Throughout a flight, the pilot will need to keep the nose attitude of the aircraft in different positions for extended periods of time. Initially, the aircraft is in a nose high attitude while climbing but after leveling off the nose attitude is kept lower. When a descent is started, the nose attitude is again lowered.
The different attitudes are selected and held through use of the elevator. However, the forces needed on the control column to keep the nose in the chosen attitude can be significant and would be tiring for a pilot to hold for any length of time.
A small moveable tab, known as the trim tab, is hinged on the trailing edge of the elevator. When the tab is deflected upwards, a downwards force is created at the trailing edge of the elevator, meaning less forward pressure is needed on the control column to maintain a lower nose attitude.
Conversely, when the trim tab is deflected downwards, an upwards force is created at the trailing edge of the elevator, reducing the backwards pressure needed on the control column in order to maintain a higher nose attitude.
When the pilot moves the elevator, a simple trim tab like this will remain in the same position relative to the elevator.
The trim tab is connected to a trim wheel in the cockpit and rotating the wheel incrementally adjusts the trim tab. Once you have placed the nose in the desired attitude with forward or backward pressure on the control column, rotate the trim wheel until no pressure is needed on the control column to maintain your selected attitude.
This is much easier said than done and many student pilots struggle with precise trim control at first. It can help to let go of the control column and see if the nose pitches up or down. If so, you will need to tweak the trim setting slightly.
Many small aircraft also have fixed metal trim tabs attached to the ailerons and rudder. These work on the same principle as regular trim tabs but are not adjustable from the cockpit.
Instead, they can only be adjusted by hand on the ground by bending the metal tab slightly. This helps to remove any continuous control force on the ailerons or rudder that is required in flight.