Ball v, string....,
Years ago we had to teach instrument flying in the RAF Gazelle. The aircraft drifted off heading to the right if the ball was centred, quite noticeable. However, if an En Route Supplement (20 mm thick book) was jammed under the right hand end of the instrument panel, it lifted it up on its rubber mounts and the aircraft stayed on heading. Problem was caused by the heavy extension on the right hand end of the IF panel, pulling it off level. The Army versions had a smaller panel and didn't have the problem.
Answer is to know your aircraft.
