Trim tabs and servo tabs on control surfaces

[Ayahuasca]

How do they work?

Yes, I get that the pilot can adjust the trim tab, and that the trim tab will push the larger control into the airflow, and that the larger control surface directs the movement of the aircraft.

And that the trim tab can be used to take pressure off the control surface so there less force on the controls.

So far, so good.

Where I get stuck is... the small trim tab, exposed to the airflow, moves a much larger control surface. But, but, ... but... the force exerted on the much larger control surface must be many times greater than that exerted by the trim tab, so how can the small trim tab ‘hold’ the larger control surface against that greater force?

[LimaDelta]

Because the lift from the control surface is not acting on the surface itself, it is acting on the whole horizontal or vertical stabiliser assembly. The centre of lift will be forward of the hinge.

I think. I'm a yacht engineer and PPL(A).

Cue an aerodynamicist to prove me wrong.

[Tony1963]

The trim tab, as with an elevator etc, will change the overall lift by moving only a small amount itself. Just like a small adjustment on your steering wheel. It doesn’t have to overcome all of the larger surface’s force.

[Mave]

LimaDelta said:

Because the lift from the control surface is not acting on the surface itself, it is acting on the whole horizontal or vertical stabiliser assembly. The centre of lift will be forward of the hinge.

I think. I'm a yacht engineer and PPL(A).

Cue an aerodynamicist to prove me wrong.

Yep, another way of looking at it is to imagine you positioned the hinge at the centre of pressure. There would be no restoring moment about the hinge. So the trim tan can be used to move the main surface without it fighting back 👍