you have to enter nose down input but the loss of weight does make the plane want to climb. It is not really as extreme as you would think. I was on a run where we went down a canyon and they wanted to climb out after the run because of terrain and he just put some nose up input and we popped right over the ridge without any problem. An experienced tanker pilot uses the effect to thier advantage. Now these are all round motor (real Airplanes) so I dont know much about the heavies. That sure was a fun time in my life as an avgeek.

747 does pump out the water anyway, so the CG is controlled by the way the tanks are emptied. Older and smaller tankers drop water literally.