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
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.
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