‘Depends on the aircraft’ is the rather veiled answer!
Aircraft like the P-38 had a mass balance which was fixed to the outside of the control surface – in the P38’s case the best example is on the elevator. A lead based weight supported above and forward of the hinge line.
Steel is more common now for this type of balance, HOWEVER this type of balance has long been superceded…
Another type of mass balance can be bolted directly to the control surface, again forward of the hinge piont, but with minimal aerodynamic loss – it is shielded from the airflow by the fixed aerofoil.
There is also a balance method which utilises differences in pressure to assist the balance of the control surfaces – a plate extends forward of the control surface into the aerofoil section and is acted upon by the differing pressures above and below the aerofoil. Moving the control surface alters these pressures which in turn assist the movement of the control surface.
Movement assistance can be given by extending the control surface forward so the when the surface is moved, the leading edge sticks out into the airflow in the opposite sence to the main part of the surface.
Balance horns work in the same way as the above – think Spitfire rudders…
Here endeth the history lesson…
Now then, modern stuff! The internal mass balances you refer to, again depends on the aircraft type. The internal balances are not designed to be removable components (They can be removed, but are a nightmare to do! Its easier to change the surface). Often lead or steel based, but dependant on the size of the surface can just be milled aluminium. Corrosion is not an issue as they are heavily coated in JC5 or equivalent jointing compound, (PRC, etc). Reading between the lines, I can see an arguement for more hazardous substances, but the extremely dense usually radiocative materials, are overkill for the job.
Modern airliners do not use mass balances like this – the use of powered flying controls has negated the use of balances.
As for when… pass.
😀
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