Understanding Trimming & Rigging

[AKar]

I recent discussions regarding the T-6's "ground adjustable" trim tabs reminded me of several older topics on trimming and turning tendencies and everything. So I decided to write a very brief introduction on what are the differences in between trimming and rigging, in this context. While it may seem obvious on a first glance, these "ground adjustable" tabs tend to make the difference a bit fuzzy.

An airplane is designed and built to fly with predictable characteristics throughout its envelope. Airplane also has flight controls to alter how it flies. In principle, rigging adjusts the former, while trimming adjusts the latter.

One way to put it is this:

• • Trimming is used to relieve control pressure necessary to achieve desired flight.
  • Rigging is used to achieve intended flight characteristics with fixed stick, and to achieve intended flight control ranges of movement.

In general, "fixed stick" is not a simple topic on its own.

Another way to put it would be this:

• • When you carefully neutralize the physical ailerons at the wings on ground, and adjust the stick to be at center position concurrently, that is rigging.
  • When you alter your aileron trim tab on ground to relieve your need to hold the stick at desired position at a given airspeed, that is trimming.

So, we can make yet another one:

• • Trimming affects the airplane's flight via flight controls.
  • Rigging affects the airplane.

An airplane is a relatively large collection of structures, all them together affecting the flight. Thereby it is essential, for their design to reproduce itself well enough on an actual individual airplane, that they all are how the designer intended. To accomplish this, it is important that an airplane is correctly rigged. That may include (but is not necessarily limited to):

• • having the wings and stabilizers at their designed positions in relation to their fuselage references;
  • having the flight controls moving correctly and symmetricly throughout their designed ranges of movement, and having the correct limits of movement;
  • having the cockpit controls properly positioned in relation to the control surfaces' positions.

This alone makes the airplane to behave, more or less, within a given tolerance, like its designers intended it to. But it is not enough to "make it fly straight all the time".

Of course, every airplane is an individual, every loading differs and no flight is alike when it comes to the power settings, air speeds and so on. That's why there is a pilot, to put in some control effort to make the airplane do what (s)he desires. For those control forces that change significantly, and on constant basis throughout the envelope, there are typically pilot-adjustable trims. Those are in place to relief any constant, applied force required to hold the desired flight. For those control forces that change not-so significantly, and on not-so constant basis, there often are ground-adjustable trim tabs, to balance an individual airplane to one's individual needs. Of note is that it would always be a compromise - no real aircraft can be perfectly steady thorough its envelope, and one must either set up his tabs for the flight conditions the perfect relieve is most desired in, or one must invest in an airplane having three axes of pilot-adjustable trims - and learn how to trim her properly!

Then again, every airplane is an individual. There are given manufacturing tolerances. They get damaged, they get repaired, and in cases of multi-engine airplanes, they get a pair or more of individually acting engines. They gain weight here and lose aerodynamic efficiency there - again regained better in this place, when an STC is bolted in - having its own tolerances and everything. So, a real airplane tends to have its individual traits. Typically, trims are used to adjust those out: it is not that rare really to have an airliner that requires significant trimming for hands-free flight, with hands-free results being even larger than those on A2A 172 on takeoff climb. It is not even unheard to have a fly-by-wire fighter with a mention in its tech log that it requires atypical trimming due to certain repair for instance. As these traits are most often comparatively small, and tend to change their effects throughout the flight envelope, trimming is the primary way to deal with these.

However, sometimes an airplane can have some constant, abnormal flight characteristics, requiring noticeable and linear-throughout-the-envelope amounts of that abnormal trimming, for instance. A reason can be a disassembly + assembly, or a major repair...modification or whatever! Then we would bring up the manufacturer's instructions on how to deal with wing-heaviness for instance. The starting point would be to rig the airplane to its specs, checking that each angle matches the design datum. Also, it is important to know how the airplane is supposed to behave, as per its type design - not by just some report. Thereby, a knowledgeable and experienced test pilot would be needed. If it was confirmed that the plane was rigged to the nominals, and that did not relieve the need for unquestionably abnormal trimming/control inputs, then it would be time to alter the rigging from the nominal.

Altering the rigging is always a maintenance action, performed in accordance with approved maintenance data, returned to service by approved certifying staff, and in principle by definition, is an action taken to keep the airplane within its design characteristics. It is not an action to alter it therefrom.

Trimming instead is a pilot-made or in-line kind of action, done by means made available without any need to alter the construction.

I may make an illustrated article to point out the topic further in the future, but that depends. Also, definitions may vary some, so be aware.

-Esa