Why does an airplane need trim, and what does it do during the flight? Does an autopilot adjust the trim automatically?
The purpose of trimming is to free the pilot from having to exert a constant pressure on the controls. This is often used to maintain straight and level flight, however trimming can also be used at any phase of flight - for example to maintain a constant rate of climb or descent.
The most basic form, as found on most light aircraft is elevator trim. Usually operated by a wheel, it moves the elevator up or down by a small amount in the same sense as the yoke (back to go up, forward to go down). This can be used to settle the aircraft into straight flight.
Another form of trim is rudder trim. Often found in larger light aircraft and twin-engined aircraft it can be used to adjust for a crosswind to keep the aircraft flying straight. On multi-engine aircraft it can also be used to trim out the differential thrust caused by one engine failing.
In short: trimming neutralizes the force required to keep control surfaces in a specific position.
Most (if not all) aircraft have some sort of elevator trim control. For example, when the pilot has to keep pulling back on the yoke/stick during a climb, trimming "nose-up" will neutralize that force. The elevator will then remain in the same position without any force required on the part of the pilot. Trimming is usually done by means of a Trim tab on the control surface and is controlled by a trim wheel.
Elevator trimming is used:
- During climbs: to maintain a constant air speed and rate of climb
- During descents: to maintain a constant air speed and glide path
- During level flight: to maintain altitude and speed
Although elevator trim is the most common, trimming can also be done on rudder (quite common) and ailerons (only on larger aircraft).
Rudder trim is used to maintain coordinated flight without rudder input by the pilot. Many single engine planes with powerful engines require rudder trim to offset the "left-turning tendency" caused by P-factor and propellor wash hitting the rudder. Aircraft without adjustable rudder trim will usually have a fixed trim tab on the rudder.
Most autopilots will control the elevator trim wheel because the servos controlling the elevator could easily be overpowered by the required force.
It's important to note that in most cases (especially on small airplanes) trimming doesn't actually move the control surfaces. It changes the force required to deflect the control surfaces. Although there are many types of trimming devices, the common trim tab makes the airflow do all the work. The trim tab causes the airflow to push the control surface in a specific position. On larger aircraft with hydraulically operated control surfaces, trim tabs are less common (see also this answer to another question)
Trimming is changing the stability of an aircraft in such a way that when no control inputs are given, the aircraft maintains its attitude and speed. It allows the pilot to release the control input without the aircraft deviating from the intended path. In general the aircraft is trimmed for straight and level flight, but it can also be trimmed for descent or even for turns.
There are various ways in which an aircraft can be trimmed. Trimming most commonly refers to elevator trim. This involves changing the angle of incidence of a trim tab on the elevator in such a way that when the control column is released the aircraft does not pitch up or down by itself. In large aircraft, the angle of whole horizontal stabilizer can be changed to trim the pitch tendency of the aircraft. Aircraft that have a fuel tank in the tail can move the centre of gravity by transferring fuel between forward and aft fuel tanks. Use of CG trim not only affects the pitch tendency of the aircraft but also reduces the fuel burn since less aerodynamic forces are needed to balance the aircraft, thereby reducing the drag.
Other forms of trim are aileron trim and rudder trim. These are used to nullify any asymmetric characteristics of the aircraft. These characteristics can be caused for example due effects of propeller wash, mechanical deformation of the aircraft (or poor manufacturing tolerances) or engine out situations.
On most larger aircraft the autopilot uses elevator trim or horizontal stabilizer trim to control the aircraft in pitch in addition to elevator control. For turning, ailerons and rudder are used, the asymmetric trim is not used by the autopilot.
Autopilot theoretically could not care about the trim but if you disconnected it and plane was out of trim in could depart flight path rapidly.
Say you have trimmed an Aircraft than forces on aircraft i.e Lift == Weight & Thrust == Drag. So aircraft is flying (straight and level) or climbing or descending at constant speed or rate(of climb or descend).
Simple to remmember, Trimmed Aircraft means Lift = Weight and Thrust = Drag thats it.
There are lot of way to trim an aircraft: Trim Tabs on control surface, complete surface moves to trim corresponding control surface (ex: stabilizer moves in response to pitch trim command) etc.