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Most light GA aircraft, like this C172, have pitch trim tabs on one side of the elevator only. Why not on both? Doesn't the asymmetry add inefficiency in the form of a slight rolling moment?Skyhawk

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A small propeller driven plane like that will have a bit of adverse yaw, rolling to the left, because of propeller effects (P-Factor, slipstream, torque, etc)

The trim tab's effect is minimal because it is very small and is close to the centerline of the plane. Any drag it creates causes a slight roll and yaw to the right, specifically to counteract the propeller effects.

That keeps the overall effect almost un-detectable.

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    $\begingroup$ I agree, it wouldn't always roll right. But the fact that it is placed on the right is a good point. Regardless of roll, the (small) added drag on the right side could help counteract the aircraft's natural left turning tendencies. $\endgroup$
    – Geoff
    Commented Apr 23, 2018 at 15:38
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    $\begingroup$ The tab is on the left of citabrias. Presumably the effect is so minor that it doesn't even matter what side it's on? $\endgroup$
    – Kirk Woll
    Commented Apr 24, 2018 at 13:31
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    $\begingroup$ @KirkWoll no I'm talking about the pitch-trim, but let me clarify to say adverse effects of the prop in totality... I could explain but this already says it well...quora.com/… $\endgroup$
    – spacegirl1923
    Commented Apr 26, 2018 at 21:05
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    $\begingroup$ That is a really good quora explaination, @spacegirl1923. It covers some aspects not in my original answer. Thx. $\endgroup$
    – abelenky
    Commented Apr 26, 2018 at 21:07
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    $\begingroup$ @KirkWoll given how quickly you responded I know you didn't read the link I posted... $\endgroup$
    – spacegirl1923
    Commented Apr 26, 2018 at 21:07
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The reason the trim is only on one side is cost saving; one trim tab is cheaper to build and simpler (therefore cheaper) to maintain.

As for the rolling motion yes, the trim tab will cause a slight rolling motion. Very, very slight to the point you won't detect it for the following reasons:

  1. The trim tab is small. Compare the surface area of the trim tab to the aileron, there just isn't enough of it to make much difference
  2. Small lever arm: the ailerons are positioned far out on the wing so they have a longer lever arm, this decreases the force required to roll the airplane across the longitudinal axis. The trim tab is located almost on the longitudinal axis, so will have very little leverage
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    $\begingroup$ Wouldn't most of the (minimal) torque produced by the trim tab also be counteracted by the pilot typically sitting in the left seat? (at least, in a single-user configuration) $\endgroup$
    – 0xdd
    Commented Apr 23, 2018 at 20:03
  • $\begingroup$ The effect is so minimal I don't think it's a factor. There's plenty of airplanes that have it on the left, it's more of an engineering question. $\endgroup$
    – GdD
    Commented Apr 24, 2018 at 15:54
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Besides the aforementioned cost, consider how the additional unnecessary complexity can add risk to the free movement of the elevator and rudder, should problems occur with its linkages.

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  • $\begingroup$ It's all up to costs, in a sense. A more complex mechanism is more costly to realise if one wants to achieve the same level of safety and flawlessness as a simpler one. $\endgroup$
    – DaG
    Commented Apr 24, 2018 at 14:21

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