@ColinTheMathmo
Ah, rabbit-hole explorer, eh? I confess I'm pretty deep in this warren myself. I believe it's for trim, is used in normal flight (but isn't itself the elevator, which might be what you were asking, that's a separate control surface), and has no flaps (basically they're on the wings). Happy to be corrected; I write in a relatively amateur capacity. You may enjoy this thread: https://homebuiltairplanes.com/threads/designing-a-removable-one-piece-horizontal-stabilizer-with-advanced-composites-self-study.37764 Fair warning: it may lead to wanting to build little airplanes.

*edited to add: I think post #31 in that thread (first post of page 2) is particularly pertinent to your question. I believe the author spent much of their career in vibration engineering (I don't know the proper name for their subspecialty, but anyway, suffice to say they're a switched-on cookie).

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