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I know that historically pilots used to trim an aircraft to relieve continuous application of force during climb/cruise/descent, and at that trim tabs existed on control surfaces (elevator, ailerons, etc) which could be used to hold the position of the control surface without the pilot applying any force. But now, we don't usually have trim tabs, and with fly-by-wire systems forces have been reduced on the pilot.

Does trimming do anything other than reduce pilot workload? Also:

  1. Do modern aircraft still follow this concept of trim to reduce pilot's continuous force on the flight controls?

  2. Apart from pilot workload and fuel efficiency (I know that trimming an aircraft can produce drag), what other benefits does trim offer?

  3. Without trim tabs, how is trimming accomplished?

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    $\begingroup$ Many of us are still flying planes built in the '60s and '70s, if not before, so 'modern aircraft' and fly-by-wire are concepts that apply to a very limited range of pilots. $\endgroup$
    – jamesqf
    Commented Apr 2, 2015 at 19:00
  • $\begingroup$ Trimming a plane is like doing wheel alignment on your car. Sure, you can drive an un-aligned car but having to constantly hold the steering wheel left while driving in a straight line gets annoying (and dangerous) after a while. $\endgroup$
    – slebetman
    Commented Jul 15, 2015 at 20:39
  • $\begingroup$ You may find relevant pieces of answers in this question $\endgroup$
    – Manu H
    Commented Aug 21, 2017 at 17:43

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Almost all airplanes have trim (I'm fairly certain that the Wright Flyer didn't, so I can't say all airplanes do), but a lot of newer designs don't use trim tabs because they are less aerodynamically efficient than alternative designs.

For instance, the Falcon 50 & 900 that I fly doesn't have a single trim tab on it. Normally the flight controls are moved by hydraulics and a small electrical actuator is used to adjust the position of the aileron and rudder for trim purposes. Pitch trim is accomplished by moving the front of the entire horizontal stabilizer in order to adjust the angle and amount of lift produced by the tail. This is done so that the elevator can be neutralized and not create the additional drag caused by being in the airflow.

On FBW airplanes, they may or may not have manual trim that is used (under normal circumstances) by the pilot. Trimming is still taking place on FBW airplanes without manual trim, but it is simply automated.

Boeing typically designs their FBW airplanes so that they behave like a "normal" airplane, and require the pilot to trim in order to relieve the control forces. This is all completely artificial though, programmed simply to give feedback to the pilot.

Airbus on the other hand doesn't give any feedback to the pilot through the side stick so there are no control forces to relieve. When you release the side stick, it centers and the aircraft maintains its current pitch and bank angle. In this case, the autopilot automatically trims the airplane for the current conditions, and the pilot doesn't need to worry about it.

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  • $\begingroup$ It might be worth noting that while the trim in Airbus is normally operated automatically, the trim wheel has direct hydraulic link to the stabilizer and is used as backup for case of total electrical failure (which I've never heard of happening in practice). $\endgroup$
    – Jan Hudec
    Commented Jun 2, 2014 at 8:05
  • $\begingroup$ This is control surface trimming. It might also be useful to mention weight-based trimming such as tank trimming, which is even more aerodynamically efficient as it doesn't add induced drag from changing the angle of attack of control surfaces. $\endgroup$
    – KeithS
    Commented Jul 15, 2015 at 16:51
  • $\begingroup$ Some light airplanes also have a horizontal stabilizer that pivots, rather than using a trim tab. Mooneys, for example, use a jack screw attached the the pitch control system in the cockpit and to the horizontal stabilizer to change the angle of incidence of tail, thus changing the pitch trim (by changing the angle of attack and, thus, lift [negative lift, actually] generated by the tail). $\endgroup$
    – ammPilot
    Commented Oct 10, 2016 at 18:09
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Trims function is to balance the downward directed lift from the horizontal stab against the upward directed lift from the wing. Obviously, the lift generated by the wing varies with airspeed, and as a consequence we trim for an airspeed. The other consequence is we trim for 0 control column/stick deflection, that is to say the balance of forces we trim for assumes no control input. We do this for stability.

The perception we get from that is that we trim to reduce control forces, and while that conclusion is true, I argue that it is a consequence to what we are really doing. With that said, the control column force reduction is absolutely essential, particularly when flying a high performance airplane capable of a wide range of speeds.

