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I understand the need for a trimming mechanism, but why use a secondary control surface placed on top of the first, when you could just adjust the resting position of the aileron/elevator/rudder itself?

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You have to have some way of holding the surface at other than where it wants to naturally trail in the airstream.

Your options:

  1. Hold it yourself with control pressure. Becomes a pain after a while. Some very simple homebuilt designs don't have any pitch trimming device and they have only one trim speed where they will fly hands off, the trim speed more or less controlled by stabilizer incidence; to fly faster or slower you have to hold continuous pressure on the stick.
  2. Put a spring in the control circuit, a bungee, where the spring can be tensioned with a trimming device, usually a lever or screw device with a friction lock of some sort, to hold the continuous control force in the control circuit so you don't have to. Gliders do this, and some light airplanes and homebuilts for pitch. Works really well for aileron and rudder trim and lots of GA use bungees for the rudder.
  3. For pitch trim, make the horizontal stabilizer incidence adjustable from the cockpit. The elevator itself just trails when hands off the controls. The fabric Pipers and Cessna 180/185 family do this, but you have a screw jack device and a pivoting stab attachment etc., so it's a bit complicated. The fabric Pipers also combine the screw jack with a bungee interconnecting the elevator and stab so the stab applies a bit of elevator force when it moves, giving a bit more overall authority.
  4. Use an external servo device to apply a force to the surface. You have the air blowing past, so why not stick another moveable surface at the trailing edge that can move independently of the main surface, and apply an aerodynamic servo force. A bit more drag than a bungee trim or adjustable stabilizer, but much more sensitive and versatile. Much lighter and less complex than an adjustable stabilizer.

Probably 80-90% of the airplanes with manually operated flight controls use trim tabs for pitch trim because it's just the most elegant solution taking in all the considerations - light, simple, effective. Tabs are less common for aileron or rudder until you get up to airplanes over 5000lbs or so and you tend to see bungees or nothing, or little fixed bent metal tabs that only trim out to a single flight condition.

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  • $\begingroup$ Still, it takes force to deflect the trim tab against the wind, and I don't see how is it less than the force needed to deflect the primary control surface itself. Since the force required to deflect = the force exerted by deflected trim tab. EDIT: Actually it makes sense now: the length of, let's say, the elevator is the lever on which the force coming from the trim tab acts on the elevator, so this gives a mechanical advantage requiring less force $\endgroup$ – Francis L. Dec 23 '19 at 1:58
  • $\begingroup$ They are pretty versatile. You can gear the tab to also respond to the surface movement, and can use it to boost your controls. That's a servo/trim tab, or just a servo tab if it's geared but not adjustable. You can also gear it to go the OTHER way, and work against the surface and artificially increase the resistance with surface movement, an anti-servo tab. Airplanes with all-flying tails like Cessna Cardinals and Cherokees get all of their static pitch stability from an anti-servo tab, with an adjustable neutral function for pitch trim. $\endgroup$ – John K Dec 23 '19 at 3:03

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