Ballast tank

A trim tank is a component of a water or aircraft, which serves its location: to influence (including trim ) in the surrounding medium. For this purpose, the level of this tank can be changed. Depending on the level and the position of the trim tanks in the vessel or aircraft by changing the position of the center of gravity in this vehicle.

For commercial aircraft, often regular fuel tanks be used as a trim tank, and the fuel itself serves as a trim mass. Accordingly, it can only use its trim tank an aircraft when the fuel tanks are not all full. By trimming the center of gravity of the aircraft modified. The purpose of the trim is in flight is to reduce the aerodynamic force of the horizontal stabilizer, which counteracts as output the wing lift / torque. The advantage is that buoyancy or output an aerodynamic surface generates an induced resistance and by replacing the output power of the horizontal stabilizer through the fuel weight in the tailplane, the resistance of the aircraft is therefore reduced, thus reducing the fuel consumption.

Ships, however, have special trimming tanks, which are filled with water from outside to trim the ship. When trimming, the total mass of the ship changed significantly.

Aircraft

For larger commercial aircraft is the mass of the fuel that is needed for a flight, in the same order as the empty weight of the aircraft itself and as its payload. The mass difference between full and empty fuel tank is easily tens of tons - During the flight, the amount of available fuel changes. If a plane had only one fuel tank, which would be exactly in its center of gravity, this would be no problem. This is structurally impossible. Most aircraft use spaces in their wings as fuel tanks, and have also still tanks in the fuselage between the wings (in the wing box). The center of mass of these tanks will not necessarily coincide in full state with the mass of the empty or loaded aircraft. Since (ie treble side and aileron ) can influence the position of an aircraft by means of its control surfaces, it is possible in principle to compensate the change in the trim, which is caused by the consumption of fuel during the flight in this manner. However, the air resistance and thus the fuel consumption increases as the control surfaces remain deflected continuously. For this reason, the length of at least two fuel tanks are divided in the aircraft fuselage, so that the trim can be compensated. By means of a system of remotely controlled valves and pumps can fuel between the tanks be pumped back and forth. In the transverse direction is usually the same amount of fuel consumed by the engines of both sides, thus does not need to be trimmed separately in this axis. But if an engine fails, or should only be operated with low power, fuel distribution is unbalanced. In this case must also be balanced in the transverse direction by the fuel is pumped from one side to the other.

Watercraft

Many sea-going vessels use by bulkheads separate sections of their bilge as one or more trim tank in the longitudinal direction. Seagoing cargo ships are designed so that they are good with a full load in the water; then drops her design waterline along with the actual waterline. When traveling with little or no charge to the hull would appear so far that the bow of the ship largely or completely out of the water. The tail would continue to be by the weight of the machine significantly lower in the water - but might not be deep enough to immerse the propeller sufficient. Thus, the controllability and handling characteristics of the vessel, and the fuel consumption would negatively influenced. To reduce this effect, the bilge of the ship when traveling fills with little or no charge wholly or partly with ballast water.

Some types of ships have also trim tank in order to change the trim of the ship for special purposes can. A historical example of this is the landing ships of the LST ( landing ship, tank ) of the U.S. Navy at the end of the Second World War. This sea-going vessels of more than 100 meters in length were built to spend heavy loads ( tanks and other military vehicles) from the sea on the sandy beaches at relatively low weight. They had a system of trimming tanks, which had been taken over by a former standard type of submarines. With trimming tanks in the longitudinal direction of the trim of the vessel could be adjusted so that the bottom of the vessel consistent with the inclination of the beach, in order to enable a safe landing process.

When trimming tanks on ships should be noted that this should be possible either completely full or completely empty. In only partially filled tanks wave motions in a seaway will build up ( the tank contents sloshing back and forth ), which can lead to resonance effects with the outer movements of the ship through the seas. Thus, the stability of a ship in a seaway are adversely affected. In severe storms, this can be a danger to the ship. This effect is referred to as " free surface effect" in English. It can be reduced, for example by a horizontal or vertical partition walls to be installed, which are interrupted only by the relatively small openings, and thereby damp the movement of the liquid in the tanks by structural measures.