The elevator is the central element in aircraft trim. Aircraft are designed to keep their speed behaviour stable; in other words, they strive to automatically return to their original position or speed after a disturbance. For example, if a pilot wants to reduce speed in unaccelerated level flight, he reduces power. However, without adjusting the trim, the aircraft would not simply slow down, but would instead nose down to maintain speed.
To compensate for this behaviour, the pilot must constantly pull on the controls without trim to keep the nose of the aircraft up. This effort can be neutralised by trimming the elevator, which makes flying much less strenuous. Trimming is usually done electrically via a switch on the control horn or mechanically via a trim wheel.
Automatic trim in modern aircraft
In modern aircraft, especially in Airbus fly-by-wire systems, trimming is carried out fully automatically by the computer without the pilots having to intervene. These systems constantly optimise the flight attitude and ensure the most efficient and safest flight performance.
Trimming techniques for different aircraft types
The type of trim varies depending on the aircraft type. On smaller aircraft, the elevator is often held in the desired position by spring force or by a small aerodynamic trim tab. This flap is deflected in the opposite direction and thus stabilises the rudder. On larger commercial aircraft, it is not the rudder itself that is adjusted but the entire horizontal stabiliser (trimmable horizontal stabiliser). This makes it possible to control the nose of the aircraft effectively with minimal rudder deflection, which reduces aerodynamic drag and optimises fuel consumption.
Importance of trim for different load conditions
Trim is particularly important for reacting to different load conditions and centre of gravity shifts during flight. For fully loaded or unevenly loaded aircraft, the correct trim can be crucial to keeping the aircraft stable and controllable. On long-haul flights, where fuel consumption changes the centre of gravity, the trim is continuously adjusted to ensure optimum flight conditions.
Trimming the other rudders
Not only the elevator, but also the other control surfaces such as the rudder and ailerons can be trimmed to further improve flight stability and overcome specific flying challenges, such as compensating for an engine failure.
Source references:
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