For FBW systems, the need to balance the lift on the trim and wing is still necessary and so the need for trim still exists. The trim won't have an impact on control forces in this case, but will adjust the need for deflection of the stick just as in a traditional control column or stick.

For larger airplanes without a trim tab, the horizontal stabilizer becomes a horizontal stabilator in which the entire horizontal surface moves. The whole surface rotates to change its angle of attack and the associated change in lift is how trim is accomplished.

For the other trims (aileron and rudder) in larger airplanes these can be implemented in the hydraulic actuators that actually move those control surfaces. These would cause a constant offset deflection from the control input.

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In reference to item 1 of the question, it's not only to reduce the pilot's continuous force on the flight controls but also that of the autopilot. For example, you're straight and level with the autopilot holding altitude, and as the flight progresses your center of gravity is moving forward due to fuel burn. The autopilot compensates with an up-elevator force. On 747-100s -200s there's a gauge that shows you how much up-elevator the autopilot is commanding. So, if you're about to disengage the autopilot, you'd best glance down at that gauge to anticipate the control force you're going to need. If you're in cruise, you may still want to keep an eye on it and add appropriate trim manually to save a little fuel. If you're lazy, you can ignore it, and when the up-elevator requirement reaches a certain point, the autopilot will trim it out.

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  • $\begingroup$ from my time in simulators, that can really unexpectedly bite you. Disengaging the autopilot and suddenly the nose is pointing 30 degrees down on final... $\endgroup$
    – Bassinator
    Commented Oct 15, 2014 at 23:27
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    $\begingroup$ @HCBPshenanigans See terryliittschwager.com/hajj4contest.php for a humorous tale involving such. $\endgroup$
    – Terry
    Commented Oct 15, 2014 at 23:56
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Trim still exists on new airplanes. I don't know where you got the idea that planes don't have trim tabs, or that FBW eliminated this. Any sources or evidence to support your claim?

(This article describes the trim of the 787)

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    $\begingroup$ Some jets replace trim-tabs with a movable stabilizer, which might be what he's referring to? $\endgroup$
    – falstro
    Commented Jan 22, 2014 at 13:22
  • $\begingroup$ @ abelenky: Not many modern aircraft don't have trim tabs Yup, this is part of sub-question 3. Pitch trim is done by movable stabilizer than how roll trim & yaw trim are done? $\endgroup$
    – ToUsIf
    Commented Jan 22, 2014 at 14:23
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    $\begingroup$ @Lnafziger not even for aileron trim? Guess I never stared hard enough! $\endgroup$
    – egid
    Commented Jan 22, 2014 at 17:03
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    $\begingroup$ It's been around 25 years since my initial ground school on the 747-100 and -200, but as I remember, aileron and and rudder trim are handled by the hydraulics holding those control services to whatever trim setting you select. Both ailerons and rudders are split: there are inboard and outboard ailerons, top and bottom rudders. I seem to remember that you're trimming only the bottom rudder and the inboard ailerons, but I might not have that correct. $\endgroup$
    – Terry
    Commented Jan 22, 2014 at 19:52
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    $\begingroup$ Just as a matter of curiosity, 727-100s had 3 trim switches. Each control yoke had a fast-trim switch that really moved. Then on the center console there was a slow-trim switch that could be reached by either pilot. If you wanted a really fine adjustment, you could move the large trim wheel on the center console by hand. $\endgroup$
    – Terry
    Commented Jan 22, 2014 at 19:57
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Yes, trim still exists, it's a really useful feature! Imagine flying from New York to London while gently pulling at the column for 6 hours in order for the aeroplane to maintain its flight path, that would be most inconvenient.

Trim tabs were/are used back in the days when aeroforces provided the feel forces for the pilot, nowadays the planes are too fast and big and irreversible hydraulic actuators move the control surfaces. This means that feel must now be produced by mechanical springs and dampers.

And this arrangement also provides a convenient way to introduce trim: just adjust the neutral point of the feel spring. Picture the feel spring with the stick attached to one end, and the other end is connected to the aircraft structure via an adjustable screw-jack. Push the trim button, and the jack starts to turn - a nut on its shaft shifts until the feel spring is not extended anymore.

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