When trimming is called the aligning bodies into a desired position. This term is used particularly often in connection with watercraft or aircraft . The trim forces are supposed to balance the forces that would change the position of the body.
Types of trim
A distinction is made between static and dynamic trim:
- With static trim, the center of gravity is shifted by shifting mass ( pumping fuel into different trim tanks, adding ballast ). The trim forces set in this way are largely independent of the condition of the vehicle.
- With dynamic trimming, the trimming forces are generated by adjusting or presetting aero- or hydrodynamic surfaces. These trimming forces change with the speed of the object.
Importance for the vehicle types
An aircraft is considered to be trimmed if no control forces of the pilot have to be applied to maintain its flight position in a certain aerodynamic situation. For this purpose, the control surfaces of the elevator, ailerons and rudder are aligned accordingly. The power required for this comes from electric motors and hydraulic pumps in commercial aircraft, and from simple mechanical devices such as spring trim or Flettner trim in small aircraft , which the pilot adjusts using muscle power and readjusted if necessary.
The need for readjustment arises from a change in speed, a change in the position of the landing flaps (elevator trim) and asymmetrical thrust in the event of an engine failure on a multi-engine aircraft (rudder trim). In addition, changes in the center of gravity due to movements in the passenger cabin and due to the escape of fuel lead to the need for readjustment of the elevator trim. If the aircraft is flown by hand, the pilot first initiates the maneuver with the appropriate control movement and the associated change in power or the corresponding flap change and then sets the trim so that he no longer has to use any further force to maintain this flight position. When using a modern autopilot, the autopilot usually performs this task at the same time.
With the Flettner trim , a small trim tab attached to the end of the elevator is adjusted so that the rudder itself produces an effect in the opposite direction. To get an aircraft z. B. to trim top-heavy, the elevator must be deflected downwards. This can be achieved by deflecting the Flettner trim tab upwards.
In shipping , the term “trimming” refers to the “twisting of the ship around the transverse axis” or the creation of the desired floating position with regard to this rotation around the transverse axis.
The trim can be influenced by several factors:
- Weight trimming is the influencing of the swimming position through the distribution of moving weights such as cargo, fuel or ballast, rarely also of the crew and any passengers in the longitudinal direction of the ship.
Dynamic trim refers to the trimming through measures that are dependent on the speed or the requested drive power of the vehicle, i.e. do not influence the swimming position at rest:
- Trim tabs are often found on the stern of smaller motor boats. In the rest position, they lie horizontally; if they are tilted down, they create buoyancy at the stern and at the same time push the bow deeper into the water.
- Trim devices on outboard motors allow the engine to be tilted forwards or backwards. If the motor is tilted forwards, the drive power no longer acts directly backwards, but also slightly downwards. This pushes the stern down and at the same time raises the bow. This trimming makes it possible to influence the sailing characteristics of the boat.
- Influence of the trim through basic effects occurs, for example, when large ships sail in shallow waters and the hull is sucked down by pressure changes under the ship
In general, one tries to trim a vehicle to be neutral, that is, to let it float as level as possible. Under certain circumstances, however, an fore or aft trim can be advantageous:
- Smaller vehicles aim for a tail-heavy trim when traveling at high speed and in heavy seas or when being towed in order to avoid undercutting (diving into a wave ahead).
- Fast sailing and motor boats aim for a stern-heavy trim in order to lift the front part of the hull out of the water. This reduces the area wetted by the water and reduces the water resistance. This enables a higher speed.
- Under certain conditions, sailboats are trimmed to be a bit bulky to allow the bow to submerge further and thus lengthen the waterline. This can enable higher speeds (see torso speed ).
According to the German standard DIN 70000 “Road vehicles; Terms of Driving Dynamics ”from August 1983, Trimm is “ the constant stationary reference state of the movement variables for the analysis of the lateral dynamic properties ”of a multi-lane road vehicle; the trim "can be determined by specifying the driving speed and path radius or centripetal acceleration and steering wheel angle".
- Götsch, Ernst - Luftfahrzeugtechnik , Motorbuch Verlag, Stuttgart 2003, ISBN 3-613-02006-8
- Federal Institute for Hydraulic Engineering: Interaction between seagoing ships and shipping routes - ship dynamics. Accessed on September 25, 2017.
- Mercury Marine: Operational Manual pp. 59/60, accessed November 21, 2014.
- Bundesanstalt für Wasserbau: Influence of ship length and keel clearance on squat and trim of very large container ships.Retrieved on November 21, 2014.