Is trimming aircraft to relieve pilot of applying force on control stick/Pedal still applicable?

ToUsIf
  • Is trimming aircraft to relieve pilot of applying force on control stick/Pedal still applicable? ToUsIf

    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?

  • 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)

  • 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.

  • 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.

  • 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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