Trim tank
A trim tank is a component of a watercraft or aircraft, which is used to influence its position (also: trim ) in the surrounding medium. The fill level of this tank can be changed for this purpose. Depending on the fill level and the position of the trim tank in the watercraft or aircraft, the position of the center of gravity in this vehicle changes.
In commercial aircraft - introduced in the Junkers F 13 - regular fuel tanks are often used as trim tanks, and the fuel itself serves as the trim mass. Accordingly, an aircraft can only use its trim tanks if the fuel tanks are not all completely full. Trimming changes the aircraft's center of gravity. The purpose of the trim in flight is to reduce the aerodynamic force of the horizontal stabilizer, which acts as a downforce against the wing lift / moment. The advantage is that the lift or downforce of an aerodynamically effective surface generates an induced drag and by replacing the downforce of the horizontal stabilizer with the fuel weight in the horizontal stabilizer, the aircraft's drag is reduced and thus fuel consumption is reduced.
Ships, on the other hand, have special trim tanks that are filled with water from outside the ship for trimming. When trimming, the total mass of the ship changes significantly.
Aircraft
In larger commercial aircraft, the mass of fuel required for a flight is in the same order of magnitude as the empty mass of the aircraft itself and its payload. In the course of the flight, the amount of fuel available changes - the mass difference between full and empty fuel tanks is easily several tens of tons. If an airplane only had one fuel tank that was exactly in its center of mass, this would not be a problem. However, this is constructively impossible. Most aircraft use spaces in their wings as fuel tanks, and they also have tanks in the fuselage between the wings (in the wing box). The center of mass of these tanks will not necessarily coincide with the center of mass of the empty or loaded aircraft when full. Since the position of an aircraft can be influenced by means of its control surfaces ( i.e. elevator , rudder and ailerons ), it is in principle possible to compensate for the change in trim that is caused by the consumption of fuel during flight. However, the air resistance and thus the fuel consumption increases if the control surfaces are constantly deflected. For this reason, at least two fuel tanks are divided lengthways in the aircraft fuselage so that the trim can be compensated. A system of remotely controlled valves and pumps allows fuel to be pumped back and forth between the tanks. In the transverse direction, the same amount of fuel is normally used by the engines on both sides, so there is no need to trim separately in this axis. However, if an engine fails or is only operated at a lower power, the fuel distribution becomes asymmetrical. In this case, you must also balance in the transverse direction by pumping fuel from one side to the other.
Watercraft
Many ocean-going vessels use bulkheads of their bilge as one or more trim tanks in the longitudinal direction. Seagoing cargo ships are designed in such a way that they lie well in the water when fully loaded; then their construction waterline coincides with the actual waterline. When sailing with little or no cargo, the hull would emerge so far that the bow of the ship largely or completely protrudes from the water. The tail would continue to be due to the weight of the machine significantly lower in the water - but may not be deep enough to the propeller to immerse sufficient. This would negatively affect the controllability and sailing characteristics of the ship, as well as fuel consumption. In order to reduce this effect, the bilge of the ship is completely or partially filled with ballast water when sailing with little or no cargo .
Some types of ships also have trim tanks in order to be able to change the trim of the ship for special purposes. A historical example of this are the LST (landing ship, tank) landing ships of the US Navy towards the end of the Second World War . These seagoing ships of a good 100 meters in length were built to carry heavy loads (tanks and other military vehicles) from the sea onto sandy beaches with a relatively low dead weight. They had a system of trim tanks that had been adopted from a standard type of submarine at the time. With trim tanks in the longitudinal direction, the trim of the ship could be adjusted so that the bottom of the ship coincided with the slope of the beach for a safe landing.
In the case of trim tanks on seagoing ships, it should be noted that these should be either completely full or completely empty if possible. In only partially filled tanks, wave movements will build up in the sea (the tank content sloshes back and forth), which can lead to resonance effects with the external movements of the ship through the sea. This can have a negative impact on the stability of a seagoing vessel in rough seas. In strong storms this can be dangerous for the ship. This effect is known as the "free surface effect". It can be reduced by structural measures in the tanks, for example by installing horizontal or vertical partitions ( baffles ) which are only broken through with relatively small openings and thus dampen the movement of the liquid .
Individual evidence
- ↑ Sebastian Steinke: Return - Rimowa builds Junkers F 13 . In: Volker K. Thomalla (Hrsg.): Classics of aviation . tape 7/2015 . Motor Presse Stuttgart GmbH & Co. KG, Stuttgart 2015 ( Klassiker-der-luftfahrt.de [accessed on January 30, 2019]).
- ↑ RS Crenshaw Jr. (Ed.): Naval Shiphandling . 2nd Edition. Naval Institute Press, Annapolis MD 1960.