So i worked on a ship and i know we had various tanks that could be filled and emptied to modify our CoG
in an airplane that could be extremely dangerous and could have several thousand kilo’s of sewage after a long flight how do they maintain such a critical factor for flight?
EDIT:
I understand that flying is different from floating i was merely using it as an example of a system i knew and how it worked
In: 8
The amount of freshwater and sewage on board a commercial airliner is minimal compared to the mass of the jet fuel, which at tens of thousands of liters per flight can weigh tens of thousands of kilos, making up to a quarter of the plane’s takeoff weight. The trim of the aircraft is maintained in part by actively pumping fuel between various tanks, which have plentiful anti-slosh baffles built in.
Most of the tanks with variable weight liquids such as the fuel tanks and such are located in the center of the airframe. For example the interior of the wings are almost full of fuel tanks. So they just need to make sure to load the cargo evenly before takeoff. They do also have the ability to transfer fuel between different fuel tanks.
A ship needs a certain amount of wait to keep stable. If the ship is too light it will sit higher in the water and the weight of the superstructure may be enough to tip it around. This is why you have ballast tanks for ships. However airplanes can fly just as stable no matter how light they are. There is some center of gravity issues, mostly front and back. However nothing like for ships. And most of their cargo is centered around the center of gravity anyway.
The real answer is they adjust the elevator (movable horizontal surface on the tail) to compensate.
So, the CG moves forward, the elevator provides more downward lift to compensate.
CG moved back(aft), elevator adjusts to provide less negative lift.
As others have said, poop in the shoot vs. poop in the toilet doesn’t change the weight of the plane. Same with the weight of soda in a can vs. soda you have drank. Slight shift due to location, but the overall size doesn’t affect it much.
[Tank wasn’t tied down in takeoff and shifts…causing crash](https://www.theguardian.com/world/video/2013/may/01/747-cargo-plane-crash-bagram-airbase-video)
This is what happens when the shift is large enough.
Main reason that it’s easier on a plane than on a ship: plane is flying, not floating.
When a plane flies, there’s a lot of air that you can redirect with control surfaces, aka ailerons ruder and elevator, and the big boy, the horizontal stabilizer. The huge tail plane or horizontal stabilizer, is pushing the tail down all the time while the front of the plane is heavier than the rear. This is done on purpose as the plane will nose down automatically if you stall a wing. It makes flight safe. Now, the horizontal stabilizer is. It fix, you can move the entire huge thing 3 degrees up (pushes nose down) and 10 degrees down (pushes nose up). That’s an enormous amount and it allows you to trim the plane with a very big force. This is enough to compensate wide shifts in weight fed or aft.
For roll, you have ailerons at the wing tips and all the variables weight very close to centerline (aircraft body is quite narrow compared to wings). Ailerons can easily compensate for a lot of things.
If things go sour in the roll department, you can transfer fuel between left and right wing tanks, that’s around 5-80k pounds.
Take all of this, and all you need is to load the aircraft to take off with a proper center of gravity, with the goal to have the plane fly perfectly balanced with all the above corrections set to neutral.
At this point, you can use all of the above corrections to restore a balance. These corrections are sized to deal with engine loss, wing icing, bad weather, and structural damages like losing an entire landing gear (up to 3000kg), or an engine detaching from the plane. You can’t poop enough to get to that scale of asymmetry, but you can try.
Las point: poop is stored close to the toilet and freshwater too. In theory one is emptying and the other is filling.
I get why you ask, on a boat you have a lot more balance issues, less ways to address it and the free surface effect that may allow massive shifts in center of mass during a flood. So I bet you have to create proper safety margins and keep adjusting your ship to keep it perfectly at the safest balance point.
They don’t quite, as far as black water is concerned. There’s a simple way to offset this, which I’ll explain in a moment.
The wastewater tank is usually about 100-200 liters depending. If half full that’s about the weight of one passenger of 75kg. It would be unusual for all the passengers on a jumbo jet to produce 150 liters of waste in an 10 hr flight. So this is basically like if two flight attendants moved from the front to the back of the plane. While this isn’t exactly negligable, it’s not a big deal for the aircraft controls to deal with.
Main source of weight onboard is the passengers and fuel.
I believe that on some aircraft, excess black water can be dumped overboard in an emergency, however don’t quote me on this. laws in most countries prohibit random dumping of sewage so you could only do that over the ocean.
The fresh water is accounted for. Typically there are a pair of tanks fore and aft holding several tens of liters. This is enough to supply water for flushing, washing hands, and making coffee or tea. Again that’s not a very significant amount of weight compared to potentially 100-200 tons of fuel.
On airliners, there are trim tanks in the rear horizontal stabilizers, as well as the wings. Sometimes in the vertical tailfin as well though that’s unusual.
On a 747 for example, the tanks in the rear stabilizers can carry an impressive 12500 liters when full, which is roughly 11 metric tons of Jet-A fuel.
Typically such tanks are kept only partly filled, and fuel is transferred into or from the rear tanks from the series of wing tanks to adjust gross weight balance by shifting the center of gravity forward or aft. This can be done by the autopilot on some aircraft, on a long flight it would be necessary to adjust several times to account for fuel consumed from the wing tanks.
On the ground this is done based on weight sensors in the landing gear struts. In the air it’s based on a number of inputs such as angle of attack, elevon flap angle airspeed, and others.
Typically this is also done before landing as weight balance has a particular influence on the margin of error at low flight speed and high angle of attack. Specifically if the aircraft is too tail heavy then a greater nose up angle is necessary which reduces the safety margin for error before stall occurs. This kind of weight balance issue was notorious on the MD-11 tri-jet which was designed to be more tail heavy than typical airliners. While this results in fuel savings during cruise at was believed to be a major factor in several MD-11 crashes.
In an emergency such as an engine failure, pilots may choose to dump extra fuel overboard to accomplish a number of things. Such as offsetting the asymmetric thrust from a single engine and reducing flight speed. This would be done after the pilots have determined the best place to make an emergency landing, and obtained clearance there. This isn’t commonly done though. On some very large aircraft the max takeoff weight with full fuel is actually significantly more than the max allowable landing weight. Therefore if the aircraft needed to land soon after takeoff because of some emergency, they wpuld have to jettison most of the fuel before landing.
>in an airplane that could be extremely dangerous and could have several thousand kilo’s of sewage after a long flight
Assuming everyone aboard takes a dump and pee on a 16 hour flight, that’s still maybe half a tonne of sewage at most, spread out between several washroom points.
In comparison, the Boeing 747 variants can hold at least 180 tonnes of fuel, mostly concentrated at the wing area.
That half ton would be nothing, you’ll probably get more CoG shift from half a dozen people getting up from their seats and walking around the cabin. If there is a problem, the pilot or autopilot can just adjust by trimming the flight controls.
I got to wondering about something like this when the captain announced that we were going to be landing soon. he suggested that you visit the lav now if you need it since the seatbelt light will be going on in a few minutes.
I was sitting near the center and at least 1000Kg of passengers headed to the rear lav. I wondered if they could feel that shift if they were hand flying.
Latest Answers