Hidden Forces: Trim

Tim Takeoff
10.05.2019
2 pictures
3 minutes

Getting an airliner off the ground is one thing. Steering it gently and safely to its destination through narrow airspaces and under any conditions is quite another. One of the most important systems for both pilots and planes is the trim system.

Ever since the dawn of flight, aircraft designers have used mechanical aids to reduce the forces transmitted from the control surfaces to the control column. Pilots can use springs, cables or electrically powered aids to fine-tune the aircraft’s flying position relative to one or more of its axes. Trim systems have attracted negative attention recently, not least because of the problems with the Boeing 737 Max, in which an automatic trim system led to two catastrophic crashes. You can read more about the causes of the accidents here.

The axes

When we talk about the “axes” of an aircraft, we are talking about three imaginary “bars”. The longitudinal axis runs from the plane’s nose to its tail. The pilot can use the ailerons on the wing to make the plane “roll” around this axis. Perpendicular to this axis is the “yaw axis”, which is controlled using the rudder on the plane’s tail. Finally, the pilot uses the elevator to control the plane’s climb and descent by altering the angle of the plane’s “pitch” around its lateral axis, which (metaphorically speaking) runs from one wing tip to the other.

Trimming using the elevator

In smaller planes without autopilots, such as gliders or smaller powered aircraft, trimming the elevator alone is often sufficient. The pilot simply pulls on the elevator controls to climb or pushes to descend. If the pilot wants to increase the flying speed, the elevator must also be extended slightly downwards in order to prevent the plane from climbing instead of accelerating. 

Widely used technology

Trim systems save pilots from having to exert continuous physical strength to counteract the forces on the yoke. As there are different types of elevator, there is a wide variety of diverse systems. An adjustable horizontal stabilizer, for example, can be controlled either via a spring in the control system or hydraulically. A “damped” elevator often uses what are known as a “servo tab” for trimming. In such cases, an additional small surface is mounted on the elevator. The trim system extends this small tab out in the opposite direction of the elevator. This holds the main elevator in position and minimises the control forces needed.

Trimming an airliner’s elevator unit  

Airliners normally use a combination of several hydraulically operated systems. The elevator itself consists of two interdependent systems. One of these is a kind of adjustable horizontal stabilizer which uses a spindle to hydraulically position the entire stabilizer at certain angles. The other is a damped elevator. This is in direct contact with the pilot’s yoke and converts his or her actions directly into changes in the plane’s position in the air. In modern “fly-by-wire” aircraft, these commands are transmitted electronically. The control column of a Boeing, for example, includes trim switches which are operated by the pilot.

Aerodynamic perfection

From a practical perspective, if the pilot wants to climb, he or she will first pull on the yoke. The elevator will extend upwards, raising the plane’s nose. As the pilot does not want to have to keep pulling on the yoke, he or she now uses the trim function. This causes the entire elevator to move the stabiliser into a different position. The elevator no longer needs to be extended, as the entire surface is now perfectly positioned within the air current. Using the trim function frees the pilot to attend to other tasks, as well as reducing the plane’s aerodynamic resistance.

Automatic and redundant

If we then switch on the autopilot, it will do basically the same thing. It always works with the elevator first, before using the stabiliser to perfectly trim the aircraft’s flying position. In most cases, the elevator itself is connected with two independent hydraulic circuits. As a fail-safe, the elevators in modern fly-by-wire planes are also connected with the pilot’s yoke using emergency control cables.

An airliner, of course, can be trimmed around all three axes. Each axis can, therefore, be finely tuned during flight in order to keep the resistance low or to counteract small discrepancies in the weight distribution. This ensures precise control of the steering and – not least – a smooth ride for passengers.

by Tim Takeoff

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