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When the ball of a turn indicator is centered, the...
  • A
    forces are along the normal axis.
  • B
    forces are through the gravitational center.
  • C
    forces are along the lateral axis.
  • D
    forces are along the aircraft pitch axis.

Refer to figures.

The Slip Indicator

It is desirable that turns should be properly balanced, with no side slip or skid. This implies that the angle of bank should be correct for the TAS and rate of turn. The slip indicator gives a direct indication of the state of balance of the turn.

This comprises a solid ball in a curved tube containing liquid with damps out the unwanted oscillations. The heavy ball behaves like a pendulum, with the centre of curvature of the tube acting as the effective point of suspension.

Operating Principles
Consider first the aircraft in level flight with lift L balancing weight W (figure 1). The weight W of the ball in the tube acts downwards and is exactly balanced by the equal and opposite reaction of the base of the tube on the ball, acting upwards towards the centre of curvature of the tube. If the wings are level, the ball will lie just between the two vertical lines etched on the tube.

Now let us consider a balanced turn to the left (figure 2). The aircraft with lift L equal and opposite to the resultant of aircraft weight W and centrifugal force C, the latter being proportional to TAS and rate of turn.
The ball is also subject to a centrifugal force depending on TAS and rate of turn, so it rolls outwards, taking up a new equilibrium position such that the reaction of the base of the tube on the ball is again exactly balanced, this time by the resultant of ball weight W and centrifugal force C.

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